---

THE RARE EARTHS

[v]

PREFACE

During the thirty years which have elapsed since Dr. Auer’s application of the rare earths to the production of artificial light, the incandescent mantle industry has developed to an extent which gives it a prominent place among those chemical industries which may be considered essential to modern civilisation. This technical development has in turn assisted and stimulated the scientific examination of the elements of this group, with the result that ordered and accurate knowledge is beginning to replace the confused and uncertain data which had been collected by earlier workers in the field. These advances have served to emphasise the scientific interest and importance of the rare earth group, and the difficulty of bringing it into relation with the other elements. The relatively scant attention devoted to the study of this province of inorganic chemistry by teachers and students in England is probably due no less to the difficulty in classification, and the uncertainty with regard to the homogeneity and individuality of the various members of the family—an uncertainty by no means entirely removed even now—than to the fact that the very extensive literature on the subject is somewhat confused and difficult of access, especially to those unfamiliar with the French and German languages.

The present work is intended to give a general but fairly comprehensive account of the rare earth group. In accordance with general usage, the elements zirconium and thorium have been included, though these are now recognised as falling outside the limits of the rare earth group proper. The inclusion of titanium, which chemically is so far removed from the cerium and yttrium elements, has been considered desirable, not only on account of its general occurrence in the rare earth minerals, and its position in Group IVB with zirconium, cerium, and thorium, but also on account of its increasing chemical and[vi] technical interest, and its use in the ordinary quantitative laboratory operations.

Though the nature of the matter embraced has rendered the division into three parts desirable, the whole subject has been treated primarily from the chemical standpoint. In view, however, of the occurrence of considerable quantities of monazite within the British Empire, and of the possibility that in the near future the Brazilian fields will not remain the sole source of thorium nitrate, stress has been laid on the technical aspect, which is more especially developed as regards the production of monazite and the incandescent mantle industry in Chapters VII and XVII-XX.

In the preparation of Part I full use has been made of Dana’s indispensable ‘System of Mineralogy,’ as well as of the encyclopædic ‘Handbuch’ of Hintze, whilst for Part II the excellent monograph of R. J. Meyer, in Abegg’s ‘Handbuch,’ Vol. III, Div. I, and the work of the same author and Hauser, ‘Die Analyse der seltenen Erden und der Erdsäuren,’ Vols. XIV-XV of ‘Die Chemische Analyse,’ have been of service.

I have great pleasure in expressing my gratitude to Mr. A. Hutchinson, of Pembroke College, Cambridge, who has kindly read for me the manuscript of Part I, and suggested improvements; to Dr. H. J. H. Fenton, of Christ’s College, who has given me similar assistance in Part II; and to Dr. S. Ruhemann, of Gonville and Caius College, who has read Parts II and III. I am also greatly indebted to Mr. E. J. Holmyard, of Sidney Sussex College, who helped me with the preparation of Part II; and to Mr. H. M. Spiers, of Gonville and Caius College, who read the proofs for me with special thoroughness and care.

I have also to thank Professor Soddy and his publishers, Messrs. Longmans, Green & Co., for kind permission to reproduce from ‘The Chemistry of the Radio-Elements’ the diagram on p. 138.

S. I. LEVY.

[vii]

CONTENTS

[ix]

TABLE OF ABBREVIATIONS EMPLOYED IN THE REFERENCES

[xii-
xiii]

INTRODUCTION
By Sir William Crookes, O.M., F.E.S.

The mysterious group of substances to which have been given the title of “rare earths” has long been the subject of my special study, and no one knows better the magnitude of the difficulties encountered in the investigation, or realises more clearly the comparative insignificance of the knowledge we have acquired. The rare earths constitute the most striking example of the association of chemical substances with others which are closely allied to themselves, and from which they are separable only with extreme difficulty. They form a group to themselves, sharply demarcated from the other elements, and it is my belief that by following the study of them to the utmost limits, we may arrive at the explanation of what the chemical elements really are and how they originated, and discover the reasons for their properties and mutual relations. When this knowledge has been wrested from Nature chemistry will be established upon an entirely new basis. We shall be set free from the need for experiment, knowing a priori what the result of each and every experiment must be; and our knowledge then will as much transcend our present scientific systems as the knowledge of the skilled mathematician of the present day exceeds that of primitive man, counting upon his fingers. The great problem of the nature and genesis of the elements is approaching solution, and when the consummation is reached it will undoubtedly be found that the study of the rare earths has been an important factor in bringing it about.

[xiv]

There has long been a need for a work in the English language dealing historically and descriptively with these substances, and Mr. Levy’s book is well fitted to fill the gap. The chapters on the technical applications of the rare earths are particularly valuable, and the chemical aspect of the incandescent lighting industry is admirably treated. The author is to be congratulated upon having successfully achieved an important and useful piece of work.

WILLIAM CROOKES.

December 1914.

[1]

THE RARE EARTHS

PART I
OCCURRENCE OF THE RARE EARTHS

CHAPTER I
THE NATURE OF THE MINERALS AND THEIR MODE OF OCCURRENCE

The history of the rare earth minerals begins in the year 1751, when the Swedish mineralogist Cronstedt described a new mineral, which he had found intimately mixed with chalcopyrite[1] in the quarry of Bastnäs, near Ryddarhyttan, in the province of Westmannland, Sweden. Cronstedt gave the mineral the name Tung-sten (heavy stone); but as the name Tenn-spat (heavy spar, or heavy mineral) had already been selected by Wallerius (1747) for a new species from Bohemia, believed to contain tin, the choice was not a happy one. More than fifty years after its discovery, a new earth, now known as ceria, was isolated from Cronstedt’s mineral, for which at the same time the name Cerite was proposed.[2] Meanwhile, however, the Finnish chemist Johann Gadolin had observed, in the year 1794, a new earth in a mineral discovered by Arrhenius at Ytterby in Sweden in 1788; he called the new oxide Ytterbia, and the mineral in which he observed it, Ytterbite. The discovery was confirmed in 1797 by Ekeberg, who suggested the names Yttria and Gadolinite for the oxide[2] and mineral respectively; these names were accepted by Klaproth, and soon came into general use.[3] Whilst then Cerite was the first of the rare earth minerals to be discovered, it was in Gadolinite that new elements were first recognised, and the chemistry of the rare earths began in 1794 with Gadolin’s observation.

During the nineteenth century a considerable number of rare earth minerals was discovered and analysed; the quantities of the minerals observed, however, were so small that the name ‘Rare earths,’ applied to the new oxides found, was in every sense justified. Until the year 1885, though by that time the scientific interest of the group had been fully demonstrated by the discovery of several new elements, it was supposed that the minerals were almost entirely confined to a few scattered localities in Scandinavia and the Ural mountains. In that year Dr. Auer von Welsbach announced his application of the rare earths to the manufacture of incandescent mantles. Immediately there was a great demand for raw material for the preparation of thoria and ceria. The agents of the Welsbach Company visited all the important mining centres of Europe and America, intent on a search which shortly made it clear that the metals of the so-called ‘rare earths’ are really quite widely distributed in nature. The chief commercial deposits are the monazite sands of the Carolinas, the Idaho basin, and Brazil, the gem-gravels of Ceylon, and the remarkable deposits of gadolinite and allied minerals at Barringer Hill in Texas.

Whilst deposits of commercial importance are not very common, improved scientific methods and more careful search have shown that in traces the rare earths are of exceedingly wide distribution. Sir William Crookes has shown that yttria earths are often present in calcite and in coral; whilst Headden[4] noted that quite considerable amounts (up to 0·03 per cent.) were present in a yellow phosphorescent variety of calcite from Colorado. Similarly Humphreys[5] found that fluorspar usually contains traces of yttrium, whilst one or two phosphorescent[3] varieties contain quantities varying up to 0·05 per cent. The presence of yttria elements in phosphorescent varieties of calcite is interesting, and some connection has been suggested; there is, however, no positive ground for the belief in such a relation.

More recently Eberhard[6] has found very considerable quantities of rare earths in cassiterite (tin dioxide, SnO₂) and wolframite [an iron manganese tungstate, (Fe,Mn)WO₄]. A specimen of wolframite from the Erzgebirge was found to contain nearly 0·4 per cent. of rare earths, over half of this quantity being scandium oxide. A process which is readily susceptible of commercial application has been worked out by R. J. Meyer,[7] for the extraction of scandia and the yttria earths from the mixed oxides left after the treatment of wolframite for tungstic acid.

Using the spectroscopic method, which is capable of detecting one part of scandia in twenty thousand, Eberhard (loc. cit.) has found that minute quantities of scandia and yttria earths are present in almost all the commoner rocks and minerals. The minerals richest in scandium were beryl, cassiterite, wolfram, the zircon minerals, and the titanates and columbates of the ceria and yttria oxides. These results are in agreement with the observations of Sir William Crookes,[8] who has made the study of scandium especially his own. From the fact that scandium was often observed unaccompanied by any other member of the rare earth group, Eberhard rather favours Urbain’s conclusion[9] that scandium may not be a member of the rare earth family. Spectroscopic examination has also shown the existence of some of the rare earth elements in the sun and stars (see Europium, p. 189).

In view of this extraordinarily wide distribution of the rare earths in the mineral world, it is but natural that they should be found also in the vegetable and animal kingdoms. Tschernik[10] found 10 per cent. of rare earths in the ash of a coal from Kutais, in the Caucasus, and smaller quantities[4] have been found in the ashes of various plants; members of the group have also been identified in the human body.

Apart from the general occurrence in traces throughout the mineral kingdom, the minerals in which the rare earths occur are not very common; and though of fairly wide distribution, they are found usually only in small quantities. The earliest known locality, and the most fruitful in regard to number of species, has been the southern part of the Scandinavian peninsula;[11] the minerals occur in the numerous pegmatite veins traversing the granitic country-rock. The mining district round Miask, in the Ural mountains, has also long been known as a fruitful source. Other districts in Europe are the Harz and Erzgebirge, the Laacher See in Prussia, Joachimsthal in Bohemia, Dauphiné, Cornwall, etc. In the United States numerous localities are known; the chief are in the Carolinas and Georgia, Idaho, Oregon, California, Texas, Colorado, Virginia, Pennsylvania and Connecticut. Many of the southern provinces of Brazil also furnish important sources; the famous diamond fields of Minas Geraes, Matto-Grosso, Goyaz and the surrounding provinces yield numerous species, whilst the sands along the southern coasts of Bahia are rich in monazite, and form to-day the most important source of the mineral. Monazite, as well as other rare earth minerals, occurs also in South Africa. An interesting species, plumboniobite (q.v.), has recently been found in German East Africa. From Australia numerous occurrences are reported, whilst in Canada only a few districts are known to yield members of the group. In Asia important localities are Ceylon—the famous gem-gravels being the most accessible source—and one or two districts in Japan; monazite has been reported recently in considerable quantities near Travancore, India.[12] A more extended search will doubtless show that they occur in many other places.

For several reasons, the rare earth minerals[13] form a group[5] of the highest scientific interest. In the first place, they are generally of very complex composition, more especially with regard to their rare earth content. Thus, whilst it sometimes happens that one or other of the two groups of oxides (the ceria and yttria groups) may predominate to the complete exclusion of the second, it is no uncommon thing for a species to contain almost all the elements of the rare earth family. On the other hand, it is very uncommon for as much as 50 per cent. of the rare earth content to consist of any one oxide. The usual case is that a mineral contains chiefly yttria earths with some ceria earths, or vice versâ, the two sub-groups being almost always complex mixtures of several oxides, in which occasionally one may predominate. The remarkable similarity in chemical behaviour of the rare earth elements, and the difficulty of separating them, correspond to this peculiarity in their occurrence.

A second point of even greater interest is that the rare earth minerals are as a general rule strongly radio-active; further, it only occasionally happens that any mineral in which the rare earths do not form an important constituent has more than the feeblest activity; the exceptions being, of course, those uranium minerals which do not contain rare earths. The connection may be pushed even further; for whilst it appears that hardly any rock or mineral possesses absolutely no radio-activity, it is equally worthy of notice that traces of the rare earths, if not quite universal in the mineral world, are yet normally found in the majority of common minerals. As a natural consequence of their activity, the rare earth minerals are also as a rule rich in helium. These facts and the problems which they open up will be treated more fully in a later chapter.

A point of further interest is that of the age of the rare earth minerals. Except in a few cases where they are obviously of secondary formation, these minerals are among the oldest[6] known to us. They occur usually in igneous rocks, particularly in granites which have been considerably metamorphosed. Where erosion has occurred, they are found in deposits of such a nature as to leave very little doubt that the original rock was of plutonic formation and of very considerable age. Whilst it is true, however, that the rare earth minerals are generally of very great antiquity (none of the primary minerals being of more recent date than the palæozoic age), Eberhard has pointed out that the age and nature of common rocks seem to have absolutely no influence on the traces of scandia and yttria oxides which they contain. The geological evidence shows that the rare earth minerals are on the whole exceedingly stable, and that they have been generally formed during the pegmatitic alteration of granites. As early as the year 1840, Scheerer drew attention to these facts, and to the extreme age of the rare earth minerals; but so far his observation seems to have attracted little attention, and no explanation has been put forward.

In the following chapters no attempt is made to treat the rare earth minerals fully. An alphabetical list of all the minerals of any importance which contain rare earths, titanium, zirconium or thorium is given, and of these several are selected for fuller treatment. The basis of selection has been somewhat arbitrary. Those species which are of mineralogical importance, as well as those to which any special historical, scientific or commercial interest attaches, have of course been singled out; in addition, the more recently discovered species have occasionally been considered worthy of separate mention.[14]

It is now being realised that some knowledge of crystallography is essential to the chemist, and for this reason short accounts of the crystallography of the selected types have been given. Apart from this, every effort has been made to render the mineralogy intelligible to the student of chemistry who has devoted no attention previously to this subject, and also to stimulate an interest in the problems of mineral[7] chemistry, unfortunately too often ignored by our present-day teachers. The rare earth minerals afford good examples of some phenomena of great interest to the chemist, as, e.g. Isomorphism and Solid Solution, Dimorphism, Isodimorphism, and Molecular Change, and in one or two cases these are treated rather fully.

No special advantages are claimed for the system of classification, which is merely one of convenience. The minerals are divided into five groups:—

(1) The Silicates, which are grouped into three sub-divisions.

(2) The Titano-silicates and the Titanates.

(3) The Tantalo-columbates, sub-divided into those free from titanium and those in which titanium is present.

(4) The Oxides and Carbonates.

(5) The Halides and Phosphates.

A separate chapter has been devoted to the monazite sands, and another to the radio-active properties of the minerals.

[30]

CHAPTER II
THE SILICATES

(a) Silicates of the Yttrium and Cerium Metals

Cerite.

—Cerite is a silicate of the cerium metals, with small amounts of lime, ferrous oxide and water. Hintze gives the formula H₃(Ca,Fe)Ce₃Si₃O₁₃,[15] which Groth interprets as a basic metasilicate (Ca,Fe)[CeO]Ce₂(OH)₃(SiO₃)₃, i.e. a basic salt of the acid H₆Si₃O₉, a polymer of metasilicic acid, H₂SiO₃.

Crystals are not very common, the mineral usually occurring granular or massive.

Cerite is infusible before the blowpipe. It is attacked[31] readily by sulphuric acid, less easily by hydrochloric acid, with which it gives a gelatinous mass. Rammelsberg[16] found that the silica left behind on treatment of the powdered granular variety with the latter acid contained a variable proportion of bases, which he obtained and estimated after fusing the siliceous residue with sodium carbonate. From the different proportions of the earths in the part attacked by the acid and that left in the silica, he remarks, ‘It would almost appear that Cerite is a mixture of silicates which are not all attacked with the same ease by hydrochloric acid.’ Apparently without previous knowledge of this observation, Welsbach[17] noticed the same thing in 1884. He concluded that ordinary granular ‘cerite’ is a mixture of several minerals, among which there are at least two which contain rare earths. Of these, one, the chief constituent of the aggregate, is probably identical with the crystallised mineral, and is characterised by the readiness and completeness with which it is attacked by hydrochloric acid. The other does not react, with hydrochloric, but is readily attacked by sulphuric acid; it contains yttria earths, in addition to the ceria earths. In the extraction of ceria earths from the mineral aggregate, Welsbach used hydrochloric acid, so leaving this second mineral unchanged; but to avoid loss of the rare earths, sulphuric acid is more commonly employed for the decomposition.

Though of great historical interest, cerite is of very small importance for the extraction of rare earths at the present time, on account of its very rare occurrence. The mineral seems to be almost entirely confined to the Bastnäs quarry near Ryddarhyttan, Sweden, where it is found with the rare earth silicate allanite (q.v.), biotite, hornblende, bismuth glance, chalcopyrite, etc. Here it was observed in 1751 by Cronstedt, who called it Tungsten (vide supra, p. 1). In 1781 Scheele examined a specimen of Wallerius’s ‘Tenn-spat’ from Bipsberg, Dalecarlia, and found Tungstic Oxide (Acid), WO₃, in it.[18] After Scheele’s work, the Ryddarhyttan mineral[32] was known as Red Tungsten, until Bergmann (1780) and d’Elhuyar (1784) showed that the two minerals were chemically distinct. They considered the red variety to be a silicate of iron and calcium, the rare earths being mistaken for lime. In 1804 Klaproth examined it, and found a new earth; he called the mineral ‘Ochroite,’ from its colour. In the same year, but independently of Klaproth, Berzelius and Hisinger made the same discovery; they called the mineral Cerite and the new metal Cerium, in honour of the discovery of the minor planet Ceres by Piazzi in 1801.

The analyses of cerite made in the earlier part of the nineteenth century resulted in some confusion. Klaproth in 1807 found 34·5 per cent. SiO₂ in a specimen (his Ochroite); Vauquelin in 1805, and Hisinger in 1810, found 17·0 and 18·0 per cent. respectively.[19] Hermann[20] called attention to this discrepancy in 1843 (and again in 1861), and declared that the two could not be the same. For Klaproth’s mineral he proposed to revive the name Ochroite, whilst from his own analyses he proposed for the cerite of Berzelius the name Lanthanocerite, having found carbon dioxide and lanthanum, with much less cerium, in the latter.[21] In 1861 Kenngott partly explained these results by showing that the sample of cerite which Hermann had analysed contained Lanthanite[22]; but the extraordinarily high percentage of silica obtained by Klaproth remained unexplained. It may have been due to impurities of high silica content in the specimen he examined.

Cerite contains from 59·4 to 71·8 per cent. of rare earths (oxides), the amount and nature of which vary with the precise locality. The oxides consist chiefly of ceria, lanthana, and didymia (praseodymia and neodymia), the complexity of the so-called ceria having been shown by Mosander in the case of ceria separated from gadolinite as well as from cerite; but yttria earths are also found to a small extent in the mineral.

[33]

It is remarkable that neither thorium nor uranium has been found in cerite, which is thus practically unique among the rare earth minerals.

This anomaly becomes even more marked in view of the very high percentage of inert gases found by Tschernik[23] in a related mineral from Batoum. This is a very complex mineral in which the basic part is represented by rare earths, chiefly ceria earths (50·8 per cent.) with water (3·4 per cent.), and oxides of iron, calcium and copper (6·8 per cent.); the acidic oxides being silica (6·6 per cent.), zirconia (11·6 per cent.), and titanium dioxide (14·7 per cent.), with phosphorus pentoxide (3·2 per cent.), and sulphuric anhydride (1·7 per cent.). Traces of thoria are present, but no uranium; very considerable quantities (up to 1 per cent.?) of helium were found.

It is somewhat heavier than cerite (sp. gr. 5·08), but otherwise resembles it closely.

Gadolinite (Ytterbite).

—Gadolinite is a silicate of iron, beryllium, and the yttria earths, of the formula 2BeO,FeO,Y₂O₃,2SiO₂, which may be written FeBe₂Y₂Si₂O₁₀. According to Groth, it is a basic orthosilicate, Be₂Fe(YO)₂(SiO₄)₂, derived from the acid H₈Si₂O₈. The beryllium content varies considerably, and some authors recognise two varieties of the mineral, one rich, and one poor in beryllium; but Scheerer pointed out in 1840 that iron and beryllium are probably vicarious constituents.

Analysis gives silica 21·8 to 25·3 per cent.; yttria earths 22 to 47 per cent.; ceria earths 5 to 31 per cent. In a variety from Ytterby, the rare earth Scandia was first found, forming up to 0·02 per cent. of the mineral. Small quantities of thoria, ThO₂ may be present, and traces of helium were found by Ramsay, Collie, and Travers. According to Strutt it contains also uranium and radium. Like cerite, it does not often occur crystalline, being usually found in amorphous masses.

Gadolinite is of common occurrence in the pegmatite veins of the Scandinavian granite. It was first found in a felspar quarry on the island of Ytterby, near Stockholm, by a Lieutenant Arrhenius[24]; it is also found, together with a large number of other rare earth minerals, at Fahlun. It occurs in Norway on the islands of Hitterö and Malö, and in Germany in the Riesengebirge and the Harz. Probably the largest deposit is that in Texas, at Barringer Hill, near Bluffton, on the west bank of the Colorado River, Llano County, now owned and worked by the Nernst Light Company of Pittsburg; in 1904 a mass of very pure gadolinite weighing 200 lb. was found here.[25]

In the same place a decomposition product of gadolinite was discovered by Hidden and Mackintosh in 1889. They named it Yttrialite or Green Gadolinite. It contains no beryllium, and twice as much silica as the parent mineral, and approximates to the formula R₂O₃,2SiO₂, where R₂O₃ is chiefly yttria oxides; it is thus similar in composition to the newly found scandium silicate, Thortveitite (q.v.). It is amorphous and massive; and is often found in continuous growth with gadolinite. Pieces up to 10 lb. in weight have been obtained.

[35]

As stated above, Gadolinite was discovered by Arrhenius in 1788. Geijer examined it in the same year, and described it as a black zeolite. In 1794 it was analysed by Gadolin, who declared it to be a silicate of iron, aluminium, and a new element which he called Ytterbium. In 1797 Ekeberg examined it, and confirmed the discovery. He proposed the name Gadolinite for the mineral, and Yttria for the new earth; these names were accepted by Klaproth, who examined it with Vauquelin in 1800, and by the French crystallographer Haüy. In 1802 Ekeberg showed that the oxide originally taken for alumina was in reality beryllia; in 1816 Berzelius showed that ceria was present with the yttria.[26] About 1838 Mosander began his classical work on the earths in gadolinite. In that year he announced the separation of Lanthana,[27] and in 1842 that of Didymia, which he had actually discovered eighteen months earlier. In the latter year he announced[28] the separation of erbia and terbia. In 1842 also Scheerer[29] declared that the yttria from gadolinite was a mixture of earths, from its different behaviour on heating in closed and open vessels; but when Mosander announced the discovery of didymia (the announcement appears to have been hastened indeed by Scheerer’s observation) it was agreed that the colouration observed was probably due to that earth. The further history of these earths must be continued elsewhere (vide p. 111).

The behaviour of gadolinite on heating is of great interest. When heated uniformly, in closed or open vessels, the mineral suddenly glows very strongly at a definite temperature (according to Hofmann and Zerban[30] at 430°C.), with considerable alteration in properties. The amorphous variety exhibits the phenomenon much more markedly than the crystalline form. The change in the two cases is entirely distinct, the only effect in common being that both varieties are rendered insoluble in acids after the glowing. The amorphous variety, in the act of glowing, changes to the crystalline form.

[36]

This phenomenon of phosphorescence, or glowing, on heating, with a change in properties, was first observed by Berzelius in 1816. He found that the oxides of many metals, e.g. chromium, tantalum, and rhodium, became denser and insoluble in acids after being heated. Later in the same year he observed the glowing, with a similar change in properties, in the case of a gadolinite from Fahlun.[31] Apparently without knowledge of this observation, Wollaston published a similar account of the glowing of a gadolinite in 1825. In 1840 Scheerer noted an almost identical change in the case of the mineral allanite (q.v.). Scheerer made a careful study of the phenomena in the cases of allanite and gadolinite.[32] In each case he found that the variety of lower specific gravity showed, on heating, a very strong phosphorescence, accompanied by change of colour and optical properties, and a marked increase of specific gravity. Gadolinite suffered no appreciable loss of weight, but allanite had lost a little water after the change. Careful measurement of the specific gravity before and after the change showed, in the case of two varieties of gadolinite and one of allanite, that the volume had decreased in the ratio 1 : 0·94. Scheerer assumed that this ratio was constant for all such cases, and advanced a general explanation. We know now that numerous cases of similar phenomena occur, in which the change of volume is quite different; but Scheerer’s explanation is so ingenious, and so foreshadows some modern theories, that it is given here in full.

He ascribes the alteration to ‘interatomic change, involving change of relative position of atoms and decrease of interatomic distances.’ (Scheerer and the chemists of that period understood by atoms the ultimate particles of a body, making no distinction between elements and compounds; in this case he meant by atoms what we mean by molecules, and the word ‘molecule’ has therefore been substituted for ‘atom’ in what follows.) The change is simply one of closer packing of the molecules, which take up a more stable position with liberation of energy as heat and light. He imagines[37] his molecules as uniform spheres arranged in horizontal layers, as shown in Fig. 1. In placing one layer vertically over another there are three possible arrangements, of which only two concern us. In the arrangement for closest packing, B, say, a molecule of any one layer touches three molecules in each of the layers above and below, which with the six it touches in its own layer make twelve altogether. In the next closest arrangement, A, say, a molecule of any one layer touches only two molecules in each of the layers above and below it, so that one molecule is in contact with ten others altogether.

Fig. 1

Now it can be shown that the volumes of equal numbers of molecules in the arrangements A and B will be to one another as the height, H, of an equilateral triangle, to the height, h, of a regular tetrahedron whose edges are equal to the sides of the triangle, a length R (which will be equal to the diameter of a molecule).

Then H = ¹⁄₂R√3, h = R√²⁄₃.

That is, the volume changes in the ratio 1 to 0·943, the amorphous variety of gadolinite consisting of molecules in arrangement A, which go over to the closer packed arrangement B in the change to the crystalline form.

More extended work has shown that this ingenious and interesting explanation is not of general application. Thus[38] H. Rose[33] found that samarskite (q.v.) exhibited the phenomenon of glowing, but that the specific gravity was actually less after the change than it was before, i.e. there was an increase of volume. Damour observed glowing in the case of zircon from Ceylon (q.v.) with increase of density, the volume change being from 1 to 0·922, i.e. even greater than for gadolinite. Again, Hauser[34] observed in the case of his new rare earth mineral risörite a sudden change at a red heat, the mineral losing water, becoming very brittle, and increasing very considerably in specific gravity (the volume changing from 1 to 0·90 approximately), but without glowing. Ramsay and Travers[35] found that fergusonite (q.v.) glowed strongly when heated to 500°-600°, with decrease of specific gravity (5·62 before to 5·37 after), evolution of all its helium, and very considerable evolution of heat; they suggested that helium was present in combination, in an endothermic compound decomposed by heat, but in view of the properties of helium, this hypothesis seems hardly tenable.

It appears unlikely that any one explanation can cover all these interesting facts; there are in each case peculiar factors to be taken into account. In 1841, Regnault,[36] considering the case of the oxides observed by Berzelius, inferred that the development of light and heat denoted that the bodies possessed a lower specific heat after the change than before. The experimental difficulties encountered in attempting to dry the oxides prevented him from confirming this view. He measured the specific heats of the minerals calcite and aragonite (CaCO₃), and of the two allotropic modifications of phosphorus, but could observe no appreciable differences. H. Rose (vide supra) showed by experiment that considerable heat was evolved on the glowing of gadolinite, with a decrease of about one-fourteenth in the specific heat. In the case of samarskite there was, however, no appreciable evolution of heat, nor could he determine any difference in the specific heats before and after glowing.

[39]

Probably the only inference that can be safely drawn is that in most cases the change is due to some molecular re-arrangement. The evolution of water, helium, etc., in some cases, may possibly be due to intramolecular change, but on the one hand the current view at present is that the helium is mechanically held in radio-active minerals, and on the other hand it is not known that the water evolved is water of constitution; in an intermolecular change at fairly high temperature, these might be evolved without disruption of the true mineral molecules. The question of the energy involved, and consequently of the specific heats, appears to depend on factors peculiar to each case, of which at present no accurate conception can be formed; and the change in specific gravity is probably bound up with these. The loss of solubility in acids is a factor not always connected with glowing, as it is frequently observed in the laboratory after ignition of compounds, but here again no adequate explanation is forthcoming.

The possibility of chemical change in one or two cases, however, must not be ignored. Thus ammonium magnesium phosphate, NH₄MgPO₄, on heating glows, and is converted to magnesium pyrophosphate, according to the equation:

2NH₄MgPO₄ = Mg₂P₂O₇ + H₂O + 2NH₃

A case possibly analogous to this is that of the mineral sipylite (q.v.), R´´´₂Cb₂O₈, with ‘basic water’ (i.e. R´´´ partially replaced by H). Before the blowpipe this decrepitates with loss of water, and glows brilliantly. The specific gravity after the change does not appear to have been determined. Mallet explains the glow as due to a change to the pyrocolumbate.

Similar explanations may possibly hold in the cases of allanite and risörite, but it must be remembered that we are really ignorant of the part played by the water in these minerals.

Allanite.

—Allanite, or Orthite, as it is often called, is a mineral of the epidote family, containing rare earths. The general formula for Epidote is H₂O,4R´´O,3R´´´´₂O₃,6SiO₂, where R´´ is a divalent and R´´´ a trivalent metal, or vicarious series of metals. In the case of Allanite, R´´ = (Fe´´,Ca), R´´´ = (Al,Fe´´´,E), where E stands for metals of the cerium[40] and yttrium groups (Engström’s formula). Groth formulates it as a basic salt, R´´´₃(OH)R´´₂Si₃O₁₂, of the acid H₁₂Si₃O₁₂ (= 3H₄SiO₄).

Crystals are fairly common, but the mineral usually occurs massive or in rounded grains.

On heating, allanite becomes amorphous and isotropic with increase of specific gravity (cf. Gadolinite). Before the blowpipe it loses water, and melts to a black magnetic glass, many varieties phosphorescing strongly (vide supra). With hydrochloric acid it gelatinises, unless previously heated strongly, in which case it is not attacked.

Analyses show that the rare earth content varies considerably (vicariously as regards ferric iron and aluminium), ceria earths varying from 3·6 to 51·1 per cent. and yttria earths from traces up to 4·7 per cent.[37] Thoria is usually present, 0 to 3·5 per cent. In 1909 Fromme[38] found small quantities of beryllia in the mineral, and in 1911 Meyer[39] found amounts of scandium oxide up to 1 per cent. It contains traces of uranium, and is weakly radioactive. Ramsay, Collie and Travers found no helium (1895), but in 1905 Strutt found[41] radium in it, so that the presence of helium seems a priori probable.

Many varieties of the mineral are known, differing in habit, colour, water content, specific gravity, etc., and the percentage composition varies very much by reason of vicarious replacement of the bases. Goldschmidt[40] has found ‘Epidote-orthites’ which are isomorphous mixtures of orthite with an iron epidote; he concludes that most orthites are probably similar solid solutions, and in this way accounts to a large extent for the varying composition.

Allanite is of very wide distribution, though it is not often found in large quantities. The usual occurrence in pegmatitic veins in granites, syenites and other acid plutonic rocks has been often noted, e.g. in many parts of Sweden and Norway. It is found also in the extinct crater now forming the Laacher See, near Coblenz, Germany, and at Impilaks, near Lake Ladoga, on the border of Finland; a mass of the pure mineral weighing 300 lb. was recently discovered at Barringer Hill, (cf. under Gadolinite), and it occurs in large quantities in Amherst Co., Virginia. It is an accessory constituent of many acid volcanic and hypabyssal rocks, and has been found also in limestone, and in magnetic iron ores. On account of its exceedingly wide distribution, and the variations in appearance and composition, it has been repeatedly described under various names, varieties being constantly mistaken for new mineral species.

Its history is rather curious.[41] In 1806 the Danish mineralogist Giesecke made a protracted voyage to Greenland, collecting minerals and rocks; he remained there until 1813. In 1808 he sent off his first collection by ship to Copenhagen; on the voyage the ship was taken by an English privateer, and the cargo landed and sold at Leith. The minerals were bought by Allan, a Scotch mineralogist, who recognised, that they were from Greenland by the presence of cryolite, at that time only known to occur in Greenland. He mistook the mineral subsequently named after him for gadolinite, and sent it to[42] Thomson for analysis.[42] Thomson recognised it as a new mineral, and named it Allanite (1810). In 1815 Hisinger described a mineral from Ryddarhyttan, Sweden, which he called Cerin; Leonhard (1821) and Hauy (1822) showed that this was identical with Allanite. In 1818 Berzelius described two varieties of a mineral from Finbo, near Fahlun, Sweden, which he called Orthite, and Pyrorthite; these were eventually shown by Scheerer (1844) to be varieties of Allanite. In 1824 the French mineralogist Lévy described a mineral from Arendal, Norway, which he named Bucklandite, in honour of the English naturalist; in 1825 this was identified with a ‘black zeolite’ from the Laacher See by G. Rose, and in 1828 both were shown by Hermann to have the same composition as orthite or allanite. The list might be extended at will; the Tautolite of Kokscharow (1847), the Bodenite of Breithaupt (1844), the Muromontite of Kemdt (1848), and the Vasite of Bahr (1863) have all been shown to be varieties of the same bewildering mineral.

Hellandite.

—Hellandite[43] is a mixed silicate of rare earths with lime, magnesia, alumina, ferric and manganic oxides, with considerable quantities of water. The formula approximates to 3H₂O,2R´´O,3R´´´´₂O₃,4SiO₂, where R´´ = (Ca,Mg,Th2)—Thorium being able to replace two atoms of calcium or magnesium—and R´´´ = (Al,Fe´´´,Mn´´´ and rare earth metals). This may be written as a basic orthosilicate, R´´₂[R´´´´(OH)]₆(SiO₄)₄, a basic salt of the acid H₁₆Si₄O₁₆ (= 4H₄SiO₄). This composition puts it in the class containing topaz and some rarer silicates.

The mineral is crystalline, the crystals being well developed, but often dull and opaque by alteration (hydration).

The mineral dissolves easily in hydrochloric acid, with evolution of chlorine; it is less soluble in nitric and sulphuric acids. It readily fuses to a yellow mass.

It was first discovered by Brögger at Lindvikskollan, in 1903, and later, in larger quantities, at Kragerö in Norway. It occurs in pegmatite veins in granite.

Thalénite.

[44]—A silicate of yttria earths with water and small quantities of alumina, ferric oxide, carbon dioxide and alkalies. The ratio of rare earths to silica gives the formula R₂O₃,2SiO₂, or R₂Si₂O₇; if the water be included, the formula becomes H₂R₄Si₄O₁₅. The presence of both water and carbon dioxide indicates, however, that the mineral has been somewhat altered, and the simpler formula R₂Si₂O₇, (cf. Thortveitite, below) probably expresses the composition of the original mineral. It contains considerable quantities of nitrogen and helium, though uranium and thorium appear to be absent.

The ‘average atomic weight’ of the rare earth metals is 99, from which it appears that these consist chiefly of yttrium, with a smaller quantity of the metals of higher atomic weight.

[44]

It was discovered in 1898 by Benedicts, accompanying fluocerite (q.v.) in a quartz quarry at Oesterby in Dalekarlia.

Thortveitite.

[45]—A silicate of yttria earths, chiefly scandia, of the formula R₂O₃,2SiO₂. Scandia forms about 37 per cent. of the whole (R. J. Meyer); yttria with small quantities of the other yttria earths forms the bulk of the remainder of the bases, the ceria group being almost completely absent. Ferric oxide (with traces of manganic oxide and alumina) forms about 3 per cent. Thorium is present only in traces, and radioactivity is barely perceptible.

Thortveitite is the first mineral to be discovered in which the content of scandia is greater than 2 per cent.; in 1908 Crookes[46] examined a very large number of yttria minerals for scandia, and finally chose for extraction of the earth Wiikite (q.v.) which has a scandia content of 1·2 per cent.[47]

It is fusible with difficulty, and only partially attacked by hydrochloric acid. It was found by Thortveit, in 1910, in a pegmatite vein in granite, at Iveland, Sätersdalen, S. Norway, accompanied by euxenite, monazite, beryl, and the usual vein-materials (quartz, felspar, etc.). It was analysed and recognised as a new mineral by Schetelig (loc. cit.).

The following minerals, of which particulars will be found in the alphabetical list, also belong to this class:

[45]

Bagrationite, Bodenite, and Muromontite, varieties of allanite with differences in composition and physical properties.

Yttrialite, a weathered variety of gadolinite.

Elpidite, Erdmannite and Cainosite, more complex silicates.

Rowlandite, a comparatively simple silicate of the yttrium metals.

Yttrogarnet, a variety of garnet containing yttrium metals.

(b) Silicates of Thorium and Zirconium

Thorite.

—Thorite and its variety Orangite are somewhat altered forms of a pure silicate of thorium, ThSiO₄, containing also small quantities of water, usually uranium, and often rare earths, with iron, lead, calcium, and aluminium. Orangite differs from thorite in its beautiful orange colour and greater specific gravity. Both varieties are radio-active.

When unaltered, the crystals are tetragonal and uniaxial, the pure mineral ThSiO₄ being isomorphous with zircon, ZrSiO₄ (q.v.). By alteration they become isotropic.

Thorite contains from 1·4 to 3·1 per cent. of rare earths. According to Nilson and Blomstrand, the uranium is present as uranium dioxide, UO₂ replacing thoria, ThO₂, but Dunstan and Blake state that the two oxides are isomorphous (see under Thorianite, p. 74), and so they might be expected to be vicarious. Thorite was discovered by Esmark in 1828, and first analysed by Berzelius,[48] who announced the discovery of a new earth in it in 1829. The name Thorite is from Thor, the god of Scandinavian mythology.

Thorite is a member of a peculiarly interesting series of isomorphous minerals, which includes Cassiterite (SnO₂), Rutile (TiO₂), Zircon (ZrSiO₄), and most probably the allied silicate Naegite, and the rare earth phosphate Xenotime[46] (q.v.), which are very similar in forms and angles. The oxide TiO₂ is itself trimorphous, being known in the three crystallographically different forms, Rutile, Anatase, and Brookite (q.v.). On account of the isomorphism of cassiterite and rutile with the two silicates, it has been suggested that the oxide formulæ be doubled and written Sn(SnO₄) and Ti(TiO₄) respectively,[49] to show the analogy with Th(SiO₄) and Zr(SiO₄). Consideration of the molecular volumes (obtained by dividing molecular weight by specific gravity, i.e. multiplying by specific volume) lends a certain amount of support to this view. It has often been observed that isomorphous compounds, and many compounds which occur in parallel growth to one another, have nearly equal molecular volumes; there are, however, many exceptions. Taking molecular volumes for the series under consideration, we have, using approximate numbers only—

It will be seen that if the numbers for cassiterite and rutile be doubled, four out of the five show very fair approximation to the constant value 40. The number 60 for thorite is quite irreconcilable with the values obtained from the other members; of course pure silicate of thorium, ThSiO₄, is not known as a mineral, but it is most unlikely that the relatively small amount of impurity in the densest specimens of orangite should have depressed the specific gravity by over two units, as would be required if the molecular volume of thorite were to show even the most approximate semblance of agreement with the others. It cannot be too often remarked, however, that very little indeed is known of the molecular formulas of minerals, and that very little reliance can be placed on such[47] figures as the above. On the contrary, it is hardly conceivable that amphoteric oxides like those of tin and titanium, occurring in the form of heavy crystalline minerals, should have molecular formulæ only double the empirical formulæ. Where agreements of the kind do occur, they must be taken as indicating approximately equal degrees of molecular complexity in the minerals concerned, rather than as affording any real insight into the molecular condition.

Zircon.

—Zircon is a silicate of zirconium, ZrSiO₄, with small quantities of other elements. Most varieties contain ferric oxide and thoria; more rarely small proportions of the yttria earths may be present. All varieties contain traces of a large number of the common metals. Traces of radium are usually present, with helium and neon,[50] and the mineral is strongly radioactive.

It is infusible before the blowpipe, but loses its colour; some varieties glow and increase in density (see p. 38). In some varieties also the colour changes or disappears rapidly on exposure to sunlight, and is often restored on keeping in the dark. These phenomena of colour change have been[48] attributed variously to alteration in the state of oxidation of the iron present, and to the presence of organic matter. It seems probable that either cause or even both may be at the root of the change in particular cases.

On account of the hardness, unalterability, and strong refraction and double refraction, good crystals of zircon are used as gems. The two gem varieties, Hyacinth and Jargon, are found chiefly in the gem gravels of Ceylon. It was in a zircon from Ceylon that Klaproth discovered the new earth, Zirconia, in 1789.[51] In 1795 he found the same earth in hyacinth, and so showed the two to be identical.

Artificial crystals of zircon have been obtained by the action of silicon tetrachloride and silicon tetrafluoride on zirconia, and by the action of zirconium tetrafluoride on silica at high temperatures.

Zircon is one of the most widely distributed minerals known, though usually it occurs in very small quantities. Good crystals have been found in New Zealand, in Ceylon, at Miask in the Urals, and in North Carolina. This last deposit has been worked commercially for the extraction of zirconia for Nernst lamps (vide p. 320). It occurs in a decomposed felspar in a pegmatite dyke in the Archæan gneiss near Zirconia, Henderson Co., and can be easily extracted by picking or washing, after crushing if necessary. Should there ever be a considerable demand for zirconia, it could doubtless be saved as a by-product in the extraction of thoria from monazite sands (q.v.), zircon being very generally found in those sands (see below).

Zircon is common in crystalline rocks, limestones, schists, syenites, granites, etc. It is a constant accessory constituent in the acid igneous rocks, especially in the more acid eruptive rocks. It is readily detected under the microscope by the pleochroic haloes with which the tiny crystals are surrounded; these have been shown by Joly to be due to alteration of the surrounding rock by the radiations emitted by the radio-active constituents of the zircon. It also occurs as a constituent of those sands which are formed by the erosion of the igneous[49] rocks in which it is enclosed, and hence it almost invariably accompanies monazite in the so-called monazite sands.

Zircon is one of the least easily altered minerals; by the prolonged action of chalybeate and other waters, during many geological ages, however, it gradually changes, losing silica and gaining lime, oxides of iron, and water. Some of these altered varieties have received special names, as, e.g. Auerbachite, Malacone, Cyrtolite, and Alvite; but none of them is of special interest.

Naegite.

[52]—This rare mineral is a silicate closely related to zircon, but of rather more complex composition. It may be represented as silicate of zirconium, ZrSiO₄ (zirconia = 55·3, silica = 20·6 per cent.), with rare earths (chiefly yttria, 9·1 per cent.), uranium (UO₃ = 3 per cent.), and thorium (ThO₂ = 5·0 per cent.), partly as silicates, partly as columbates and tantalates ((Cb,Ta)₂O₅ = 7·7 per cent.).[53]

It is tetragonal, usually occurring in globular aggregates of crystals. The measurable angles are extremely close to those of zircon, and it is probable that naegite is isomorphous with the series mentioned above under Thorite.

So far it has only been found in the ‘placer’ tin deposits or ‘gravel tin’ of Japan.

The following minerals (see list) are also to be included in this sub-class:

Alvite (Anderbergite or Cyrtolite), Auerbachite, Malacone, Oerstedite and Tachyaphaltite, altered varieties of zircon.

Calciothorite, Eucrasite and Freyalite, altered varieties of Thorite.

Pilbarite, Thorogummite and Yttrogummite, hydrated silicates of thorium with uranium and other metals.

[50]

(c) Complex Silicates

Eudialyte (Eucolyte).

—This is a complex silicate of alkalies, lime, ferrous oxide, rare earths, etc., containing chlorine and a high proportion (up to 17 per cent.) of zirconia. The empirical formula is given by Dana as Na₁₃(Ca,Fe)₆Cl(Si,Zr)₂₀O₅₂. Brögger gives the simpler metasilicate formula R´₄R´´₃Zr(SiO₃)₇, where R = (Na,K,H), R´´ = (Ca,Fe,Mn,CeOH), and Zr(OCl) may partly function as an acid in place of SiO₂. The true formula, however, is quite uncertain, as the zirconia may function either as an acidic or basic oxide. The fact that a mineral of such exceedingly complex composition occurs in perfectly well-defined crystals indicates the intricate nature of the problems to be solved in mineral chemistry.

The double refraction is strong, being positive for eudialyte, negative for the Norwegian variety, eucolyte. From careful microscopic examination, Ramsay has found that zones of positive and negative birefringence, as well as isotropic (singly-refracting) zones can occur on the same crystal, and he suggests that the mineral is really composed of two isomorphous compounds forming mixtures. In view of the continuous variation of optical properties in an isomorphous series like the felspars, such an explanation seems doubtful. The optical behaviour of minerals is very often anomalous, and the phenomena in this case are probably due to repeated twinning, with some alteration in the double refraction, or to the lamellar intergrowth of two varieties having slightly different optical properties.

[51]

On heating, the mineral evolves moisture and readily fuses. It is easily attacked even by dilute acids, being named by Strohmeyer (1819) on account of this property. The dilute hydrochloric acid solution reddens turmeric paper—a test for the presence of zirconium.

It is found in Greenland, usually embedded in felspar, in Norway, in Lapland and in Arkansas, being generally associated with minerals rich in alkalies, e.g. ægirine, ælæolite, nepheline, sodalite, arfvedsonite, etc.

Beckelite.

—This is a mineral similar in composition to eudialyte, though not so complex, and of more recent discovery.[54] It is a silicate of ceria earths and lime, in which zirconia replaces silica; the oxygen ratio (i.e. ratio of oxygen in basic oxides to oxygen in acid oxides) is 3 : 1, and the formula Ca₃R´´´₄(Si,Zr)₃O₁₅, where R = rare earth metals, chiefly of the cerium group. It is thus a salt of an acid H₁₈Si₃O₁₅ [= 3H₆SiO₅ = 3(3H₂O,SiO₂)] with zirconium and silicon vicarious.

It is soluble in hot hydrochloric acid, even after ignition; the solution gives the turmeric test for zirconium.

It was found in a dyke in an ælæolite syenite, near the Sea of Azov.

The following minerals (see list) are also to be placed in the class of mixed silicates:

Arfvedsonite and cataplejite, complex zircono-silicates.

Hiortdahlite (Guarinite) and Lavenite, zircono-silicates with fluorine.

Caryocerite, Melanocerite and Steenstrupine, complex fluosilicates.

Auerlite, Britholite, Erikite and Florencite, phospho-silicates.

Cappelenite, Homilite and Tritomite, boro-silicates.

[52]

CHAPTER III
THE TITANO-SILICATES AND TITANATES

(a) The Titano-Silicates

Yttrotitanite or Keilhauite.

—A titano-silicate of calcium, aluminium, iron and yttrium metals. The mineral is isomorphous with titanite, CaO,TiO₂,SiO₂ (q.v.), and is itself probably an isomorphous mixture of titanite with the silicate (Y,Al,Fe)₂SiO₅, where Y = yttrium metals. Its composition will then be represented by the formula m (Y,Al,Fe)₂(SiO₅) + n CaTi(SiO₅).

The mineral is fusible before the blowpipe, and is decomposed by hydrochloric acid.

It was named by Scheerer in 1844 from its composition, and by Ekeberg in the same year in honour of the Norwegian geologist Keilhau.

Titanite or Sphene.

—This species, important as an accessory mineral of many rocks, is a titano-silicate of calcium, generally containing small quantities of aluminium and iron. The approximate formula usually given, CaTiSiO₅, is unsatisfactory; some specimens contain as much as 7 per cent. of ferric oxide, others up to 2 per cent. of manganese, whilst the percentage of titanium oxide, TiO₂, varies very considerably[53] (30 to 45 per cent.). Zambonini and Nickolan have independently analysed specimens for which no satisfactory formulæ could be deduced. For specimens containing trivalent metals, Groth considers the mineral to be an isomorphous mixture of CaTiSiO₅ and R´´´₂SiO₅ (see under Yttrotitanite, above); Blomstrand, however, advances the formula 2(R´´R´´´₂O₂,TiO)O,SiO₂, where TiO is basic, and the trivalent metals occur in the divalent group R´´´₂O₂; this formula is also supported by Zambonini.

More recently the problem of the constitution has been attacked by Bruckmoser, using Tschermak’s method of determining the nature of the salts present in silicates. In this method, the mineral is digested with hydrochloric acid, at a temperature not greater than 60°, until decomposition is complete; the silicic acid formed is washed by decantation, and dried in air at a constant temperature; it is weighed at regular intervals until the weight is constant. It is stated that if a curve of times and weights be plotted, a break is observed at the point where drying ceases (for the acid is of course wet) and decomposition begins; the composition at this point, which is taken as the composition of the acid required, can be determined from the weight of the acid, and the weight of anhydrous silica present, which is determined by ignition after the weight has become constant.

Employing this method in the case of titanite, Bruckmoser claims to have obtained the acids H₂Si₂O₅ and H₂Ti₂O₅. He therefore concludes that the constitution of the mineral is represented by the formula Si₂O₅,Ti₂O₅Ca, which presumably may be written Ca(Ti,Si)₂O₅.

It is fusible with difficulty before the blowpipe. Hot concentrated hydrochloric acid decomposes it partially, with separation of silica; boiling sulphuric acid, or, better, fused potassium hydrogen sulphate, decomposes it completely.

On account of the high dispersion and refractive index, clear specimens of sphene make very beautiful gems, but the stone is not sufficiently hard to stand much wear.

The mineral was discovered in Chamouni by Pictet in 1787, and was named Pictite by Delamètherie (1797). In 1795 Klaproth analysed a specimen from Passau, and, observing the presence of titanium (which he had just discovered in rutile), proposed the name Titanite. The mineral described by de Saussure (1796) as ‘Schorl rayonnante,’ and afterwards by Hauy (1801) as Sphene (σφήν = a wedge), was shown to be identical in composition with titanite by Cordier, and also by Klaproth (1810); the crystallographic identity was proved by G. Rose (1820).

On account of the difference in colour and composition, a large number of varieties are distinguished. The ordinary yellow and brown varieties are known indifferently as sphene or titanite. Ligurite has an apple-green colour; Semeline is a greenish form named from a fancied resemblance to flax seed. Lederite is a brown variety of tabular habit; Greenovite is rose-coloured, and contains manganese. Alshedite and Eucolite-Titanite are rich in the trivalent metals; Grothite is a brown variety containing a considerable percentage of ferric iron. Yttrotitanite, which contains a high proportion of rare earths, is usually treated as a separate species (see above).[55] Titanomorphite and Leucoxene are white amorphous varieties chiefly produced by alteration of rutile and ilmenite.

Titanite is a fairly widespread mineral; as an accessory rock constituent it is common in the massive plutonic rocks in tiny crystals, readily distinguished under the microscope by the high refraction and birefringence, whilst in large embedded crystals it occurs in many granular limestones, and in plutonic acid, as well as in some metamorphic rocks. In good crystals it is found in many parts of Switzerland and the Alps, in Dauphiné, the Tyrol, Piedmont, the Urals, South Norway, and other European localities; it is also widely distributed in the United States and Canada.

The mineral is important as a valuable source of titanium.

The class of Titano-silicates is a very large one, and might be extended almost at will by the inclusion of the numerous silicates which contain titanium. Owing to the frequency with which small quantities of silica are replaced by titanium dioxide, almost all the commoner silicate minerals contain the latter oxide, so that titanium is one of the most widely distributed of the elements. Relatively very few, however, of the titanium-bearing minerals contain the element in considerable quantities, and only two or three have any importance as commercial sources of titanium compounds.

Only those additional titano-silicates which contain titanium as an important constituent are mentioned below; short accounts will be found in the alphabetical list.

Johnstrupite, Mosandrite, Rinkite, Rosenbuschite and Tscheffkinite are complex titano-silicates containing yttrium or cerium metals.

Astrophyllite, Leucosphenite, Molengraafite, Neptunite and Rhönite are complex titano-silicates free from rare earth elements.

Benitoite is a simple titano-silicate of barium; Ænigmatite and Narsarsukite contain iron and sodium; Lorenzenite has sodium and zirconium. Schorlomite is a titaniferous garnet. A variety of olivine rich in titanium (Titanium Olivine) is also known.

[56]

(b) The Titanates

Yttrocrasite.

[55]—This is a complex titanate of rare earths (chiefly yttria earths) with lime, thoria, and oxides of lead, iron, uranium, etc.; it has a considerable water content. An approximate formula is R´´O,R^ivO₂,3R´´´₂O₃,16TiO₂,6H₂O, where R´´ = (Ca,Pb,Fe), R^iv = (Th,U), and R´´´₂O₃ = rare earths. No constitutional formula can be given; it will be noticed that the amount of titanium dioxide is considerably more than is required to combine with the bases present (cf. also Delorenzite below). It is radioactive.

It is infusible, and not easily soluble in acids. Hydrofluoric acid decomposes it, and the powdered mineral is also slowly attacked by boiling concentrated sulphuric acid.

It was found in 1904 by Barringer, in Burnet Co., Texas.

Delorenzite.

[56]—A compound similar to the above, but even richer in titanium dioxide, which amounts to 66 per cent. Tin dioxide is also present, with traces of columbic anhydride. The bases are the yttria earths (almost free from ceria earths), uranium dioxide, and some ferrous oxide, the formula being 2FeO,UO₂,2Y₂O₃,24TiO₂, with a little SnO₂ replacing TiO₂. It is strongly radioactive. Its closest chemical neighbour is yttrocrasite, but in appearance and angles it closely resembles polycrase (q.v.). Its discoverer, Zambonini, therefore formulates it as a metatitanate with titanium acting also as a base—polycrase is a mixed metatitanate and metacolumbate—thus, 2FeTiO₃ + U(TiO₃)₂ + 2Y₂(TiO₃)₃ + 7(TiO)TiO₃.

Ilmenite or Menaccanite (Specular Iron Ore, Titaniferous Ironstone, etc.).

—This is a titanate of iron, usually written FeTiO₃. Its constitution has given rise to very considerable discussion[57]; not only do the relative proportions of iron and titanium vary greatly, but the iron is undoubtedly present in both the ferrous and the ferric states, and in the former state is partly replaced in some specimens by manganese and magnesium. In 1829 Mosander put forward the view that the mineral consisted of FeTiO₃, ferrous titanate, with varying proportions of ferric oxide, the forms and angles of ilmenite being very similar to those of hæmatite, Fe₂O₃. This view was disputed by H. Rose, who concluded that the mineral must have been originally an isomorphous mixture of ferric oxide, Fe₂O₃, and titanic oxide, Ti₂O₃, which on exposure to high temperature in the earth’s crust would change according to the equation

Fe₂O₃ + Ti₂O₃ = 2TiO₂ + 2FeO

so that the proportion of ferrous iron increases with the proportion of titanium dioxide, as is actually found to be the case. This condition, however, is also satisfied by Mosander’s view. The latter view was also supported by Rammelsberg, who pointed out that the presence of magnesium indicated the existence of ferrous iron as a primary constituent. Additional support is lent to this view by the discovery of Pyrophanite, MnTiO₃ (see list), which is found to be isomorphous with ilmenite, so that there can be little doubt that MgTiO₃, which can be only a titanate, would, if it existed in the crystalline form (see Geikielite in list), also be isomorphous with ilmenite. Friedel and Guérin (1876) prepared artificial titanium sesquioxide,[58] Ti₂O₃, and found it to be isomorphous with hæmatite, Fe₂O₃; they concluded that FeFeO₃, FeTiO₃ and TiTiO₃ formed an isomorphous series, and that ilmenite was a mixture of the second with the other two. In 1890 Hamberg pointed out that there was no reason to suppose that hæmatite, Fe₂O₃, contains ferrous iron, i.e. has the constitution Fe´´Fe^ivO₃, analogous to Fe´´Ti^ivO₃, since in corundum, the analogous compound of aluminium, Al₂O₃, divalent aluminium can hardly exist; nevertheless, strict analogy of constitution is not necessary for isomorphism, as shown by the case of potassium nitrate, KNO₃, and aragonite, CaCO₃, so that hæmatite, Fe₂O₃, and ferrous titanate, FeTiO₃, might form solid solutions in varying proportions without the strictly analogous formula FeFeO₃ being true for the former. The balance of opinion inclines to the constitution (mFeTiO₃ + nFe₂O₃ in isomorphous mixture) originally proposed by Mosander. The evidence in support of this view has been greatly strengthened by the recent work of Manchot,[58] which has proved the absence of titanium sesquioxide, Ti₂O₃; the mineral is therefore to be regarded as a titanate.

The mineral is infusible; when powdered, it dissolves slowly in boiling hydrochloric acid, the filtered yellow solution giving the characteristic blue colouration of titanium salts on addition of tinfoil. In fused potassium hydrogen sulphate it dissolves readily. The variation in composition can be judged from the following limits:

[59]

Ilmenite is a widely distributed mineral. In crystals it occurs chiefly at Kragerö and Arendal in Norway, at Miask in the Ilmen mountains, in Dauphiné, the St. Gothard, etc.; in the massive form at Bay St. Paul, Quebec, and other localities in America; and in sands at Menaccan in Cornwall, Iserwiese in Bohemia, Puy de Dôme, dép. Haute Loire, France, and in Brazil, Australia, and New Zealand.

The mineral was discovered at Menaccan in Cornwall by McGregor, about 1790. He described it as containing iron and a new oxide; the unknown oxide was obtained in 1795 from rutile by Klaproth, who gave the name Titanium to the new metal it contained.

Short descriptions of the following titanates are also given (see list):

Davidite and Knopite; these are complex titanates containing elements of the cerium and yttrium groups.

Arizonite and Pseudobrookite—ferric titanates.

Perovskite, calcium titanate, and its variety Hydrotitanite.

Pyrophanite, a manganese titanate isomorphous with ilmenite, and Senaite, a species intermediate in composition between these two.

Geikielite, the magnesium analogue of ilmenite, with the variety Picroilmenite, which is rich in iron.

Uhligite, a titanate of zirconium, calcium and aluminium.

Derbylite, Lewisite and Mauzeliite, an interesting series of titano-antimonates.

Warwickite, a boro-titanate.

[60]

CHAPTER IV
THE TANTALO-COLUMBATES

(a) Tantalo-Columbates containing no Titanium Dioxide

Samarskite, Yttro-ilmenite or Eytlandite (Urano-tantalite).

—Samarskite is a tantalo-columbate[59] of the rare earth metals, with iron, calcium, and uranium.

Rammelsberg gives the formula R´´₃R´´´₂(Cb,Ta)₆O₂₁, where R´´ = (Fe´´,Ca,UO₂), and R´´´ = rare earth metals. Groth regards it as essentially a pyrocolumbate (tantalate) of rare earth metals R₄[(Cb,Ta)₂O₇]₃ the iron, calcium and uranium being more or less accessory constituents. Des Cloizeaux considers the formula indefinite. The mineral has also been found to contain tin, thorium, germanium, and helium. The yttria earths usually predominate (11·9 to 18·9 per cent.), the percentage of ceria earths being low (2·4 to 5·2 per cent.). The yttria earths contain the very rare oxide samaria.

The mineral is radio-active.

Before the blowpipe it fuses at the edges; with borax it gives an iron bead. It is decomposed by boiling concentrated sulphuric acid, better by fusion with potassium hydrogen sulphate, and leaching the residue with dilute hydrochloric acid—this leaves the insoluble oxides Cb₂O₅ and Ta₂O₅. On heating it glows, with decrease in specific gravity (cf. p. 38).

Samarskite occurs with other columbo-tantalates in felspar, or in veins in granite, near Miask in the Urals, near Quebec in Canada, and in Mitchell County, North Carolina. From the last-named locality, masses up to twenty pounds in weight have been obtained.

The mineral was first discovered in the Urals by Ewreinoff, captain of a corps of Russian mountain engineers. He sent a specimen for identification to the mineralogist Gustave Rose, who pronounced it to be a tantalate of uranium containing manganese, and called it Urano-tantalite.[60] In 1847 the chemist Heinrich Rose, brother of Gustave, in the course of his researches on tantalic ‘acid’ (oxide), analysed a specimen. He found the composition given above, and renamed it Samarskite,[61] in honour of the Russian engineer who furnished him with the specimen for analysis.

In 1907, Brögger[62] announced that Annerödite, of which he had published an account as a new species in 1881, was a parallel growth of the mineral columbite, (Fe,Mn)Cb₂O₆, on samarskite.

Both minerals are orthorhombic, but they are not isomorphous. The mistake was due to the fact that whilst the crystallographic data were determined from the upper crystals of columbite, the crystals of samarskite were used for analysis.

[62]

Plumboniobite.

[63]—This is a recently discovered mineral closely related to samarskite and yttrotantalite (q.v.). It is essentially a columbate[64] of yttrium metals, lead and uranium, with water, ferrous oxide, titanium dioxide, stannic oxide, alumina, lime, and cuprous oxide. The formula given is R´´₂Cb₂O₇,R´´´´₄(Cb₂O₇)₃, where R´´ = (Fe,Pb,Ca,UO), and R´´´ = Al and yttria metals, with isomorphous (?) metatitanate. The mineral is radio-active, and gives considerable quantities of gas on being heated with sulphuric acid (carbon dioxide 0·19, helium and nitrogen 0·22 per cent.). The yttria earths are rich in the oxides of gadolinium and samarium, and the mineral should prove a valuable source of these elements. It is remarkable that the ceria earths are almost entirely absent.

It occurs with mica and pitchblende in pegmatite veins in granite, at Morogoro, in the Uluguru Mountains, German East Africa.

Yttrotantalite.

—This is a tantalo-columbate similar in composition to Samarskite, and isomorphous with it; though, as the name implies, the acidic oxide is chiefly tantalum pentoxide, the percentage of columbic anhydride being much lower than in the latter mineral. It is a pyro-salt of the formula R´´R´´´₂(Cb,Ta)₄O₁₄ + 4H₂O,[65] where R´´ = (Fe,Ca) and R´´´ = rare earth (chiefly yttrium) metals (Rammelsberg). Strutt found thorium and radium in it. The manner in which the water is combined in this, as in many other minerals, is at present undetermined.

[63]

It is found at Ytterby in Sweden, and in South Norway.

Fergusonite, Tyrite, or Bragite.

—A columbate and tantalate of the rare earth metals, with uranium, iron, calcium, etc. The general formula is that of an ortho-compound, R₂O₃,(Cb,Ta)₂O₅ or R(Cb,Ta)O₄, where R = metals of the rare earths, chiefly of the yttrium group. Brögger includes the other constituents in the more complex formula (Th,U)(Si,Sn)O₄ + 12R(Cb,Ta)O₄; but the simpler formula agrees quite well with specimens from the most widely separated localities, and is usually adopted. The mineral is radio-active and contains helium.

Fergusonite was discovered by Hartwell. It occurs with samarskite, and often with gadolinite and allanite, in Norway and Sweden, the Carolinas, Texas, the Urals, W. Australia, etc.

On heating it glows suddenly between 500° and 600°C.,[66] losing all its helium, and with decrease in density (5·619 to 5·375). At the same time it gives out a considerable amount of heat—8·09 C.[67] for 1 gm. (see p. 38).

Sipylite.

—Essentially a columbate of rare earth metals, with oxides of tantalum, tungsten, zirconium, uranium, iron[64] and calcium, and some water. Mallet, the discoverer, gives the formula as R₂O₃,Cb₂O₅, the basic oxides including, besides the rare earths, Cb₂O₅ with Ta₂O₅ and WO₃, and some water. An alternative formula, making it a complex pyro-salt, is also given, but from its great similarity in form and angles to fergusonite, the first formula is preferred. Strutt finds that it contains not only uranium, radium and helium, but also thorium in considerable quantity (ThO₂ = 4·9 per cent.), a fact which had been overlooked by Mallet. The rare earths contain a high proportion of erbia.

Its behaviour on heating has been already mentioned (see p. 39); it is infusible. Boiling hydrochloric acid partially dissolves it; the solution gives the turmeric test for zirconium, and on diluting and adding metallic tin a sapphire-blue colour is developed, due to the columbium present. Boiling concentrated sulphuric acid decomposes it slowly.

It is found in Amhurst Co., Virginia, adherent to the allanite which occurs there in large quantities. It was discovered there by Mallet in 1877, who named it, on account of the columbium (niobium) present, from Sipylus, one of the sons of Niobe.[68]

In this class, also, are to be included the following minerals (see list):

Nohlite and Vietinghofite, varieties of Samarskite.

Hjelmite and Kochelite, minerals closely related to Yttrotantalite and Fergusonite respectively.

Koppite, Loranskite, Microlite and Rogersite, complex tantalo-columbates containing elements of the cerium or yttrium groups.

[65]

(b) Tantalo-Columbates containing Titanium Dioxide

Æschynite.

—A columbate and titanate of the cerium metals, with thorium, calcium, iron, etc. From the results of an analysis on a specimen from Hitterö, Norway, Tschernik proposed the rather formidable formula

2(2Ce₂O₃,3TiO₂),4(ThO₂,TiO₂),Y₂(CbO₃)₆,3(CaO,TiO₂),3Fe(CbO₃)₂,Fe(TaO₃)₂,6TiO₂.

This can be simplified to Y(CbO₃)₃ + ThTiO₄ + ³⁄₂TiO₂, in which Y represents rare earth metals partially replaced (2 atoms) by ferrous iron (3 atoms), whilst thorium can be partially replaced by (2 atoms of) ferrous iron or calcium. Strutt found it to contain the uranium-radium combination and helium.

It occurs at Miask, in the Urals, at Hitterö in Norway, and at Fredriksvarn. The variety from the last locality is called Polymignite; it was shown by Rose to be probably identical with Æschynite. Æschynite was discovered by Berzelius at Miask and named by him from the Greek αίσχύνη, shame, from the fact that its composition could not at that time be determined.

If the ceria earths be largely replaced by yttria earths, a variety very similar in appearance and angles, but approximating to polycrase (q.v.) in composition, is obtained. This mineral was found in 1879, and referred to Æschynite; analysis subsequently showed its true composition, and it was named Blomstrandine (q.v.) by Brögger in 1907.

[66]

The Isodimorphous Series Euxenite, Polycrase, Blomstrandine, and Priorite.

Euxenite and Polycrase are members of an isomorphous series and vary considerably in composition. The composition of the series is that of mixed columbates and titanates of yttria earths (with, as usual, some ceria earths), with uranium and zirconium, and water. Before the isomorphous relation was recognised, Rammelsberg gave for Euxenite the formula R´´´(CbO₃)₃,R´´´₂(TiO₃)₃,1¹⁄₂H₂O. The ratio of the acidic oxides, Cb₂O₅ : TiO₂, is here 1 : 2. This is the greatest value of the ratio, which varies for the series between 1 : 2 and 1 : 5.[69] The end members, the pure metacolumbate and pure metatitanate respectively, are unknown; all the members occurring in nature are to be regarded as mixtures of these within the limits set by the ratios ¹⁄₂ and ¹⁄₅. Brögger[70] suggests that the name Euxenite be retained for all members for which the ratio is between ¹⁄₂ and ¹⁄₃, whilst for those minerals in which it is less than ¹⁄₄ the name Polycrase be kept; these views have been supported by Lange, who has analysed members of the series.

The members of this isomorphous series, however, are themselves dimorphous, that is, can each crystallise in two different ways. The second form corresponding to the Euxenites is known as Priorite, whilst that corresponding to Polycrase is known as Blomstrandine; and these second forms are themselves members of a parallel isomorphous series of the same chemical composition, of course, as the first series. It is, perhaps, undesirable to cite this as a typical example of an isodimorphous series, since no end members of unmixed composition are known. A perfect example of such a series is furnished by the oxides of antimony and arsenic. Each of these compounds exists in two distinct crystalline varieties, antimony trioxide, Sb₂O₃, as Valentinite (orthorhombic) and Senarmontite (cubic), arsenic trioxide, As₂O₃, as Claudetite (orthorhombic) and Arsenolite (cubic); and these two modifications[67] are isomorphous with one another, senarmontite with arsenolite, and valentinite with claudetite.

In the case we are considering, the name Euxenite is applied to one crystalline modification (A) of a number of isomorphous compounds within certain limits of composition, the name Priorite to the second crystalline modification (B) of the same compounds; the name Polycrase is applied to compounds having the crystal form A, and a composition varying within a second set of limits in the same chemical series, whilst this second set of compounds in the crystalline form B is known as Blomstrandine.

Stated as concisely as possible, the relationship is as follows: Each member of this chemical series of continuously varying composition can crystallise in two forms, which are the same for every member. The two varieties at one end of the series are called euxenite and priorite, at the other end polycrase and blomstrandine.

Thus, whilst euxenite and priorite, at the one end, and polycrase and blomstrandine at the other, have the same compositions, euxenite and polycrase have the same crystalline form, whilst priorite and blomstrandine have the same second crystalline form.

All four minerals have the same bright black appearance, and bright conchoidal fracture; they are all four isotropic, probably as a result of hydration. All are orthorhombic, but the measurements for euxenite and polycrase are different from those for blomstrandine and priorite. The two latter are not so widely distributed as the two former. Blomstrandine occurs at Hitterö, Arendal, and other localities in Norway; priorite is found in Swaziland, South Africa.

The crystal system of the Polycrase-Euxenite series is orthorhombic, but Dana gives slightly different axial ratios for the two minerals. This, though Brögger gives the same values for both, is by no means incompatible with isomorphism, as a glance at the axial ratios for the minerals aragonite, strontianite, witherite, etc., of the series of the orthorhombic carbonates, will show.

Brögger’s ratios for the two are a : b : c = 0·3789 : 1[68] : 0·3527; Dana gives for polycrase 0·3462 : 1 : 0·3124, for euxenite 0·364 : 1 : 0·303.

Euxenite.

The mineral is infusible and with difficulty soluble in acids. It occurs in many localities in Scandinavia (Hitterö, Arendal, Brevig, etc.), in North Carolina, South Australia, etc. It was discovered by Scheerer at Jölster, in Norway, in 1839.

The Euxenite-Polycrase series was studied by Hauser and Wirth in 1909,[71] in an endeavour to establish their theory that the proportions in which the various earths and acids occur in this group of minerals is subject to definite laws beyond the ordinary laws of combination. Thus of the erbia earths they state that the proportion of holmia and dysprosia increases relatively to erbia as titanium dioxide increases, i.e. as we pass from the euxenites to the polycrases; at the same time scandia and yttria increase relatively to the other yttria earths (the terbia group), whilst in the ceria group samaria and praseodymia decrease relatively to the others. Thus samaria is found in appreciable quantities only when the titanium content is low. The original paper must be consulted for full details.

It was stated above that zirconium was found in euxenite in 1879. In 1901 Hofmann and Prandtl[72] declared that zirconia was an unfailing constituent of the mineral, and that it was always accompanied by a new oxide, which they named Euxenia (‘Euxenerde’). This was characterised by the solubility of its oxalate in acid solutions, the insolubility of[69] the precipitated hydroxide in excess of alkali, and the gradual precipitation by hydrogen peroxide from a slightly acid solution of its salts. In their paper quoted above, Hauser and Wirth state that zirconia is never present in typical euxenites. In a second paper[73] they state that after exhaustive treatment of every known zirconia mineral, they can find no trace whatever of the ‘new earth,’ and conclude that Hofmann and Prandtl must have made some experimental error. During this examination, they observed radioactivity in some minerals which contained no traces of uranium or thorium.

Risörite.

[74]—A columbate of yttria earths, with titanium; ferric oxide, alumina, lime and lead monoxide are present in small quantities. It resembles fergusonite in composition, but differs in the almost complete absence of uranium, the high loss on ignition, and the amount of titanium present, which is here considerable (TiO₂ = 6·5 per cent.). Hauser regards it as an orthocolumbate, R´´´(Cb,Ta)O₄, with an isomorphous admixture of metatitanate, R´´´₂(TiO₃)₃.

The rare earths are chiefly yttria, with some erbia earths and a little terbia; ceria, lanthana and didymia are also present. The mineral contains a considerable amount of helium, which is remarkable in view of the very small content of uranium and thorium (cf. Thalenite). It is radioactive, the active constituent being precipitated with the lead (and to a very small extent with the rare earths).

It is infusible, but at a red heat it loses much water, and becomes very brittle, with increase of specific gravity; no glowing is observed. It is attacked by boiling concentrated sulphuric acid, and by fused potassium bisulphate; also by hydrofluoric acid (40 per cent.), with separation of the insoluble rare earth fluorides.

The mineral was found in a granite-pegmatite at Risör, South Norway.

Wiikite.

[75]—A mineral of very complex composition, for which no definite formula can be assigned. Its chemical nature may be understood from the following analytical data:

Columbic and tantalic anhydrides = 16·0; Dioxides of titanium and zirconium = 23·4; Silica = 17·0; Ceria = 2·5; Yttria = 7·6; Scandia = 1·2; Thoria = 5·5; Ferrous oxide = 15·5; Uranic oxide = 3·6; water (and gas) = 5·8 per cent.

Traces of lime, magnesia, stannic oxide and sulphur are also present.

The mineral is infusible; on heating, helium, sulphuretted hydrogen and water vapour are given off, and a white sublimate is formed. The evolution of gas is almost explosive, the mineral breaking with a curious fracture.

Wiikite is partially attacked by acids, readily by fused potassium bisulphate. It is radioactive.

The mineral was found with monazite in a felspar quarry at Impilaks, Lake Ladoga, Finland. It is important as the source of scandium used by Sir William Crookes in his investigations of that element; some specimens of the mineral contain over 1 per cent. of the oxide (see p. 44).

The following related minerals, of which descriptions are given in the alphabetical list, are to be included here:

Arrhenite, Chalcolamprite, Endeiolite and Wöhlerite, are complex tantalo-columbates containing silica.

[71]

Hainite contains both silicon and titanium.

Dysanalyte is a titano-columbate believed by Hauser[76] to be merely an impure form of perovskite (see p. 14).

Ilmenorutile and Strüverite are closely allied minerals believed by Prior[77] and Schaller[78] to be isomorphous mixtures of rutile with Tapiolite or Mossite (ferrous tantalo-columbates).

Pyrochlore is a complex titano-columbate containing elements of the cerium or yttrium groups.

Blomstrandite is an hydrated titano-columbate of rare earth elements, with calcium and uranium; it must not be confused with blomstrandine.

[72]

CHAPTER V
THE OXIDES AND CARBONATES

(a) The Oxides

Uraninite or Pitchblende.

—Uraninite consists essentially of oxides of uranium (UO₂ + UO₃ = 75 to 85 per cent.), associated with thoria, zirconia, rare earths, beryllia, and oxides of lead. Traces of lime, iron oxides, silica, bismuth, and arsenic are also sometimes present, with water in widely varying quantities. Nitrogen and helium are always found in it, and, of course, radium. Groth regards pitchblende as uranous uranate U^iv(U^viO₄)₂, the uranium in the acidic radicle being hexavalent and in the basic radicle tetravalent, and in the latter condition partially replaced by lead, thorium, and rare earths.

Szilard[79] regards it rather as a loose compound or even a solid solution of oxides of thorium and uranium,[80] with small quantities of other oxides, he having obtained apparently homogeneous (though non-crystalline) bodies by dissolving thorium hydroxide in solutions of uranium salts and evaporating to dryness.

The cubic form of the crystalline varieties has been taken as indicating that the mineral is really a spinel,[81] but it is difficult to see how the general formula of that group can be considered comparable to the uranyl uranate formula, UO₂,UO₃, for pitchblende.

[73]

It is infusible before the blowpipe, but readily soluble in nitric acid.

The mineral occurs both as a primary and secondary constituent of rocks; as a primary mineral it is found in Norway, North Carolina, etc.; as a secondary species it occurs in the massive and hydrated form, with ores of lead, silver, tin, etc., in Saxony and Cornwall, and at the celebrated mine of Joachimsthal, in Bohemia. The latter deposits, consisting of the massive and altered varieties, for which the name Pitchblende is generally reserved, have been much used as a source of radium, especially those at Joachimsthal, and the Cornwall ore.

Several varieties of uraninite have been distinguished by special names. Crystalline varieties from Anneröd and Arendal in Norway are known as Bröggerite and Cleveite respectively; Nivenite is a third form. In these varieties uranium oxides have been replaced to a considerable extent by the rare earths and thoria. An amorphous variety of doubtful composition, produced by alteration, is known as Gummite; Uranosphærite is a similar altered form.

Thorianite.

[82]—This interesting mineral consists chiefly of thoria, ThO₂ (55-79 per cent.), with oxides of uranium (11-32 per cent.), and ceria oxides (1-8 per cent.); oxides of lead and iron are also present in small quantities, and zirconia with silica, probably due to associated zircon.

Helium is present, and the mineral is strongly radioactive. A careful analysis by Hahn[83] shows traces of many metals; the same chemist has also separated an extremely active[74] component, 250,000 times as active as thorium nitrate, which he calls Radiothorium.

The composition has been accounted for (Dunstan and Jones, loc. cit.) on the hypothesis that thoria (ThO₂) and uranous oxide (UO₂) are isomorphous, the mineral being really a solid solution. Whilst, however, the crystal system of the natural body is really rhombohedral (vide infra) the two pure oxides appear to be cubic. Thus Troost and Ouvrard[84] obtained artificial thoria in minute octahedra; and, similarly, Hillebrand[85] obtained uranous oxide in octahedra by reduction of uranyl chloride, UO₂Cl₂, though his work seems to be open to objection. On the other hand, the same author[86] found that uranous oxide and thoria, fused together in almost any proportions, gave a homogeneous body crystallising in octahedra (cf. Szilard, Compt. rend. 1907, 145, 463, quoted under Uraninite). The probability of the isomorphism of the oxides is strengthened by the observation of isomorphism in the sulphates. As early as 1886, Rammelsberg showed that uranous sulphate, U(SO₄)₂, crystallises with nine molecules of water and is isomorphous with the corresponding thorium sulphate, Th(SO₄)₂,9H₂O; and six years later, Hillebrand and Melville[87] obtained mixed crystals of the two sulphates which were exceedingly close in forms and angles to those of pure uranous sulphate. It is then at least probable that the two oxides are isomorphous, though the point cannot be regarded as satisfactorily proved, by reason of the anomalous crystal forms of the naturally occurring mixtures, thorianite and uraninite. The recent results of Kobayashi[88] point to the conclusion that different varieties of thorianite may exist, in each of which the oxides of thorium and uranium bear definite simple ratios to one another.

Thorianite is infusible, incandescing before the blowpipe. When powdered, it dissolves readily in nitric and sulphuric acids, with evolution of helium. Gray[89] has shown that the helium content can be reduced by 28 per cent. by fine grinding, thus showing that part at least of the gas must be mechanically held.

Thorianite was found in Ceylon, being originally mistaken for pitchblende. A sample was supplied by the discoverer, Mr. Holland, to the officers of the Mineral Survey, by whom it was sent to London for examination. Its composition was determined by Dunstan, who named it. It was found in the river gravels (gem-gravels), the matrix being a pegmatite granite. It is a valuable source of thorium nitrate for incandescent mantles, one ton of the mineral (with thoria content of 70 per cent.) having been sold for £1500; but the supply is small and unreliable.

Baddeleyite.

[90]— Baddeleyite consists of almost pure zirconia (ZrO₂ = 96·5 per cent.) with small quantities of ferric oxide, alumina, lime, magnesia, alkalies and silica. Thoria and rare earths are present in traces, uranium is absent; the mineral is not radioactive, and contains only traces of helium.

[76]

The mineral is insoluble in acids, readily soluble in fused potassium hydrogen sulphate. Before the blowpipe it is almost infusible; it dissolves in the fused borax bead, rapid cooling causing separation of crystals. If a bead containing zirconia be heated until the borax is partially volatilised, zirconia crystallises on cooling in tetragonal crystals, isomorphous with those of rutile.[91]

The mineral was discovered in 1892 by Hussak and L. Fletcher independently. The former, who obtained it from the pyroxenite sand of São Paulo, South Brazil, believed it to be a tantalo-columbate, and called it Brasilite. Fletcher found it in a gem-gravel from Rakwana, Ceylon, and named it Baddeleyite. An analysis by Blomstrand of Hussak’s mineral showed it to be identical with the Ceylon mineral, and Hussak withdrew his name and accepted Fletcher’s. It has recently been found[92] in a corundum-syenite, near Bozeman, Montana, U.S.A.

The mineral now comes on the market in commercial quantities; pure zirconia almost entirely free from iron can be obtained by leaching with acids. The pure oxide is extraordinarily refractory, and promises to be of great use for crucibles, furnace linings, etc. (vide p. 324).

[77]

Rutile.

—Titanium dioxide, TiO₂, occurs crystallised in nature in the three minerals Rutile, Brookite, and Anatase (Octahedrite), which therefore form a trimorphous series. They are all stable minerals, though rutile appears the most stable, being occasionally found in pseudomorphs after the other two. The family is remarkable in that it is not unusual to find two of them occurring together—an uncommon phenomenon with polymorphous minerals.

Rutile often contains small quantities of iron and chromium. The ferriferous varieties are distinguished as Nigrine, which is black, with 2-3 per cent. ferric oxide, and Ilmenorutile, with up to 10 per cent. of ferric oxide, and specific gravity up to 5·13.

The mineral is insoluble in acids, but can be dissolved after fusion with alkalies or alkali carbonates.

Rutile is a member of the isomorphous series, cassiterite, zircon, etc. (see under Thorite), and in particular it has the colour, appearance, and twinning of cassiterite, from which, however, it is readily distinguished by its lower specific gravity. In this connection it is interesting to note that an apparently pure specimen, quite free from inclusions, was found (1904) to contain 1·7 per cent. of tin dioxide.[93]

[78]

As an accessory rock mineral, and also as an important constituent of many sands, rutile is of very wide distribution. It occurs, usually imbedded in quartz or felspar, in many granites, syenites, gneisses, slates, and allied rocks; in acicular crystals penetrating quartz it forms the ‘Veneris Crinis’ of Pliny. At Risör and other localities in Norway, it is found in the massive form, and it is largely worked at Risör as a source of titanium. It occurs in all the countries of Europe, and largely in America. Arendal, Kragerö, and Risör, in Norway, the Binnenthal, the Urals, the St. Gothard, Castile, Magnet Cove in Arkansas, Alexander Co. in N. Carolina, Barre and Shelburne in Massachusetts, and Chester Co. in Pennsylvania are the chief localities.

It was in this mineral that the element titanium was first recognised by Klaproth (1795).

Anatase (Octahedrite)

 is the second crystalline modification of titanium dioxide.

It is found at Bourg d’Oisans in Dauphiné, and in Norway, the Urals, Brazil, etc. In Switzerland it occurs as the variety Wiserine, which was at one time believed to be xenotime. It was named Octahedrite by de Saussure, in 1796, from the prevailing habit, and Oisanite, from its occurrence in Dauphiné, by Delamètherie, in 1797. The name anatase (ανατασις =[79] erection) was proposed by Haüy, being intended to denote that the vertical axis (c : a) is greater than that of rutile, the other tetragonal modification of the dioxide.

Brookite

, the third form of this compound, is orthorhombic.

The chief localities are Bourg d’Oisans, Miask, the St. Gothard, the Tyrol, Magnet Cove in Arkansas, and Tremadoc in Wales.

Titanium dioxide can be obtained crystalline by the action of steam on titanium tetrafluoride, TiF₄, at high temperatures; it is stated that by varying the temperature of the reaction, any one of the three crystalline modifications can be obtained.

The only other minerals which need be mentioned in this class (see list) are:

Zirkelite, a complicated mixture of oxides, in which thoria, zirconia, and titanium dioxide act as acidic oxides, and

Mackintoshite, a mixture of several oxides, of which those of thorium and uranium are the most important.

(b) The Carbonates

Lanthanite, Hydrocerite.

—This mineral is a carbonate of ceria earths, chiefly lanthana, of the formula La₂(CO₃)₃,9H₂O.

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Lanthanite occurs with cerite at Bastnäs, and at Bethlehem, Pennsylvania.

Morton[94] states that he prepared a crystalline didymium carbonate in the laboratory, of the formula Di₂(CO₃)₃,8H₂O, which was isomorphous with lanthanite; he concluded that the latter had only eight instead of nine molecules of water.

Parisite (Synchisite), and Cordylite.

—Parisite is a fluocarbonate of calcium and cerium metals; Cordylite is an analogous compound in which barium replaces calcium, and is isomorphous with Parisite. The formula of Parisite is CaR₂F₂(CO₃)₃, where R = cerium metals. Groth formulates this as (CaF)(RF)R(CO₃)₃, Penfield and Warren as (RF)₂Ca(CO₃)₃, whilst Schilling gives Ce₂(CO₃)₃,CaF₂. Analogous formulæ may be proposed for Cordylite, BaR₂F₂(CO₃)₃. Since the two minerals are very similar in crystallographic properties, one description will be sufficient for both. The following are Dana’s data for Parisite:

Both minerals are characteristic pneumatolytic species of the riebeckite-ægirine rocks. Parisite was discovered by Paris in the emerald mines of the Muso valley, Colombia, in 1835, and first correctly analysed by Bunsen in 1845. Before the blowpipe it glows, remaining infusible (the glow does not appear to have been investigated in this case).

Cordylite was discovered by Flink in 1900, in Greenland.

The so-called Synchisite was discovered by Nordenskiöld who correctly described it as Parisite. Flink found it in Greenland, and announced it as a new species, with the formula R₂F₂Ca₂(CO₃)₄, i.e. the formula for parisite plus one molecule of calcium carbonate, CaCO₃. From its extraordinary resemblance to parisite in physical and crystallographic properties, Palache and Warren[95] believe that the specimens selected by Flink for analysis must have consisted, in reality, of parisite with admixed calcium carbonate. This conclusion has now been confirmed by Quercigh, by a careful comparison of the optical properties.[96] The minerals are usually found together, the chief localities being S. Norway, the gold districts of the Urals, Narsarsuk in S. Greenland, and Montana, U.S.A.

The following rare earth carbonates are described in the alphabetical list:

Ancylite, a basic hydrated carbonate.

Tengerite, a hydrated carbonate formed by the weathering of gadolinite.

Kischtimite, a fluo-carbonate related to parisite.

Bastnäsite (Harmatite) and Weibyite, hydrated fluocarbonates of the cerium elements.

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CHAPTER VI
THE PHOSPHATES AND HALIDES

(a) The Phosphates

Monazite, Phosphocerite.

—Monazite, by far the most important, commercially, of all the rare earth minerals, is essentially an orthophosphate of the ceria earths, of the formula R´´´PO₄.[97] The yttria earths are usually present in small quantities. Silica and thoria, in quantities varying from traces up to 6 per cent. of the former and from 1 to 20 per cent. of the latter, are invariable constituents; it is almost entirely to the percentage of thoria that the mineral owes its commercial value. The following also are common constituents, though usually in very small quantities only—stannic, ferric and manganous oxides, alumina, lime, magnesia, zirconia and water. Helium was observed in it by Tilden, and by Ramsay, Collie and Travers.[98] Boltwood[99] and Zerban[100] found uranium in it; the latter attributed this to impurities, the former regarded it as an essential constituent. Strutt[101] found uranium in a pure monazite. Haitinger and Peters[102] detected radium, their result being confirmed by Boltwood and Strutt.

Monazite is with difficulty soluble in acids; before the blowpipe it is infusible; when moistened with sulphuric acid it colours the flame greenish-blue.

The mineral often occurs massive, yielding angular fragments, but is most common in rolled grains. It occurs in the gneiss of the Carolinas and Georgia, and in sands derived from the gneiss, in Idaho and many of the Pacific States; in Brazil, at various localities in the provinces of Minas Geraes, Bahia, Espirito Santo; in Queensland, Australia; in Madagascar; in Ceylon; near Travancore in India; in the Urals; in Scandinavia, etc. The deposits of commercial value will be treated more fully in the next chapter. It is of wide distribution as an accessory constituent of granites, diorites, and gneisses.

Monazite was first described, under the name Turnerite, by Lévy,[103] in 1823; the specimen was from the collection of the English chemist Turner, who thought it a variety of sphene (titanite), and was named after him at the suggestion of the mineralogist Heuland. The specimen was stated to have been found in Dauphiné, but in spite of considerable examination of the question, the precise locality is still unknown. The resemblance between Turnerite and the mineral later described as monazite (μοναζειν = to be solitary) was pointed out by[84] Dana in 1866, and confirmed by Pisani, 1877. The name Monazite was first used by Breithaupt[104] in describing a mineral found by Menge (1826) accompanying zircon in a granite from Miask in the Urals. Breithaupt concluded, from the high specific gravity, that the mineral contained a heavy metallic oxide. It was again described as Mengite by Brooke[105] in 1831. It was re-discovered by Shephard[106] in South Carolina in 1837, and described by him under the name Edwardsite, a variety from Connecticut being called Eremite. To Shephard belongs the honour of having discovered its true nature; after analysis he described it as a ‘Basic Sesquiphosphate of the Protoxide of Cerium,’ giving the formula (modern notation) 3CeO,2P₂O₅, and finding also zirconia, alumina, and silica in it (his specimen was probably very impure). Gustav Rose[107] showed this to be identical with monazite in 1840. In 1846 Wöhler described, under the name Cryptolite, a variety of tetragonal habit closely resembling zircon. This occurs at Arendal in Norway, enclosed by apatite, in the granite; it may be obtained by treatment with dilute nitric acid, which dissolves the apatite.

The question of the manner in which the thorium is combined in monazite is of considerable importance, in view of the fact that it is to this element that the mineral owes its commercial value. The amount present varies from traces up to over 20 per cent., but the usual value is between 5 and 7 per cent. The first explanation of its presence was advanced by Dunnington[108] who suggested, on the result of only one analysis, that orangite (ThSiO₄) was present mechanically mixed with the monazite. Penfield[109] supported this suggestion, and stated that in three analyses of pure material he found the ratio of rare earths to phosphorus pentoxide and that of thoria to silica exactly equal to unity, though the actual amounts of thoria varied considerably. He also quotes[85] an analysis made by Rammelsberg in 1877, in which no thoria was found, to show that it is not an essential constituent. In a microscopic examination he found dark resinous particles scattered throughout the section; after moistening with hydrochloric acid, warming, and washing, these dark spots became white, and could be stained with fuchsine, the monazite remaining unaffected throughout. He concluded that these particles were thorite or orangite.

Blomstrand[110] disputed Penfield’s conclusions. In twelve analyses of monazite from various parts of Scandinavia he never once found either thoria or silica absent. Of these twelve analyses, two give the ratio of thoria to silica, ThO₂ : SiO₂, exactly unity, in seven cases the ratio is not greater than 1·25, in five cases it varies considerably. He summed up his results in three statements:

An exhaustive examination of the question has been made more recently by Kress and Metzger.[111] They made in all over fifty analyses, using thirty different specimens of monazite; they estimated silica both as quartz and as silicate silica, and determined thorium by the fumarate method—the other investigators had used the thiosulphate method of Hermann (vide p. 286). Their results may be summarised as follows:

They conclude that thorium is present as phosphate, and is an essential constituent, but that there is always some admixed silicate, most probably a felspar.

Xenotime.

—Chemically this mineral is closely allied to monazite, being an orthophosphate of rare earths, containing silica and thoria; whereas, however, in monazite the content of yttria earths does not rise above 4 per cent., in xenotime these constitute by far the greater part of the bases, the content of ceria earths ranging from 8·2 to 11 per cent. The yttria earths, chiefly oxides of yttrium and the erbium group, vary from 54·1 to 64·7 per cent. There are traces of zirconia; Ramsay, Collie and Travers detected helium, whilst Boltwood, and also Strutt, found uranium and radium. It also appears to contain traces of sulphuric anhydride.

It is insoluble in acids, and infusible before the blowpipe; when moistened with sulphuric acid, however, it turns the flame bluish-green, like most mineral phosphates (vide monazite).

It is not so widely distributed as monazite, but is not uncommon. It often occurs with zircon—to which it is very closely allied in crystal form, if the two are not actually isomorphous—in parallel growth, in granitic rocks. The diamond sands of Diamantina, Brazil, form the richest source of the mineral, but it is also found in Scandinavia, at Hitterö, Åro, etc.

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The mineral is of considerable importance, chemically, on account of the high percentage of erbia earths.

In the works of Bauer, Rosenbusch, Weinschenk, Schilling and Iddings will be found accounts of a mineral named ‘Hussakite.’ These accounts rested on the work of Kraus and Reitinger,[112] who in 1901 announced the discovery of a new species. The crystals were obtained as a specimen of xenotime by Prof. Muthmann from Dr. E. Hussak, in São Paulo, and had the crystallographic properties of that mineral. On analysis, the amount of sulphur trioxide present was found to be remarkably high (6·3 per cent.), and Kraus and Reitinger concluded that the substance was distinct from xenotime. They announced it as a new mineral, with the name Hussakite, and the formula 3R₂O₃,3P₂O₅,SO₃ or 6RPO₄,SO₃, and stated that by the action of dilute alkalies the sulphur trioxide could be easily and completely removed. They therefore regarded xenotime as a pseudomorph[113] after hussakite, the sulphur trioxide having been removed from the latter by the action of the alkaline waters of the earth’s crust. In support of this view, they gave analyses of opaque crystals from a Bahia sand represented as containing 2·6 to 2·7 per cent. of sulphur trioxide, and so as being intermediate forms produced during the change.

The latter conclusion was quickly challenged by Brögger, who found no sulphur trioxide in a perfectly fresh and transparent xenotime from Åro in Scandinavia. Brögger concluded that the Hussakite of Kraus and Reitinger was an independent species of the formula 5YPO₄,(YSO₄)PO₃, and that xenotime was not derived from it.

Basing his work on the barium chloride test given by Kraus and Reitinger (see below) Rösler[114] declared that ‘Hussakite’ was a common accessory constituent of igneous rocks, having been previously mistaken for zircon, which it resembles in appearance and optical properties.

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In 1907 Hussak[115] published a paper in which he showed that the mineral named after him was not a new species at all, but a xenotime of prismatic habit. Analyses made at his request by Florence in Brazil, G. T. Prior in London, and Tschernik in St. Petersburg, confirmed the original values given by Gorceix (sulphur trioxide up to 0·25 per cent.). He mentions Brögger’s analysis of the Norwegian specimen in which Kraus and Reitinger had found 2-3 per cent. of sulphur trioxide, but in which Brögger found none. He explains the results of Kraus and Reitinger as due to the addition of barium chloride to the acidified solution of the carbonate fusion of the mineral, by which barium phosphate was precipitated; this was dried and weighed as barium sulphate. Rösler’s tests are declared doubtful; xenotime is not a widely spread rock constituent, the mineral in question being really zircon.

In face of these results, there can be little doubt that the name ‘hussakite’ is unnecessary and undesirable, since the mineral to which it was applied is proved to be xenotime.

In the alphabetical list, particulars of the following rare earth phosphates will be found:

Castelnaudite, a variety of xenotime containing zirconia.

Churchite and Rhabdophane (Scovillite), hydrated phosphates.

Gorceixite, an alumino-phosphate of alkaline and ceria earths.

Retzian, an hydrated arsenate of manganese, calcium and rare earth metals.

(b) The Halides

Yttrocerite.

—This mineral is a fluoride of calcium and rare earth metals, with water. A recent analysis by Tschernik[116] gives the formula Ce₂F₆,2Y₂F₆,9CaF₂,2H₂O. Putting the rare earth metals together, this gives 6RF₃,9CaF₂,2H₂O, or R₂Ca₃F₁₂,²⁄₃H₂O. Yttrocerite is of interest since it was probably in the analysis of this mineral by the discoverers, Berzelius and Gahn, that the double sulphate method of separating the yttria from the ceria earths was first employed[117] (vide p. 156).

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Yttrofluorite.

[118]—This is a fluoride of varying composition, very similar to yttrocerite, but characterised by the absence of water, and the very small ceria content (1·7 per cent.). It is thus a fluoride of calcium and the yttrium metals.

It is very similar to fluorspar (except that the octahedral cleavage of the latter is very good), and is regarded by Vogt as an isomorphous mixture of the latter with yttrium fluoride (or with a double yttrium calcium fluoride, which is less probable). This view would account for the variations in composition, and also for the remarkable frequency with which traces of rare earths are found in fluorspar (vide p. 2). Yttrocerite is regarded as a similar isomorphous mixture, but containing cerium metals in addition to the yttrium group.

Yttrofluorite occurs in pegmatite veins in granite in Northern Norway, with gadolinite, fergusonite, allanite, fluorspar, and the usual vein minerals.

The other members of this family (see list) are:

Fluocerite, a basic fluoride of yttrium and cerium metals.

Tysonite, a hydrated fluoride containing carbonates.

It is to be noticed that fluorine is the only member of the halogen family which occurs in nature in combination with rare earth elements. This fact is possibly connected with the great age of the rare earth minerals, and their formation during pneumatolytic metamorphism of plutonic rocks (vide Chapter I).

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CHAPTER VII
THE MONAZITE SANDS

It has been stated that monazite is a not uncommon accessory constituent of many rocks, particularly of granites, gneisses, diorites, etc. The crystalline material, of which an account has been given, is found sometimes in veins in these rocks, more often in tiny crystals disseminated throughout the mass. Most of these monazite-bearing rocks are extremely old, belonging to the Archæan or pre-Cambrian age, and probably none are of secondary (Mesozoic) or later age. It follows, then, that they have been subjected to erosion during practically the whole immense period of which geology can give us any detailed knowledge. Heat, frost, wind, the action of vegetation and of percolating water, the innumerable weathering agents known to the geologist, have been at work on them during countless ages, breaking, crushing, dissolving; rains, brooks, rivers, even ocean-waves have dissolved or washed away the fragments, sorted them out unerringly according to density, and re-deposited them, now in a river-bed, now at the base of some sea cliff, now in a wide alluvial plain from which the water has long since retired. It is in deposits of this nature that the monazite has been concentrated. Its relatively high specific gravity (about 5·0) has secured its separation from the lighter mica, quartz, and felspar of the parent-rock; but the heavier vein or accessory minerals have, of course, been concentrated with it. Zircon is an invariable constituent of these ‘monazite sands,’ as such deposits are called; and others almost as frequently found are rutile, ilmenite, sphene (titanite), and apatite. Common, too, are the characteristic minerals of the metamorphic rocks, garnet, epidote, sillimanite, tourmaline, etc. Rare earth minerals found in the monazite sands include xenotime, fergusonite,[91] samarskite, gadolinite, and allanite. The remaining minerals are oxides of iron and tin, with, of course, a considerable amount of quartz.

It is apparent, from what has been said above, that monazite will be concentrated with the heaviest constituents of the rocks from which it is derived. Very often, indeed usually, these rocks are precisely those in which gold occurs, disseminated sometimes in tiny particles, sometimes collected into nuggets in veins of quartz and pegmatitic minerals. The erosion of these rocks concentrates the gold with the heaviest minerals; and hence it happens that monazite is an almost universal constituent of the gold- and gem-bearing sands and gravels. In the Carolinas and in Brazil, monazite is found in the gold washings; and though in the past the two have always been extracted separately, the gold first and the monazite from the washings or tailings, there appears to be no reason why a system calculated to extract both—where, of course, the content is high enough—should not be put into operation in the future.

A chemical test affords the only reliable method of detecting monazite in a sand. A little of the sand is washed with water to remove the lighter minerals and warmed with concentrated sulphuric acid. A few drops of the liquid are poured off, evaporated to small bulk, and one drop placed on a glass plate. This is placed under a microscope and one drop of a concentrated solution of sodium acetate is added. If monazite is present in the sand, tiny pointed oval crystals of sodium cerium sulphate will separate.

On the commercial scale, monazite is extracted from the sands only, in the manner described below. An effort was made in North Carolina in 1906 by the British Monazite Company, representing the South Metropolitan Gas Light Company of London, to extract monazite from the rock in which it occurs disseminated. The rock was crushed and powdered, and the monazite separated by washing off the lighter particles on concentration tables (see below). In the same year, however, the price of thorium nitrate was suddenly lowered 50 per cent. by the German Thorium Syndicate,[92] which largely controls the Brazilian output of monazite, and the British company stopped operations in 1907. At present it may be said that only the sands are available for profitable extraction.

Up to 1895, the Carolina deposits, which were worked chiefly by the Welsbach Light Company of New York, either directly or indirectly, supplied all the demand, but in that year the Brazilian sands were first worked, and a keen struggle commenced for the market. The American companies, after keeping up a considerable output for some years, were forced to suspend operations in May 1910. The Brazil deposits, worked by the German Thorium Syndicate and the Austrian Welsbach Company, which have an agreement, now meet practically the whole demand. The Brazilian sand occurs chiefly along the shores of the southern provinces, having been concentrated by the action of the tides from the products of erosion of the cliffs; it is very uniform and considerably richer than the Carolina sand, and owing to its occurrence on the sea-shore, the cost of transporting it is very low. It is exported chiefly to Germany, recently also to the United States, and to a small extent lately to England. The method of working it is similar to that employed in Carolina—namely, concentration by washing and magnetic separation.

The North American Deposits[119]

There are two important regions in North America within which monazite sands occur; one extends over the Carolinas, and the north-western part of Georgia, the other over the Idaho basin and neighbouring counties of the Pacific Slope. It will be best to treat these separately, as the deposits are somewhat different in character.

(a) The Carolina Deposits

, including the unimportant Georgia deposits, which belong to the same field, occur over an area approaching 4000 square miles. The area is occupied chiefly by the Piedmont plateau, which is drained by a number[93] of streams rising in the South Mountains, an eastern outlier of the Blue Ridge; it is in the basins and valleys of these streams, particularly at the head-waters, that the monazite is chiefly found. The geology of the district is very complicated,[120] the rocks being very highly altered granites. The chief bed is known as the Carolina gneiss, and includes several types of gneiss, usually very much weathered. The sands, which average about 1 per cent. of monazite, are worked in and near the stream beds; they occur in the beds, and in layers 1 to 2 feet in thickness a few feet below the surface of the surrounding soil.

Concentration was formerly effected chiefly by a crude process of washing. In this process the sand is thrown on to a sort of sieve, fixed over the upper end of a long wooden trough, by one workman; a jet of water is directed on to the sieve, washing the sand through it. The heavier particles fall to the bottom of the trough, whilst the lighter are washed right through. A second workman continually turns over the sand left in the box and on the sieve; at the end of a day’s work the ‘concentrate’ is collected. This averages from 15 to 70 per cent. of monazite, according to the nature and amount of the heavy minerals accompanying it in the sand. The concentrate is dried either on rubber or oiled cloths in the sun, or on an iron plate covering a trough in which a fire is lighted. The iron minerals are then picked out by means of a magnet, and the sand filled into sacks for transport.

Before treatment for thorium nitrate, the sand is at the present day further concentrated by powerful magnetic separators. In a few cases the older method of concentration by hand-washing has been abandoned for machine concentration, the Wilfley table being sometimes employed. The principle here is exactly the same, the sand being fed into a hopper by means of a moving belt and thence on to a machine-shaken table from which running water constantly removes the particles, sorting them according to their specific gravity.

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Further separation of the dried concentrate has been effected by three kinds of separators.[121] The first was of the Edison, or fall-and-deflection type; in this the sand is allowed to flow in a thin vertical stream past a horizontal magnet, which deflects the minerals containing iron; these fall on one side of a partition, the part richer in monazite on the other. The second was an electrostatic machine; the heated sand is borne on a moving belt underneath a rotating vulcanite cylinder, excited by felt-covered rubbers; the lighter particles are attracted to the cylinder, and dropped on one side, the heavier passing on. Neither of these machines is of much value in effecting concentration, and neither is in general use.

The third, and by far the most efficient and most widely-used machine, is known as the Wetherill electro-magnetic separator. It depends on the principle, first applied by the American engineer Wetherill, that not only the iron minerals, but a large number of other minerals may be attracted if the magnetic field be sufficiently strong. In all types of this machine used in cleaning monazite concentrates, four magnetic fields of increasing intensity are traversed by the sand; the first removes magnetite, ilmenite, and the larger fragments of garnet; the second removes all the remaining garnet and ilmenite; the third removes the coarser, and the fourth the finer monazite, tailings of zircon, rutile, and silica passing on. Careful adjustment of the magnetic fields will readily give a 97-99 per cent. monazite.

Two types of this machine are in common use. In the first the magnetic fields are obtained by four successive electro-magnets, arranged so that a broad horizontally-moving belt passes between the poles of each in succession. The upper poles are ground down to a fine edge perpendicular to the direction of the belt, to secure a more powerful field. Just beneath these edges, and just above the broad belt are four rapidly driven horizontal belts moving at right angles to the first or main belt; these carry off and deposit in separate bins the minerals attracted by their respective magnets. This type is known as the Rowand separator.

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In the second type four horizontal belts are arranged in the form of descending steps, as shown in the diagram (Fig. 2). The magnets are placed at the end of each belt, and within it. The attracted mineral is held to its own belt, whilst the remainder drops on to the next; the attracted mineral falls into a bin as soon as its belt carries it out of the magnetic field. The sand to be cleaned is fed on to the first belt by means of a hopper.

The almost pure monazite so obtained is now treated chemically for its thorium. The processes proposed and in use are described in Chapter XVIII.

Fig. 2.

As already stated, the extraction of monazite in the United States has practically ceased; but the processes outlined above, which were first brought into use in the Carolinas, have been adopted for the treatment of the Brazilian sands.

(b) The Idaho Deposits.

—Monazite was first observed in placer-gold deposits in the vicinity of Boise city near the Snake river. This deposit was a gold-bearing sand derived from granite. Later the gold-bearing sands of Oregon were also found to contain monazite; these sands are rich in zircon, and contain platinum and allied metals as well as gold. The sands of the Pacific slope are the so-called black sands, derived from hornblende, and augite-granites, usually porphyritic, which are much weathered at the surface. The soil is loose[96] and is largely composed of granite fragments; the rain and streams constantly bring it down to the valleys, and continually renew the deposits. The concentrates obtained by washing are rich in well-crystallised zircon, with titanite and garnet.

In 1906 a company was formed to extract monazite from the black sand residues left after the extraction of gold. By 1909 they had erected plant and commenced operations at Centerville, and proposed to work the poorer auriferous sands for gold during the monazite washing. This, it was expected, could be done by washing the sands in boxes lined with amalgamated copper plates, which would retain the gold. Considerable amounts of monazite had already been extracted from the tailings when a disastrous fire put a stop to the operations in 1910.

Since then the production of monazite in the United States has practically ceased.

The Brazilian Deposits

Brazil first became a serious competitor in the world’s market with the United States, for the supply of monazite, in 1895. The greater percentage of thorium, the more even quality of the sands, and above all the occurrence on the sea-coast, rendered the Brazilian monazite cheaper from the beginning, so that it soon ousted the Carolina sand, and since 1910 has supplied the whole demand. The deposits at present worked lie along the coasts of Bahia, Minas Geraes and Espirito Santo, and whilst they are very rich in monazite, there is the disadvantage that their position and extent, and so also the possibility of working them, depend very largely on the variations in the tides, etc. The largest of these deposits is on the shores of a bay near the island of Alcobaca, on the southern coast of Bahia.

Monazite also occurs to a considerable extent in the diamond sands and gold-bearing sands of many of the interior provinces. In Minas Geraes it has long been known to occur at the celebrated mining centres of Diamantina and Ouro Preto, where[97] xenotime and other rare earth minerals are also found; it is also known at various localities in the surrounding mining provinces of São Paulo, Goyaz, and Matto Grosso. More recently, extensive inland deposits have been found by Freise, in the province of Espirito Santo.[122] In the plateau-basin of the Muriahé and Pomba rivers he found a sand known locally as ‘catalco’ which carries an average of 2·1 per cent. of monazite and a gold-content of 1·75 grams per ton. In the Aymoré’s mountains he found monazite, both massive and granular, in pegmatite veins in granite; analysis showed a thoria content of 9·23 per cent., which is very high. These deposits would form a very valuable and extensive source of thoria, if the difficulties of transport could be overcome.

At present, as stated above, only the beach deposits are worked. The Brazilian Government has laid a very heavy tax on all monazite exported; it is stated[123] that the German Thorium Syndicate pays 50 per cent. of its profits in royalties to the Government. In spite of this, the high quality of the sand and the low cost of transport have enabled this combine to lower the price of thorium nitrate to a point at which the Carolina sands cannot be worked, and it appears probable that the world’s markets will be supplied for some time, at least, entirely from Brazil. The methods employed in working the sand are similar to those already described.

In the last few years monazite deposits have been found in various places, notably in Australia, India, and Ceylon. In the latter locality it occurs sparingly in the gem-gravels, in association with the much more valuable thorianite and thorite, but the supply is uncertain, and the minerals cannot be worked regularly. In Australia it occurs in Victoria and in Queensland. In Victoria the deposits are poor in monazite—about 0·025 per cent.—so that working is not profitable. In Queensland it occurs in beach sands on the southern coast, with gold, platinum, and cassiterite; there seems to be no reason why these deposits should not be profitably worked when sufficient labour is forthcoming. It also occurs in North[98] Queensland, on the Walsh and Tinaroo mineral fields; here it is found massive and granular in veins in granite, associated with wolframite, molybdenite, and cassiterite.

Quite recently, deposits of considerable extent have been found near Travancore, India.[124] These sands contain about 46 per cent. of the mineral, which is itself very rich in thoria, containing about 10 per cent. of the oxide; the unconcentrated sand is therefore as valuable as a source of thorium nitrate as the ordinary Brazilian concentrates, which average 4 per cent. or less of the oxide.

Monazite has also been observed in the tin-bearing sands of Embabaan, Swaziland, South Africa, and in the province of Ottawa, Canada.

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CHAPTER VIII
RADIOACTIVITY OF THE MINERALS

In the present chapter no attempt will be made to give a complete account of all the phenomena of radioactivity which have been observed in the mineral world. There are, however, a few problems of the highest scientific interest which centre about the rare earth minerals, and mention of these can hardly be avoided in a work which professes to give a general account of the rare earth group. It is obvious that a detailed treatment cannot be given without entering into phenomena which would be quite beyond the range of the present work, and an excuse is hardly needed, therefore, for the fragmentary and abbreviated account which follows. The reader’s acquaintance with the general phenomena of radioactivity is of necessity assumed.

Radioactivity (the spontaneous emission of special radiations) was first observed by Becquerel, in 1896, in the case of potassium uranyl sulphate, and was soon found to be common to all uranium compounds, and to the metal itself. Mme. Curie showed that whilst in uranium salts the degree of activity varies directly with the percentage of uranium, in minerals containing the element the same rule does not hold. The observation that pitchblende is considerably more active than the uranium it contains led to the discovery of polonium[125] and radium in 1898. Exactly analogous phenomena were shown to hold for thorium salts and thorium-containing minerals by Mme. Curie and Schmidt in 1898, and in 1905 Hahn separated Radio-thorium from thorianite.[100] In 1899 Debierne discovered that the rare earths precipitated from the solution obtained on treatment of pitchblende are associated with another extremely active body, which he named Actinium; Giesel found that in the separation of the rare earths this remains with lanthanum. In 1903 Ramsay and Soddy experimentally confirmed the prediction of Rutherford and Soddy, that radium would be found to produce helium continuously. The discovery of these remarkable phenomena has modified many fundamental physical conceptions, and has opened up a new field of scientific enquiry, which is being developed with unexampled rapidity.

It has been mentioned, in the accounts of the rare earth minerals given above, that almost all these minerals are radioactive, i.e. have the property of emitting specific radiations. Moreover, radioactivity, to any considerable extent at least, is, with a few important exceptions, confined to the minerals which have been already described. It has been shown by many investigators, chief amongst whom are Strutt and Boltwood, that the activity is usually due to the presence of uranium or thorium, or both.[126]

After the discovery of helium in Cleveite (a variety of pitchblende, vide p. 13) in 1895 by Ramsay, a large number of minerals were examined for this gas, and it was found that almost all the rare earth minerals contain helium. The fact that these minerals are also for the most part radioactive, naturally suggested some relation between the activity and the presence of helium, and led directly to the discovery that radium is continuously producing helium; and it became apparent that helium has been accumulating in these minerals since their formation, by the decay of radioactive elements. The question of the origin of helium in minerals will be touched on again.

In 1904 Boltwood advanced the theory that radium is produced by the degradation of uranium, the parent-element having, however, a much greater half-life period. If uranium continuously produces radium, whilst the latter decays much[101] more rapidly than the former, it must follow that in minerals containing uranium a state of equilibrium is reached between uranium and radium, and the ratio of these two in all minerals should therefore be constant, and independent of the geological age. Boltwood examined a number of the minerals of which descriptions have been given in the preceding chapters, and found the ratio to be surprisingly constant.[127] Strutt also examined a large number of minerals,[128] and whilst on the whole his results seemed to support the theory, his values for the ratio were by no means so constant as those of Boltwood. Strutt included in his examination the interesting radium-containing mineral observed by Danne at Issy l’Evêque.[129] This was a pyromorphite (lead chlorophosphate) containing neither uranium nor thorium. Danne suggested that the radium was not an original constituent, but had been introduced by the action of percolating waters. This view was confirmed by McCoy and Ross,[130] who found that the activity was entirely confined to the surface layer.

Mlle. Gleditsch has also examined the question of the uranium-radium ratio in minerals. Her earlier work[131] gave ratios which, whilst constant for each mineral species, varied in much the same manner as Strutt’s for different species, and afforded very little support to Boltwood’s theory. Her more recent results,[132] however, are much more closely in accord with the theory, which has been still further strengthened by the work of Pirret and Soddy[133] and of Marckwald and Russell.[134] It may now be regarded as firmly established that radium is in the line of direct descent from uranium.

Boltwood had assumed that the helium in radioactive minerals is produced from the uranium, during its disintegration. Strutt, however, disputed this; his experiments showed that very little helium is found even in the richest radium-uranium[102] minerals unless thorium is also present. Thus pitchblende contains a very high percentage of uranium, but relatively little helium (there is usually a considerable thorium percentage here too, so that nothing conclusive can be deduced from this). Adams[135] found that carnotite, a mineral very rich in uranium, but containing no thorium, contains no helium at all; he explained its absence by the very loose texture and permeability of the mineral, which would allow the gas to escape. Strutt concluded that whilst helium is undoubtedly produced by disintegration in the uranium series, in minerals it is produced more by thorium or, as more recent work indicates, by radio-thorium, than by uranium.

The question of the origin of helium in minerals is, however, not definitely settled, for several anomalous cases are known. Thus the yttria silicate, Thalénite (q.v.), contains quantities of helium, but no uranium or thorium is given in the analyses. Similarly, Risörite contains a relatively large quantity of helium, but only traces of uranium and thorium. In the last mineral, the active constituent is precipitated with the lead, so that no radio-thorium appears to be present. Further, Thomsen analysed a fluorspar from Ivitgut in Greenland which he found to contain 27 c.c. of helium per kilogram. This specimen contains no uranium, but gives off the thorium emanation in quantities which suggest the presence of radio-thorium; moderate quantities of thorium are also present. Since the α particle has been definitely identified as a positively charged helium atom, it appears certain that disintegration in all three series (uranium, actinium, and thorium series) produces helium, and a mineral containing a member of any of these series (which gives α rays or α ray-giving products) would also contain helium.

Even so, there is a case in which the helium content is anomalous, if not altogether beyond explanation at the present stage. In examining a large number of minerals for helium, Strutt[136] found that some samples of beryl, a beryllium aluminium silicate, contain a relatively very large amount of helium, but only traces of thorium, and was altogether[103] inactive. The absence of any active constituent renders untenable the ordinary explanations of the presence of such a surprising quantity of helium. Boltwood has put forward a suggestion which in the present state of our knowledge must be regarded as a provisional explanation. He conceives that in the concentration of beryllium from the parent magma, it may have become associated with some short-lived intermediate radioactive element, which had been altogether separated from its long-lived parent element in the process of concentration; this intermediate element, having collected in the crystallised beryl, decayed completely in the course of the great period which must have elapsed, leaving the helium to which it had given rise during its disintegration enclosed in the mineral. It is difficult to see how two substances which must be so intimately connected as a parent-element and its product could be completely separated in the process of cooling of a magma; but since so little is known of the process of crystallisation of minerals, the suggestion can hardly be rejected on geological grounds. In any case, we have here only one strongly marked exception to the very definite rule that in all cases in which helium occurs in minerals, it is accompanied by and undoubtedly produced from, a radioactive element or elements; and in the majority of cases, the helium in minerals is produced by disintegration of uranium or thorium and their products.

Strutt found that traces of helium are universal in the mineral world. His method of determining helium was approximate only. He obtained the gas content by heating the powdered mineral—a method which, as Wood has shown,[137] will only give all the gas when very high temperatures (up to 1000°C.) are employed. The gases were freed from oxygen and hydrogen by passing over a heated, partially oxidised, copper spiral, and from carbon dioxide by means of potash. Nitrogen was removed by sparking with excess of oxygen and shaking over potash; the excess of oxygen was removed by melted phosphorus. The inert gases so obtained were freed from all impurities by the use of the liquid alloy of sodium[104] and potassium for the electrodes of the spectrum tube in which the gases were examined spectroscopically.[138] Argon, if present—it seems to be a universal constituent of igneous rocks, into which it may have been absorbed from the air—was removed by charcoal at a temperature of -80°C. The helium so left was examined spectroscopically, and measured in a MacLeod gauge.

As stated, helium was found in traces in nearly all minerals, and its presence is to be attributed to traces of radium, which also appears universal. In minerals containing uranium or thorium, or rare earths (the latter are almost always accompanied by uranium and thorium), helium is found to a much greater extent, and Ramsay considers it possible that some fraction of the helium content may arise from the rare earth metals. There is, however, no positive evidence to support the conjecture. He found that the helium ratio, i.e. the volume of helium per gram of uranous oxide, UO₂, varies with the amount of thoria present; but where the latter is absent the variations are much less marked. If helium were produced in a mineral from uranium alone, and none escaped, it is obvious that the helium ratio would depend only on the age of the mineral. For minerals of about the same age, and containing no thorium, the helium ratio would be roughly constant, if no disturbing factor required consideration.

In 1905 Strutt pointed out that in all the minerals he had examined, thorium was never present unless accompanied by uranium and radium, whilst uranium and radium often occurred without thorium. He suggested that the present atomic weight of thorium, 232·5, was too low, and that it was really the parent of uranium (at. weight 238·5); he further supposed that the next permanent member in the line of descent was one of the cerium metals. These suggestions have been negatived by later work of Boltwood and Holmes. The former pointed out[139] that it was far more likely that thorium is a disintegration product of uranium of considerably[105] longer life. On the whole, however, there is very little positive evidence to connect thorium with uranium.

In the same year Boltwood (loc. cit.) drew attention to the persistent appearance of traces of lead, bismuth, barium, etc., in the radioactive minerals, and also pointed out that the variations of the ratio of helium to uranium in pitchblende might be used to determine the age of the mineral. In 1907 he suggested[140] that lead was the final product of the degradation of uranium, from which it follows that the ratio of uranium to lead should be constant for minerals of the same age (since, lead decays, if at all, at an infinitely slower rate than uranium). He collected all the available analyses, and classified the minerals dealt with into six groups according to the value of the ratio. The order given by the ratio was declared to be in accordance with the order of age as given by geological evidence.

Holmes[141] has further extended this work. He examined a number of rare earth and allied minerals from the Christiania district, which Brögger considers to be of approximately Lower Devonian age, and found the ratio of lead to uranium to approximate quite closely, for almost all the minerals examined, to 0·045. Representing the change in the usual way as

and using the data calculated by Rutherford and others for the rates of decay, he gives the age of Lower Devonian strata as about 370 million years. This figure is about twice as great as that deduced by palæontologists from the flora and fauna, and greater still than the times based on physical data, e.g. rates of cooling, precession and nutation, etc. His figures for pre-Cambrian rocks, based on the same ratio, range between 1000 and 1640 million years, the later being deduced from a thorianite from the Archæan rocks of Ceylon. Strutt’s figure for Archæan rocks is about 700 million years; this was derived from work on the helium ratio, which must now be considered.[142]

[106]

In 1898 Travers[143] had examined the effect of heat on cleveite and fergusonite, and found that about half the total helium, together with hydrogen, is given off at a bright red heat. He considered it likely that the helium was combined with a metal (though he recognised no distinction between occlusion and combination) and remarked: ‘The results of such experiments cannot therefore serve as a basis for speculation as to the origin or history of the substances in question.’ The chemical inactivity of helium, however, as well as the experiments of Moss and Gray, who showed that helium was evolved on grinding the materials,[144] indicate that the gas is mechanically bound only. This, however, introduces the difficulty, if an attempt be made to use the helium-uranium ratio to calculate the age of minerals, that the gas would be expected to escape from a porous material, so that its amount is never so great as it should be. Strutt himself found that helium escapes rapidly from powdered monazite, whilst even the solid mineral was found to evolve helium at a rate much in excess of the probable rate of production by radioactive changes. Similar results were found with thorianite, and the only conclusion, since helium is found in the minerals, is that under the conditions under which these minerals exist in the earth’s crust, this escape is checked or altogether prevented. It follows, however, that any age determined from the helium ratio must be a minimum age, since there is always the chance of loss; this of course is not the case—except where the minerals have suffered chemical changes—with the lead ratio, and may account for the discrepancies observed.

Strutt’s earlier work on the helium ratio was made with phosphate minerals (coprolites and fossil bones) of known ages. The ratios found were not in order of age, the minerals being very permeable, so that helium had probably been lost. He next turned his attention to igneous rocks, and selected zircon for the work. Here he obtained some sort of regularity in the order of age and the order given by the ratio, and assumed[107] that if helium were lost at all, it must be lost in roughly proportional amounts by reason of the similarity in conditions. Geological criticism tends to lessen the trustworthiness of the conclusions; it is pointed out that the age of a specimen of zircon is not necessarily that of the rock in which it occurs, for zircon is an extremely stable mineral, and might survive unchanged several fusions and re-crystallisations of the magma. Strutt replies to this that at the temperature of fusion of a rock, zircon would certainly give up its accumulated helium, so that the age determined from the helium content would be that of the last fusion, i.e. the age as given by geological data. On the other hand, our ignorance of the real mechanism of the crystallisation of a magma, and especially of the amount and effect of the pressures obtaining, robs this reply of its force, and the objection must be counted valid.

In still later work Strutt used sphene and thorianite, and his results agree as well as can be expected. The sphenes used were all from Archæan rocks, except one, which was from a Tertiary volcanic deposit of the Laacher See, near Coblenz (the lake is in the crater of an extinct volcano). In this case the helium ratio was very much smaller (about ¹⁄₄₀₀₀ of the values for Archæan rocks) indicating the (comparatively) extremely recent formation of the deposit.

The most recent results in the study of radioactivity point to the conclusion that elements which differ in atomic weight and radioactive properties may be chemically identical, or at least chemically inseparable; such elements have been termed isotopes. The end product of the thorium series of radio-elements should have an atomic weight of about 208·4, and it has been suggested that the element actually produced in this series of changes may be bismuth. The latest results, however, rather point to the conclusion that disintegration in the thorium series gives rise to an isotope of lead. If this hypothesis be true, the lead derived from a mineral rich in thorium and poor in uranium should have an atomic weight appreciably higher than that of ordinary lead. Experiments to test this conclusion have recently been carried out by Soddy and Hyman.[145]

[108]

These authors have made analyses of Ceylon thorite, which they find to contain 0·35 per cent. of lead; from the ratio of thorium to uranium in the mineral, they calculate that the lead should have an atomic weight of 208·2, that of ordinary lead being 207·1. Preliminary comparative experiments on 1 gram of pure lead chloride extracted from the mineral point to an atomic weight for the thorite lead of 208·4, a result surprisingly in accord with theory. More extended experiments on this most interesting question are in progress.

The present chapter would be incomplete without a reference to the interesting work of Goldsmidt on radioactivity as an aid in identifying mineral species.[146] He describes a simple method by which the activity of a mineral may be rapidly and easily measured to a sufficient degree of approximation, and shows how the determination enables a line to be drawn on a diagram already mapped out; this line will intersect an area on the diagram which corresponds to the particular mineral. Owing to lack of analytical data, and to the great difficulty of determining with accuracy small quantities of uranium and thorium, the method is at present of scientific interest only; but it is capable of development, and its development would be of undoubted value in the further study of this branch of radioactivity.

In order to make this part of the subject as clear as possible, the chief points in this chapter are summarised as follows:

1. Radioactivity is only observed to an appreciable extent in some rather rare minerals. These minerals as a rule contain radium, uranium, thorium, rare earths, and helium.

2. The helium has been produced during geological time by the degradation of one or more members of the three series of active elements (the Uranium, Actinium, and Thorium series).

3. Radium is a degradation product of uranium, and itself is degraded continuously; the final product of degradation is probably lead.

4. The age of minerals has been calculated from the ratio[109] of lead to uranium; the figures obtained are much greater than those put forward by geologists and physicists.

5. The helium ratio has also been used, but appears less trustworthy, owing to escape of helium, and uncertainty as to geological age of the minerals employed.

6. Some connection between radioactivity and the presence of the yttrium or cerium metals appears highly probable, but no satisfactory theories have been advanced on this point; it has been shown that actinium is very closely allied to lanthanum.

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PART II
THE CHEMISTRY OF THE ELEMENTS

CHAPTER IX
GENERAL PROPERTIES OF THE CERIUM AND YTTRIUM GROUPS

The chemistry of the rare earth elements begins in the year 1794, with Gadolin’s discovery of the new oxide ‘Ytterbia,’ for which the name Yttria was subsequently proposed by Ekeberg, and generally adopted (see Chapter I, and under Gadolinite, p. 35). The discovery of Ceria followed in 1804 (see under Cerite, p. 32). The classical work of Mosander, carried out between 1838 and 1842, showed the complex nature of the new oxides. From ceria he separated three new earths, Ceria proper, Lanthana, and Didymia. Yttria was shown to be a mixture of at least three oxides, for which the names Yttria, Erbia, and Terbia were proposed. These oxides were believed to have the general formula RO, by analogy with the alkaline earths, which they were found to resemble in many respects, notably in their strongly basic character.

The properties of the new oxides were examined during the next twenty years by many chemists, the chief workers being Marignac, Rammelsberg, and Hermann, but the next important advance was the investigation of the absorption spectra of solutions of the rare earth salts, first suggested by Gladstone in 1856, and developed more fully by Bunsen and Kirchhoff in 1860 and the following few years. The introduction of the methods of spectrum analysis furnished a very[112] delicate and valuable method of examining and identifying the various oxides, and so greatly assisted the laborious processes of separation.

Sixteen elements (excluding thorium and zirconium) are at the present time recognised as belonging to the rare earth group. With one or two exceptions, these show the closest resemblance to one another, both in chemical behaviour and in the properties of their compounds, so that the difficulties of separating and purifying them are very great. They may be said to form a series, in which the properties vary continuously but gradually from member to member, so that no sharp differences are anywhere perceptible. The method of division into groups is, therefore, almost entirely one of convenience, and has arisen from the course which the separations have followed.

The elements are divided into two chief families or groups, that of the cerium metals and that of the yttrium metals respectively. The cerium elements are separated by a process depending on the relative insolubility of their alkali double sulphates; in this group are included cerium, lanthanum, praseodymium, neodymium, and samarium. The yttrium family is further divided into four sub-groups: the first consists of scandium and yttrium; the second or terbium group of europium, gadolinium, and terbium; the third or erbium group of dysprosium, holmium, erbium, and thulium; and the fourth or ytterbium group of ytterbium and lutecium—the element celtium, recently discovered by Urbain, will also fall into this sub-group, but the discovery awaits confirmation. Whilst scandium and yttrium fall into somewhat abnormal positions, corresponding to their low atomic weights, the terbium elements occupy an intermediate position between the cerium elements and the remaining yttrium elements, or yttrium group proper, and so are frequently classified as a third or intermediate group.

This list does not include all the names which have been put forward to designate what have been claimed from time to time as new elements; whilst the individuality of some of those included is not yet fully established, and the homogeneity[113] of others has been called in question. The uncertainty is more pronounced among the yttrium elements than among the cerium elements; owing to the opportunities for investigation furnished by the commercial treatment of monazite, the chemistry of the cerium group may be regarded as complete.

In the following table the elements are arranged in order of increasing atomic weight, and it can be seen at once how closely the division into groups follows this order:

In their chemical relations, the rare earth elements may be placed between the metals of the alkaline earths, and the trivalent metals iron, aluminium, and chromium. With the exceptions of cerium in the ceric salts, and of samarium and europium in the recently discovered dichlorides, they are uniformly trivalent, but the oxides are very strong bases, and the salts very slightly hydrolysed in dilute solutions; generally, therefore, they resemble the calcium family rather than the aluminium group. Among the common salts, the[114] oxalates, phosphates, chromates, iodates, fluorides, carbonates, tartrates, and borates are almost insoluble; the sulphates are only sparingly soluble at ordinary temperatures. Among the double salts, the alkali double sulphates are of great importance from their employment for separations; the tendency to the formation of complex salts is greater among the yttrium than among the cerium elements, increasing with the atomic weight, and with the decrease in basic strength of the oxides.

The great similarity in chemical behaviour of the rare earth elements is apparent not only in the similarity in composition, solubility and chemical properties of the salts—which is so great that the general account of the compounds which follows applies almost in its entirety to each member of the group—but also in the crystallographic relations between corresponding compounds. Many of the salt hydrates form isomorphous series; the sulphate octohydrates, for example, appear to be isomorphous throughout the whole group, and probably the relation would be found to apply even more completely than is generally accepted, if the necessary data were forthcoming. Of great interest and practical importance is the isomorphism between the nitrates and double nitrates of the cerium elements and bismuth, which has been utilised with such valuable results in the processes of fractional crystallisation.

The Metals.

—The earlier attempts to reduce compounds of the rare earth elements to the metallic condition, by means of metallic sodium or potassium, did not yield pure products; nor did the use of aluminium or magnesium lead to results of practical importance. The metals were first obtained in a coherent physical condition by Hillebrand and Norton,[147] by electrolysis of the fused chlorides. These investigators obtained cerium, lanthanum, and the so-called didymium, and measured their specific heats; their results confirmed the atomic weights assigned to the elements by Mendelejeff, except in the case of lanthanum. Their method has since been elaborated by Muthmann, Hofer and Weiss,[148] who have[115] prepared large quantities of the cerium elements in the pure state. More recently, Hirsch has prepared metallic cerium in large quantities,[149] and has studied its properties.

By electrolytic reduction of the mixed chlorides of the cerium elements, a mixture known as ‘Misch metal’ is obtained; this has powerful reducing properties, and, like aluminium, reduces the oxides of iron, chromium, etc., with great development of heat.[150] The yttrium metals have not yet been obtained in the pure state, the electrolytic method giving unsatisfactory results on account of the high melting-points of the metals, and the volatile nature of their chlorides.

The cerium metals are white or slightly yellowish in colour, and are moderately stable in dry air. In moist air they tarnish slowly, lanthanum, as the most positive, being most readily oxidised. The melting-points and specific gravities are as follows:

The metals decompose water slowly in the cold, but rapidly at the boiling-point, with evolution of hydrogen. They have a great affinity for oxygen, the heats of formation of the oxides being of the order of those of alumina and magnesia:

[116]

In consequence of the high values of the heats of combustion, the metals have powerful reducing properties.

The cerium metals form alloys with magnesium, zinc, aluminium, and iron, and combine with boron and silicon. The alloys of cerium, and the metal itself, are remarkable for their property of emitting brilliant sparks when scratched (see Chapter XXI). Cerium also forms an amalgam with mercury.

The metals burn brilliantly when heated in oxygen, and dissolve readily in dilute mineral acids. When heated to a temperature of 200°-300° in a current of hydrogen, they absorb the gas very readily, forming the hydrides. These compounds are also obtained by heating the oxides with magnesium in a current of hydrogen. They were first prepared by Winkler,[152] who deduced from his analyses the general formula RH₂; the more recent work of Muthmann and Beck,[153] however, points to the formula RH₃.

If nitrogen be substituted for hydrogen in either of the above methods of preparation, nitrides of the general formula RN are obtained; cerium nitride, however, cannot be obtained by heating the element in the gas.[154] These compounds are also obtained when the carbides are heated in ammonia. They are amorphous solids, which yield ammonia when acted upon by water.

Hydroxides.

—The hydroxides are thrown down as gelatinous precipitates on the addition of alkalies to hot dilute solutions of the salts; precipitation in the cold, or in strong solution, usually gives a basic salt, or an hydroxide mixed with a large quantity of basic salt. The hydroxides are insoluble in excess of precipitant, but the precipitation is inhibited by the presence of some organic hydroxy-acids.[155]

The hydroxides are insoluble in water, but dissolve very readily in acids. The most basic of them absorb carbon dioxide from the air; lanthanum hydroxide is exceptional in that it colours litmus blue.

Whilst hydrogen peroxide in neutral solution does not[117] react with rare earth salts,[156] alkalies in presence of this reagent precipitate gelatinous hydrated peroxides, which are very unstable, decomposing on standing, or on treatment with acids, with evolution of oxygen. The general formula R₄O₉ + xH₂O was proposed for these compounds by Cleve, but more recently the formula R(OOH)(OH)₂ has been advanced.[157]

Oxides.

—In their most stable state of oxidation, the rare earth elements are generally trivalent. In the case of cerium, the dioxide, CeO₂, is more stable than the sesquioxide Ce₂O₃, but the ceric salts are unstable, and are very readily reduced to cerous compounds, corresponding to the oxide Ce₂O₃. Higher oxides are known with certainty among the other elements only in the cases of praseodymium and terbium, but these do not give rise to salts.

The oxides R₂O₃ are fairly strong bases, being comparable in strength to the alkaline earths, and far more strongly basic than alumina and oxides of other trivalent elements; thus they liberate ammonia from ammonium compounds, whilst the salts they form with strong acids are not easily hydrolysed. Their relative strengths as bases are expressed in the following series, in which the elements are placed in order of diminishing electropositive character:[158]

La, Ce´´, Pr, Nd, Yt, Eu, Gd, Sa, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Ce^iv.

It will be seen that, with the exception of scandium and yttrium, the metals of the cerium and yttrium groups become less electropositive as the atomic weight increases.

This arrangement is obtained by ascertaining the order in[118] which the various hydroxides are precipitated from a solution by gradual addition of a dilute solution of a strong base. The weakest base is precipitated first, and the strongest last; those intermediate in strength are thrown down in ascending order of strength. Similar results may be obtained by the fractional decomposition of the nitrates by heat; in this case the nitrate of the weakest base is decomposed at the lowest temperature. This order is also confirmed, as far as the data are available, by measurements of the equivalent conductivities of solutions of the salts (see, for example, p. 122).

Quite recently, a very different order has been obtained from a consideration of the dissociation tensions, and of the heats of dissociation of the anhydrous sulphates.[159] In the following table the elements are arranged in the order of the increase of the dissociation tension (T) measured at 900°, which is the same as the order of decrease of the heats of dissociation (Q):

It will be observed that the order is very different from the order of increase of atomic weight, the positions of lutecium and ytterbium being especially surprising; these elements are generally considered to be among the least electropositive of the whole series. The anomalous position of cerium is probably due to the fact that the sulphate on decomposition leaves the[119] dioxide, and not the sesquioxide, as with the other elements; this would undoubtedly affect the values. The heats of dissociation are the greatest yet observed for the sulphates of trivalent metals, a further evidence of the strongly basic nature of the oxides.

Ignited lanthana resembles quicklime in that it readily absorbs carbon dioxide from the air, and hisses when slaked with water; as the basicity becomes weaker, the affinity for water and carbon dioxide becomes less marked. All the oxides are soluble in dilute acids, even after prolonged ignition; but the ease with which solution occurs is naturally much influenced by the treatment to which the oxide has been subjected, as well as by its strength as a base.

The rare earth oxides are capable of existing in more than one modification, the compounds obtained by ignition of the hydroxides differing in appearance and reactivity from those prepared by ignition of the oxalates or nitrates, and so on; they are probably highly polymerised. Cerium dioxide, CeO₂, is remarkable for its power of combining with the other oxides, R₂O₃, of the rare earth metals. The pure dioxide is insoluble in nitric acid, but mixtures of earths containing up to 50 per cent. of the dioxide dissolve readily. The various colours of mixtures of the ceria earths may sometimes be attributed to a similar combination,[160] and there can be little doubt that the dioxide sometimes functions as an acid in the rare earth minerals.

Sulphides.

—These compounds cannot be prepared in the wet way, that is, by the action of hydrogen sulphide or ammonium sulphide on the salts in solution; the former reagent gives no precipitate, the latter throws down the hydroxides. In this behaviour, the rare earth elements resemble aluminium and chromium.

The normal sulphides, R₂S₃, are obtained by reduction of the anhydrous sulphates, or from the oxides at high temperatures,[120] by treatment with hydrogen sulphide. They are strongly coloured compounds, fairly stable towards cold water, but readily hydrolysed on boiling.

Disulphides, RS₂, are known in the cases of cerium, lanthanum, and praseodymium; these are to be regarded as polysulphides, since on treatment with dilute acids they yield hydrogen persulphide, H₂S₂.

Carbides.

—By reduction of the oxides with carbon in the electric furnace, Moissan obtained the carbides in the form of microscopic yellow crystals. They have the general formula RC₂, and are attacked by water and dilute acids, with evolution of very complex mixtures of gases.[161] The principal product is acetylene, with various higher homologues, and in smaller quantities ethylene and ethane and their homologues. No methane is formed,[162] but hydrogen is always present, the olefines and paraffins probably arising from its action on the acetylenic hydrocarbons. The relation of the rare earth elements to the calcium group is here very close; calcium carbide when attacked by water yields pure acetylene, whereas aluminium carbide gives pure methane.

Halogen Salts.

—The halides of the rare earth elements show a close analogy with the corresponding compounds of the alkaline earth elements. The fluorides are insoluble in water and dilute mineral acids, and are obtained as gelatinous precipitates by the addition of hydrofluoric acid, or a soluble fluoride, to solutions of the salts. They may be prepared in the crystalline condition by heating the carbides in a stream of fluorine, or by the action of hydrofluoric acid upon the hydroxides in aqueous suspension. The rare earth elements, as well as thorium, may be separated from zirconium by taking advantage of the insolubility of their fluorides in excess of hydrofluoric acid or alkali fluorides, since zirconium fluoride[121] is readily soluble in excess of the precipitant. The solubility of the fluorides in a large excess of concentrated acid increases with the electropositive character of the metal, the fluorides of the more negative elements being the least soluble. Thorium and scandium may, therefore, be concentrated to a large extent by repeated precipitation with hydrofluoric acid in acid solution.

The silicofluorides of the rare earth elements have been used by R. J. Meyer in the extraction of scandium from wolframite (see Chapter I and under Scandium, p. 215). They are thrown down as gelatinous precipitates on addition of potassium or sodium silicofluoride to boiling, neutral solutions of rare earth salts. In presence of mineral acids, however, they are not thrown down in the cold; on boiling, the cerium metals are precipitated as fluorides, by hydrolysis of the silicofluorides—the yttrium elements, with the exception of scandium, being held in solution by the mineral acid.

With the exception of the fluorides, the halogen salts of the rare earth metals are readily soluble in water, and crystallise from the concentrated solutions in the hydrated form. The bromides and iodides have not been so fully studied as the chlorides; they are hygroscopic salts, and decompose rather easily. The iodides have been obtained by Moissan in the anhydrous state, by the action of iodine vapour on the carbides at high temperature.

The anhydrous chlorides may be obtained by the application of any of the ordinary methods, e.g. by heating the oxides with carbon in a stream of chlorine, by heating the carbides in the same gas, by heating the sulphides or hydrated chlorides in hydrogen chloride, or by evaporating the solutions of the hydrated salts to dryness in presence of ammonium chloride, and then igniting till the latter has all been removed. As obtained by any of these methods, they are fusible at a red heat, but only slightly volatile; they are easily soluble in water or alcohol, with disengagement of heat. They are insoluble in most organic solvents, but dissolve to some extent in some bases; the chlorides of the yttrium elements, for example, are readily soluble in pyridine. With such solvents,[122] the chlorides form compounds which may be considered as derived from the hydrated forms, by replacement of the so-called water of crystallisation by the organic base.

Conductivity measurements show that the salts are not perceptibly hydrolysed in moderately dilute aqueous solutions, though the values for the equivalent conductivities vary somewhat with the variations in the electropositive character of the elements. In the following table, the equivalent conductivities of the chlorides in solutions of dilution 32 and 1024 at 25°C. are given. It will be seen that the value (λ₁₀₂₄ - λ₃₂) ÷ 10 is in all cases (except for the highly hydrolysed scandium salt) very close to 3, an experimental proof of the trivalent nature of the elements. The values for the chlorides of iron, aluminium and chromium are included; it will be seen that these elements are considerably less positive than the rare earth metals (with the exception, of course, of scandium).

From aqueous solutions the chlorides crystallise with six molecules of water, except praseodymium chloride, which has seven. The hydrated salts, when heated to 120° in the air, form insoluble oxychlorides of the general formula ROCl.

The chlorides do not show a great tendency to form double salts with other metallic chlorides; on the other hand, they readily form complex compounds with the chlorides of the less electropositive metals, e.g. tin, bismuth, gold, and platinum.

Subchlorides of samarium and europium have recently[123] been obtained; in these compounds, for the first time, rare earth metals have been shown to be capable of functioning as divalent elements.

Cyanides of the rare earth elements are not known; addition of potassium cyanide to solutions of the salts throws down the hydroxides. The platinocyanides may be obtained by double decomposition of the sulphates with barium platinocyanide. They are very stable and characteristic bodies, of the general formula R₂[Pt(CN)₄]₃, with 18 or 21 molecules of water. The compounds of the cerium elements are yellow, with a strong blue fluorescence; they crystallise in the monoclinic system. The platinocyanides of the yttrium metals are red or crimson, with a splendid green fluorescence, and crystallise in the rhombic system. Scandium platinocyanide is of great interest from the fact that it exists in two modifications, which show the characteristic appearance of the two groups of compounds respectively.

Potassium ferrocyanide precipitates potassium earth ferrocyanides of the general formula KR(FeC₆N₆),3H₂O, from neutral solutions;[163] the precipitate is somewhat soluble in excess. The ferrocyanides have been proposed for the purification of yttrium; the method is useful where rapid concentration of the element is required, yttrium ferrocyanide being far more soluble than the analogous compounds of the erbium and ytterbium metals, but the precipitates are gelatinous, and very difficult to handle.

Halogen Oxy-salts.

—Perchlorates and periodates of the rare earth elements, of the general formula R(XO₄)₃,xH₂O, have been obtained. The existence of chlorates has been observed only in the yttrium group; yttrium chlorate, Yt(ClO₃)₃,8H₂O, has been prepared by double decomposition of the sulphate with barium chlorate. The bromates are also prepared in this way. They are readily soluble compounds, of which several hydrated forms are known. They are of considerable importance for purposes of separation in the yttrium group.

[124]

The iodates are sparingly soluble bodies, precipitated by addition of the alkali compound to solutions of the rare earth salts. The rare earth iodates are soluble in nitric acid, the solubility increasing as the electropositive character of the element becomes stronger. A method for the purification of yttrium has recently been based upon this property of the iodates, whilst the fact that thorium iodate is completely insoluble in nitric acid allows of the easy separation and estimation of thorium in minerals or mixtures containing rare earth elements.

Sulphates.

—The sulphates of the rare earth elements are obtained by dissolving the oxides or hydroxides in sulphuric acid. From the solutions so obtained, various hydrated salts separate according to the temperature of crystallisation. By heating the hydrated salts to a temperature of 300°-400°, the anhydrous salts are prepared. These are extremely soluble in water at 0°, having a great tendency, which is indeed to be observed in the hydrated forms also, to form supersaturated solutions. When the temperature of such a solution is allowed to rise, larger or smaller quantities of an hydrated form separate out, the differences of solubility among the sulphate hydrates of the various elements being sometimes considerable.

The hydrated sulphates of the cerium elements have been very closely studied in connection with the purification of thorium. Cerium sulphate itself forms hydrates with 12, 9, 8, 5, and 4 molecules of water, but sulphates of the other elements generally form fewer hydrates; the commonest have 12, 8, or 4 molecules of water, and numerous cases of isomorphism are known among them. The solubility curve of the cerium sulphate hydrates is shown in the diagram. Fig. 3. The sulphates of the yttrium elements have not yet been systematically investigated, and in most cases only the octohydrates are known. Scandium sulphate is notably different from the other sulphates, in that it is considerably more soluble, and crystallises with six molecules of water.

Fig. 3.

It is an important characteristic of the rare earth elements that the solubility of the sulphates diminishes rapidly as the[125] temperature rises. The study of the various equilibrium conditions is greatly complicated by the tendency to form supersaturated solutions, and the fact that many hydrates can exist throughout considerable ranges of temperature in[126] the metastable condition; in consequence of this, also, the solubilities of many hydrates are known for temperatures far beyond the transition points. Foreign elements may be separated by taking advantage of the very great solubility of the anhydrous sulphates at 0°, and the rapid decrease in solubility with rise of temperature. For this purpose, a solution of the anhydrous sulphates saturated at 0° is prepared, and after filtration is slowly allowed to come to room temperature; the hydrated rare earth sulphates then separate, leaving in solution the foreign sulphates. This method may indeed be used instead of the oxalate separation (see p. 147).

In presence of excess of sulphuric acid, acid sulphates of the general formula R(HSO₄)₃ are formed. These are fairly stable, and must be heated to a temperature of 400°-500° to decompose them completely to the normal salts; even at that temperature, traces of acid are tenaciously retained, a fact which renders the determination of the equivalents by the sulphate method unreliable, unless special precautions are taken. On further heating, the normal sulphates pass into basic salts, R₂O₃,SO₃, and finally, at the temperature of the blowpipe flame, into the oxides. The temperatures at which these decompositions occur vary with the positive character of the elements; the most basic oxide clings most tenaciously to sulphuric anhydride, and forms the most stable acid salt. Lanthanum sulphate, for example, requires to be heated for a considerable time at a white heat if the pure oxide is required, whilst the sulphates of the less positive elements are easily decomposed at a red heat. The order of basic strength of the oxides, as determined by the ease with which the sulphates are decomposed, seems, however, to be very different from the order determined by decomposition of the nitrates (see p. 118).

With the alkali sulphates, the sulphates of the rare earth elements readily form double salts, which are of great importance in separation, on account of the great differences in solubility. The double sulphates of the cerium group are almost insoluble in excess of alkali sulphate, whereas the yttrium double sulphates, with the exception of those of the terbium metals, which occupy an intermediate position, are[127] very easily soluble. This method of separating the elements into the two main groups was first employed by Berzelius, and though a century has elapsed, it remains to-day the most efficient method of effecting the separation.

The ethylsulphates have been employed by Urbain and others in effecting separations, especially in the erbium and terbium groups. The solubilities of these salts are in the same general order as those of the alkali double sulphates, and they are especially convenient for separating the metals into the three groups of the cerium, terbium, and yttrium elements respectively. They may be prepared by double decomposition of the rare earth sulphates with barium ethylsulphate, but on account of the ease with which the alkylsulphates are hydrolysed by acids, it is essential that the solutions should be quite neutral. A more convenient method, according to James, is the treatment of the anhydrous chlorides in alcohol solution with sodium ethylsulphate dissolved in the same medium; sodium chloride is precipitated, whilst the ethylsulphates of the rare earth elements remain in solution.

The sulphites of the rare earth elements are sparingly soluble crystalline salts, of the general formula R₂(SO₃)₃,xH₂O. They are obtained by passing sulphur dioxide into a suspension of the hydroxides in water, or by double decomposition of soluble salts with alkali sulphite. They dissolve in excess of sulphurous acid, and on evaporation of the solution are deposited unchanged. They are distinguished from thorium sulphite by the fact that they form no alkali double salts. The strongly electropositive character of the rare earth metals is shown by the fact that they form normal and not basic sulphites.

The thiosulphates are readily soluble, crystalline bodies. With the exception of the ceric and scandium salts, they are not hydrolysed in boiling solution, a fact which allows of a complete separation from the readily hydrolysed thiosulphates of zirconium and thorium.

Dithionates of the commoner rare earth elements, of the general formula R₂(S₂O₆)₃,xH₂O, have been prepared by double[128] decomposition of the sulphates with barium dithionate. They are readily soluble, crystalline salts.

The selenates are soluble, crystalline salts, which separate from aqueous solutions in various hydrated forms. They resemble the sulphates in being less soluble in hot than in cold water, and numerous cases of isomorphism have been observed among the corresponding sulphate and selenate hydrates. Several alkali double selenates have been described; they show a close resemblance to the analogous double sulphates.

The selenites are amorphous, insoluble compounds, obtained by the action of selenious acid on the carbonates, or on solutions of neutral salts. Basic and acid selenites are also known.

Nitrates.

—The nitrates are crystalline, deliquescent compounds, readily soluble in water and alcohol, but less easily in nitric acid, a fact which has been of considerable importance for purposes of separation. The solubility is greatest in the case of lanthanum nitrate, diminishing through the cerium group to a minimum in gadolinium nitrate, and then increasing again. They separate from aqueous solution in the form of crystalline hydrates; in the cerium group, these have commonly the formula R(NO₃)₃,6H₂O, whilst the nitrates of the yttrium elements usually crystallise with 3 or 5 molecules of water. By carefully heating the hydrated salts, basic nitrates may be obtained, which in the yttrium group are soluble in water, and may be obtained crystalline; in the cerium group, the basic nitrates are insoluble. By further heating, insoluble ‘superbasic salts,’ and finally the oxides, are obtained in all cases. The temperatures at which these basic and superbasic compounds are formed vary with the electropositive character of the element; this fact affords a method of separation which has been very frequently employed.

An interesting series of addition compounds of the rare earth nitrates with antipyrine (dimethylphenylpyrazolone, C₁₁H₁₂ON₂) has been described recently by Kolbe.[164] Those of the cerium metals have the general formula R(NO₃)₃,3C₁₁H₁₂ON₂;[129] the yttrium nitrates appear to combine with four molecules of the base.

The tendency to form double nitrates with nitrates of the metals of Group Ia and Group IIa also varies with the basic strength of the hydroxides. In the most positive elements of the cerium group, the tendency is very pronounced, and there are a large number of stable, crystalline double salts; but the stability decreases rapidly as the atomic weight of the element rises, and in the terbium and yttrium groups crystallised double nitrates cannot be obtained. The solubility of these double salts increases rapidly in the same direction, the lanthanum double nitrates being the least soluble. For this reason, these compounds are of great importance for the purpose of separation, especially in the cerium group. Bismuth nitrate and the various bismuth double nitrates are isomorphous with the corresponding compounds of the cerium group, and the double bismuth ammonium and bismuth magnesium salts have been largely used by Urbain in the separation of samarium and the elements of the terbium group.

Phosphates.

—Addition of phosphoric acid, or an alkali phosphate to solutions of rare earth salts throws down the phosphates as gelatinous precipitates, which slowly become crystalline on standing. The precipitate is soluble in excess of phosphoric acid, and in other mineral acids, a fact of great importance in the commercial treatment of monazite. The composition of the precipitate is not known with certainty; both neutral and acid phosphates can probably be obtained according to the conditions. Double salts with the alkali phosphates can be prepared by fusion methods. The naturally occurring phosphates, monazite and xenotime, are mixtures of the orthophosphates of the cerium and yttrium elements respectively.

Phosphites are known in a few cases only; arsenates and arsenites of lanthanum have been prepared. Vanadates of some of the rare earth elements have been described.

Chromates.

—The rare earth chromates are, as a rule, sparingly soluble in water, and show considerable differences[130] of solubility amongst themselves; for this reason, they have been of some use in the separation of the cerium elements.[165] They are obtained by addition of potassium chromate to neutral solutions of rare earth salts as crystalline precipitates, of the general formula R₂(CrO₄)₃,8H₂O; with a large excess of alkali chromate, double chromates are obtained, which are more readily formed, and more soluble, in the yttrium series than in the cerium group. Addition of chromic acid or alkali bichromate to solutions of the soluble salts gives no precipitate, a fact which allows of the separation of zirconium and thorium, and of cerium in the tetravalent state, since the tetravalent elements are precipitated by both these reagents.

Ammonium molybdate throws down from neutral solution of rare earth salts gelatinous precipitates of the molybdates; the formula La₂2(HMoO₄)₆ is assigned to the lanthanum compound obtained in this way. No precipitation occurs if the solution be strongly acid; on this fact a process has recently been based for the volumetric estimation of thorium, in presence of rare earth salts, by means of ammonium molybdate (see p. 289).

Various silicotungstates and double tungstates have been described.

Carbonates.

—The more pronounced electropositive character of the rare earth elements, as contrasted with other trivalent metals, is well illustrated by the fact that they form stable neutral carbonates of the formula R₂(CO₃)₃,xH₂O. These may be obtained by passing a current of carbon dioxide through an aqueous suspension of the hydroxides, or by addition of an alkali carbonate to neutral solutions of the salts. Basic carbonates are known in the case of the less positive yttrium elements only; both these and the neutral carbonates are insoluble in water.

In presence of a large excess of alkali carbonate, double carbonates are formed. The stability as well as the solubility of these compounds increases in passing from the cerium to[131] the yttrium group, i.e. as the electropositive character becomes weaker. The double carbonates of the cerium elements are sparingly soluble, and are decomposed by water, especially on warming; they may, however, be recrystallised from alkali carbonate solution. The sodium and ammonium double salts are less soluble than the potassium compounds. The latter have the general formula R₂(CO₃)₃,K₂CO₃,12H₂O, and are of considerable importance in many processes of separation. The yttrium elements can be separated from the cerium metals, and the latter from one another, by taking advantage of the differences of solubility shown by the potassium double carbonates. If a concentrated solution of the salts in potassium carbonate solution be fractionally diluted with water, the cerium elements separate in the order: lanthanum, praseodymium, cerium, neodymium, and samarium; the more soluble yttrium compounds remain in the solution. Thorium forms double alkali carbonates which are very readily soluble in excess of alkali carbonate; this property is of great importance for the technical separation of the element.

Oxalates.

—The oxalates of the rare earth elements are of the greatest importance, on account of the fact that they are not only insoluble in water, but are also very sparingly soluble in dilute mineral acids, and in excess of oxalic acid. They can be completely precipitated even from strongly acid solutions by addition of sufficient excess of oxalic acid, or alkali oxalate, and thus afford a means of easily and completely separating the rare earth group from the commoner elements.

They are thrown down by addition of oxalic acid, or alkali oxalate, as amorphous precipitates, which rapidly become crystalline, especially if the solution is warmed. From water at normal temperatures they usually separate as the decahydrates, R₂(C₂O₄)₃,10H₂O, but hydrates with 7, 9, and 11 molecules of water of crystallisation are also known. From strongly acid solutions, mixed oxalo-salts of the general formula R(C₂O₄)X, where X = Cl, NO₃, HSO₄, etc., may be obtained. These mixed salts may also be prepared by dissolving the oxalates in concentrated solutions of the chlorides, nitrates, etc., whilst nitro-sulphates, R(SO₄)NO₃, have been[132] obtained by recrystallising the sulphates from strong nitric acid. The tendency to form salts with mixed acid radicles appears to be general.[166]

The solubilities of the oxalates in mineral acids of various concentrations have been examined by Hauser and Wirth.[167] Whilst the solubilities in water are exceedingly slight, and increase with increasing atomic weight of the elements, i.e. from the cerium to the yttrium group, in mineral acids of concentration 3-4N the solubility becomes noticeable, and is greatest for the oxalates of the most positive elements. The solubility is greatly lessened, however, if considerable excess of oxalic acid be present.

Double oxalates with the alkali oxalates can be obtained with the salts of the yttrium elements only, the oxalates of the cerium elements being almost insoluble in excess of alkali oxalate in the cold. Of the alkali double oxalates, the potassium compounds are the most soluble, but the ammonium compounds show the greatest differences in solubility; von Welsbach has employed the method of fractional crystallisation of these salts from a saturated solution of ammonium oxalate for separations in the yttrium group. The sodium double oxalates are the least soluble of these double salts.

Since the rare earth elements are almost always separated in the form of the oxalates, the methods for transforming these into soluble compounds become important. They may be ignited to oxides, and these dissolved in nitric acid; if the content of ceria is very high, the oxide mixture may become insoluble, but this difficulty may be overcome by addition of a reducing agent—hydrogen peroxide is very convenient for this purpose. The oxalates may also be dissolved directly in fuming nitric acid, care being taken to avoid loss; if the mixture contains cerium, the oxidation is hastened, ceric salts having the property of acting as oxygen carriers. By boiling for a short time with potash, the oxalates may be easily transformed into the hydroxides, which can be dissolved in dilute acids.

[133]

Formates.

—On account of the considerable differences in solubility by which they are characterised, these salts have been employed for separations. The formates of the cerium group are considerably less soluble than those of the yttrium group. They may be partly precipitated from solutions of rare earth salts by addition of alkali formate—formic acid itself causes precipitation only with salts of weak acids, e.g. the acetates—but are best prepared by dissolving the oxides in formic acid; on concentration of the solution, the formates of the cerium and terbium elements successively separate, the salts of the yttrium group remaining in solution. The separation of the terbium earths by this method was attempted by Delafontaine; his ‘new’ element, Philippium, obtained from the mother-liquors, was in reality a mixture of the terbium and yttrium elements, which cannot be completely separated by the formate method.[168]

The acetates are readily soluble in water, the yttrium salts being rather less easily soluble than those of the cerium group. They are therefore obtained by dissolving the oxides in acetic acid; addition of alkali acetate to a solution of a rare earth salt gives no precipitate, even on boiling, behaviour which is in marked contrast to the ease with which the salts of other trivalent metals are hydrolysed under these conditions. In this respect the rare earth elements differ also from the tetravalent elements zirconium and thorium (and from cerium in the tetravalent state); soluble salts of the latter, on boiling with sodium acetate, give insoluble basic acetates. Even sparingly soluble compounds of the rare earth elements are as a rule taken into solution by digestion with ammonium acetate.

Tartrates.—Addition of ammonium tartrate to a neutral solution of rare earth salts throws down an amorphous precipitate, which dissolves easily in acids, and in excess of the precipitant. In the presence of tartaric acid, precipitation of the earths by addition of sodium hydroxide is completely inhibited. Potassium hydroxide under these conditions gives a precipitate in the case of the yttrium elements, though only[134] on boiling; ammonia gives a crystalline precipitate even in the cold with this group. These precipitates are alkali double tartrates of the yttrium metals; the cerium elements give no precipitate at all. In all cases, therefore, the precipitation of the hydroxides is inhibited by the presence of tartaric acid.

A very large number of organic salts of the rare earth elements has been prepared and examined during the past two decades, in the endeavour to find some class of compounds which will allow of an easy separation of the group. The benzoates, succinates, hippurates, citrates and similar relatively simple salts first received attention, but less common acids, as e.g. the hydroxynaphthalenesulphonic acids, have also been employed.[169] The use of various organic acids for the separation and estimation of thorium in presence of the rare earths is outlined in that connection (see p. 288). More recently, the glycollates and cacodylates have been prepared. The glycollates[170] of the cerium elements have the general formula R(C₂H₃O₃)₃, and crystallise in crusts; they are more soluble than the yttrium compounds, which have the formula R(C₂H₃O₃)₃,2H₂O, and crystallise in needles. The cacodylates,[171] R₂[As(CH₃)₂O₂]₆, crystallise with 16 or 18 molecules of water, and have similar solubility relations.

The phthalates of the yttrium group have been found to be very valuable for purposes of separation by Meyer and Wuorinen.[172] The salts are readily obtained in solution by shaking together cold aqueous suspensions of the rare earth hydroxides, and phthalic acid; the clear solutions when warmed become cloudy, the organic salts hydrolysing very easily, with separation of the hydroxides. The most positive elements naturally remain longest in the solution, the weakly basic oxides accumulating in the first precipitates.

An organic compound which has proved very useful[135] in the treatment of the rare earths is acetylacetone, CH₃.CO.CH₂.CO.CH₃.[173] In its enolic form, this substance forms salts with metals, which in the case of the rare earth elements are especially characterised by the ease with which they may be obtained, and their high crystallising power. They may be prepared by double decomposition of neutral solutions of rare earth salts with ammonium acetylacetone, and crystallise readily from dilute alcohol. They have been used by Urbain in the fractionation of the yttrium group, and for determination of molecular weights by the boiling point method; Biltz[174] has shown that in solution they generally have the double formula R₂(C₅H₇O₂)₆.

The Rare Earth Elements, and the Periodic Classification

At the time of the introduction of the periodic classification the rare earth elements were generally believed to be divalent. This belief, which has persisted until quite recently,[175] was based chiefly on the electropositive character of the metals, and their general chemical resemblance to the elements of the alkaline earths; the isomorphism of the tungstates of calcium and the cerium elements, and of the molybdates of lead and the cerium elements, also supports this view. The physical evidence in favour of Mendelejeff’s view, however, is quite overwhelming; the specific heats of the metals, the equivalent conductivities of the chlorides, and molecular weight determinations by means of vapour densities and the boiling point method, prove beyond doubt that the elements are in fact trivalent.

In deciding in favour of the trivalent nature of the rare earth metals, Mendelejeff was influenced chiefly by the fact that there was no room in the table for divalent elements with the equivalent weights then assigned to the cerium and yttrium[136] elements. At that time, only the six oxides obtained by Mosander were known; of these the accepted equivalents and atomic weights were as follows:

the values for terbium being uncertain. If cerium be considered trivalent in the cerous salts, its atomic weight becomes 138, that of barium being 136. Mendelejeff placed cerium in Group IV, series 8, in the position which it still occupies; he pointed out that the accepted equivalent must be too low, and suggested that the atomic weight should be at least 140, almost exactly the value accepted to-day.

This choice left the positions in Group III, series 8, horizontally before cerium, and in Group IV, series 10, vertically below it (see figure), to be filled by the two elements, lanthanum and didymium. No chemical evidence being available to decide the choice, he provisionally assigned didymium to the first (Group III, series 8), and lanthanum to the second (Group IV, series 10) position, at the same time expressing the opinion that didymium was probably a mixture of closely related elements. Yttrium then fell into place in Group III, series 6, above didymium, and erbium in Group III, series 10, below it. To the vacant space above yttrium in Group III, series 4, he assigned the hypothetical element Eka-boron, with atomic weight 44; this space is now occupied by scandium, which corresponds almost exactly in properties to the metal described by the Russian chemist. A part of the table illustrating these positions is shown in Fig. 4.

The determination of the specific heats of the metals by Hillebrand and Norton in 1875, whilst confirming the trivalency of the elements, rendered it necessary to alter the position of lanthanum, which was placed in Group III, series 8, instead of didymium, which was thus left without a place. This first indication that all the rare earth elements could not be fitted[137] into the table without difficulties was soon followed by the discovery of several other members of the group, for which places could not easily be found.

Fig. 4.—Part of the Periodic Table, showing the positions originally assigned to the Rare Earth Elements by Mendelejeff

It was first pointed out by Brauner in 1881 that, with the exception of scandium (44·1) and yttrium (89·0), the rare earth elements form a zone of increasing atomic weight between barium (137·37) and tantalum (181·5). In 1902 he proposed[176] to consider the rare earth metals as a kind of zone or belt among the elements, comparable to the asteroids in the solar system, extending from cerium in Group IV to tantalum in Group V in a continuous series. The suggestion seems at[138] first sight contrary to the whole principle of periodic classification, but it accords very well with the anomalous position of the rare earth group among the other elements; it is very well illustrated in the accompanying Fig. 5, which shows an helical or space representation of the table.

[139]

Fig. 5.—helical representation of the periodic law

Electropositive Elements, above plane of paper, black letters on white ground. Electronegative Elements, below plane of paper, white letters on black ground. Intermediate Elements, in plane of paper, black letters on sectioned ground.

Fig. 6.—Part of the Periodic Table,
showing the Positions assigned to the Rare Earth Elements by Brauner in 1908

Brauner’s conception is also in accord with the physical properties of the elements and their compounds. These vary continuously throughout the group, and show nowhere the sudden transitions which are characteristic of other series in the table. Benedicts[177] has collected all the data bearing on the atomic volumes, and finds that those also vary continuously, with rise in the atomic weights, within quite small limits, all lying between the values for barium and tantalum. In face of all the evidence furnished by physical and chemical properties, however, Brauner[178] has recently reverted to an idea which he put forward in 1881, according to which lanthanum and cerium are placed as usual in Groups III and IV, series 8, whilst the other elements are distributed in order throughout the remaining groups, as shown in Fig. 6.

In support of this arrangement, he quotes the fact that[140] some of the elements appear to be able to form higher oxides in the presence of other oxides, which act as oxygen carriers (see pp. 174, 177-8), though these higher oxides are certainly not salt-forming. He also deduces, from the rates of hydrolysis of the sulphates, that the elements fall into two parallel series, according to the strengths of the hydroxides as bases, on which ground he justifies the distribution throughout series 8 and 9. There can be no doubt, however, that this disposition is far less in accordance with the behaviour and properties of the rare earth elements than is the first arrangement, which places them in a transition zone between barium and tantalum; it is impossible, for example, to reconcile the properties of praseodymium with those of columbium and tantalum, or to find the slightest analogy between neodymium and molybdenum or tungsten, as the second arrangement requires.

The analogy of the rare earth group to the elements of Group VIII has been pointed out by many authors.[179] On the ground that the rare earth elements cannot be spread over the table in series 8-10, Steele[180] favours the early classification of Thomsen, according to which the elements are divided into three groups. The first, corresponding to Groups I and II of Mendelejeff’s table, consists of two sub-groups, each containing seven elements[181]; the second, corresponding to the first two long series of the periodic table, has two sub-groups, each of seventeen elements, of which the first and last seven are analogous—these elements fall into the same groups in the periodic table—whilst the middle three are interperiodic. These interperiodic elements are those which Mendelejeff places in Group VIII. The third division consists of one (or two) group(s) of thirty-one elements; here again, the first and last seven are analogous, whilst the interperiodic elements, which are seventeen in number, include the rare earth metals.

Steele’s idea has been extended by Werner,[182] who has drawn up a table to illustrate it. In this classification, the elements are arranged in order of atomic weight, but arbitrary gaps are left in such a way that similar elements may fall[141] into the same vertical columns, as in the periodic table. The arrangement has the advantage that the interperiodic elements, consisting of the rare earth elements and the elements placed in Group VIII of the periodic table, here do fall in the middle of their respective periods, but it has several drawbacks, and does not represent the transition of properties from element to element so well as the helical representation of the periodic table, which brings out most clearly the true relations between the elements, and the anomalous position of the rare earth metals.

Mention must be made at this point of the theory of ‘Meta-elements’ put forward in 1888 by Sir William Crookes.[183] From his work on the cathode luminescence of some of the oxides (see next chapter), that author was led to the conclusion that several of the then-accepted rare earth elements, notably samarium and yttrium, were in reality heterogeneous, consisting of large numbers of very closely related bodies, differing so very slightly in properties that only the most refined methods could perceive the variations; for these he proposed the name Meta-elements. Though it has been proved that the differences observed by Crookes in the luminescence spectra were really due to the presence of very small quantities of impurities, his paper is of great interest, in that it contains a theory of evolution of the elements, and postulates the possibility of their decay. Modern developments in radioactivity have not only lent a curious force to these speculations, but even support his contention that a chemical element, in the ordinary sense of the word, is not necessarily homogeneous.[184] In the field of the rare earths, also, the homogeneity of elements is even now continually being called into question (see Thulium, p. 204). In any case, we have in the rare earth elements a series of bodies in which the change of properties from one member to another—and the consequent possibility of easy separation—is so very slight, and so far without parallel in the whole field of chemistry, that we are at least justified in asking whether some extension of our ordinary conception of an element is not required.

[142]

CHAPTER X
GENERAL METHODS OF SEPARATION

The chemist who sets out to prepare a pure compound of a rare earth element is faced by a great difficulty. The rare earth compounds occur in nature, as one might expect from their great similarity, as mixtures of very complex composition. After the relatively simple separation from foreign elements has been accomplished, the enormously greater difficulty of separating the elements from one another has to be encountered. So great is this difficulty, by reason of the fact that, with the sole exception of cerium, the elements show no variation in property sufficient to allow of the use of ordinary analytical methods, that even at the present day it is extremely doubtful if all the elements in the yttrium group are known to us.

The methods which can be adopted in attempting a separation are of two kinds. The first includes those processes which take advantage of the gradual variation in basic strength of the hydroxides as the atomic weight changes; the most important of these are fractional precipitation of the hydroxides, and fractional decomposition of the nitrates. Fractional precipitation of the hydroxides is generally effected by gradual addition of ammonia, soda, magnesia, or other base, to a solution of the mixed salts; such a solution may also be digested with the oxides obtained by ignition of another fraction of the rare earth compounds. If the digestion be sufficiently complete, the precipitate in each case will be richer in the less basic hydroxides, whilst the solution will be richer in the salts of the more electropositive elements.

[143]

The fractional decomposition of the nitrates is based on the fact that when a mixture of the salts is heated gradually, the nitrate of the least positive element begins to decompose first. The temperature is maintained for some time at the point at which decomposition begins; when nitrous fumes cease to be evolved the mixture is cooled, and extracted with water or dilute acids. The insoluble portion—basic or superbasic nitrate (see p. 128)—will then be richer in the less electropositive elements; the solution is evaporated, and the solid so obtained subjected to a somewhat higher temperature, and the process repeated several times. In this way, a series of fractions is obtained, in which the elements tend to distribute themselves in order of electropositive character. By a sufficient number of systematic repetitions of such steps, the elements may eventually be obtained in the form of compounds of approximate purity, which may then be refined by one of the methods of the second kind described below. Experience has shown, however, that a quicker and more complete separation may generally be effected by combining two or more methods of separation; one method will give the best separation up to certain limits, but then becomes much less valuable; the separation at this point is therefore taken up by another process. A process depending on differences of basic strength of the hydroxides is generally supplemented by a method of the second class, i.e. a process of fractional crystallisation; where the basicity method is not used (as, for example, in most of the recent processes for separation of the cerium elements), two or more different methods of fractional crystallisation will supplement one another.

The methods of the second class, which are processes of fractional crystallisation, depend on the differences in solubility which are observed in analogous compounds in passing from one member of the group to another. The value of these methods, as opposed to the methods depending on differences in basic strength, was clearly shown by Auer von Welsbach, who in 1885 succeeded in resolving Mosander’s ‘Didymium’ into two new elements, praseodymium and neodymium, by fractional crystallisation of the ammonium double nitrates;[144] since that date, much attention has been devoted to the task of finding rare earth compounds which will lend themselves to such processes. The method is extremely laborious, and may involve several thousand recrystallisations, in consequence of the generally very slight differences of solubility, and the ease with which the rare earth compounds, being almost always isomorphous with one another, form mixed crystals.

Whilst the method of fractional crystallisation has come into general use for the separation of one element from another only within the last thirty years, processes for the separation of the cerium group from the yttrium elements, depending on differences of solubility, have long been known and used. The most important of these, the double sulphate method, depends on the fact that the potassium double sulphates of the cerium metals are almost insoluble, whilst those of the terbium group are sparingly, and of the yttrium group readily soluble in a concentrated solution of potassium sulphate. The cerium elements may be thus completely removed from a solution of mixed salts by addition of a crust of potassium sulphate crystals, or of an hot concentrated solution of the same reagent. In other cases, e.g. in the double carbonate and double oxalate processes, separation is effected by taking advantage of the greater tendency to the formation of double salts possessed by the yttrium metals.

In effecting a separation of closely related bodies by fractional processes, in which a large number of repetitions of the same operation are necessary, only the most careful and systematic procedure can avoid much waste of valuable material; in these processes, the object of the chemist is to obtain pure end fractions, whilst keeping the middle fractions as small as possible. One method of procedure generally adopted is illustrated in Fig. 7, which represents a fractional crystallisation of a mixture of four or five substances, α, β, ... φ; the separations being usually conducted in such a way that subgroups of three, four or five elements are first obtained, these being then further fractionated to obtain the pure elements. In the diagram, crops of crystals are represented by crosses, the mother-liquors by circles; for the sake[145] of illustration, the process is made to appear as simple as possible.

Fig. 7

The mixture is dissolved up, and allowed to crystallise; the crystals are filtered off, the filtrate concentrated, and a second crop obtained; this is repeated until five or six crops of crystals have been obtained. These, with the mother-liquor, constitute series A. The first fraction is now recrystallised; it yields a crop of crystals, fraction 1 of series B, and a mother-liquor, which is added to fraction 2 of series A, as indicated by the dotted arrow and circle; on recrystallisation of this mixture, a crop of crystals, fraction 2 of series B, is obtained, together with a mother-liquor, which is recrystallised with fraction 3 of series A. In this way, by continued repetition, series are obtained, of which each contains one fraction more than its predecessor; the least soluble constituent is thus concentrated in the fractions represented on the left of the diagram, whilst the most soluble accumulates[146] in the mother-liquors. After a greater or smaller number of series have been traversed, according to the differences in solubility, the end fractions in each series will be pure. These are no longer fractionated, and the number of fractions in each series begins to diminish, as shown on the diagram. The middle fractions will contain the compounds of intermediate solubility; these may be separated by further fractionation on the same lines, or may perhaps be better treated by a different or modified process.

In a modification of the method, each fraction of series A is recrystallised separately, yielding a crop of crystals, and a mother-liquor; series B is then built up by adding to the crystals from fraction 2 the mother-liquor from fraction 1, to the crystals from fraction 3 the mother-liquor from fraction 2, and so on; the fractions in this series are then recrystallised separately, and the third series built up by the similar combination of the crystals and mother-liquors.

Similar systematic methods of procedure must be adopted in working out any method of fractional separation; it can at once be seen that where, as in the rare earth group, only small variations in properties exist, much time and care must be expended, if pure products are required.

Since the development of the methods of spectrum analysis, the difficulty of testing the efficiency of a method of separation, and of examining the purity of the products obtained, has been greatly lessened. The only reliable test at the disposal of the earlier chemists was the determination of the equivalent weight, which still constitutes an important check on the modern methods. Some account of the methods available for the control of the methods of separation is essential in a general account of the rare earths; but before describing these, it will be convenient to give a short description of the methods used in the extraction of the elements from the rare earth minerals.

Extraction of the Rare Earths from Minerals

With the exception of those containing large proportions of columbium, tantalum, and titanium, the rare earth minerals[147] are easily decomposed by acids. The silicates, as a general rule, can be satisfactorily treated with hydrochloric acid in the ordinary way, but for large quantities, the use of sulphuric acid is more desirable. The more refractory minerals are completely decomposed by fused alkali hydrogen sulphate; sodium bisulphate is more suitable for this purpose than the potassium compound, the sodium double sulphates of the rare earth elements being more soluble than the potassium salts. Hydrofluoric acid also attacks the refractory minerals very readily; the rare earths, in this case, are left as the insoluble fluorides.

After decomposition with sulphuric acid or bisulphate, the cold residue is extracted with water, the rare earth sulphates or double sulphates being removed in solution. Digestion with nitric acid may be necessary at this stage, if titanium, columbium, etc., are present; after filtration, the solution is evaporated to dryness, and the residue extracted with dilute hydrochloric acid. The solution is saturated with sulphuretted hydrogen to remove lead, copper, bismuth, molybdenum, etc., and treated in the usual way with ammonium chloride and ammonia. The precipitate is washed, and dissolved in hydrochloric acid, the solution heated to about 60°, and the rare earths precipitated by addition of excess of oxalic acid, which holds in solution any zirconium which may be present. In the presence of phosphates, e.g. in the treatment of monazite or xenotime, the precipitate of oxalates should be ignited to the oxides, these dissolved in acid, and a second precipitation with oxalic acid effected; this treatment is necessary to remove phosphoric acid completely.

Preliminary examination of the earth mixture.

—Before a method of separation can be decided upon, some knowledge of the composition of the mixture to be treated must be obtained. The nature of the mineral used for the extraction will, as a rule, afford useful information. It is known that in some minerals the cerium group, in others the yttrium group, predominates more or less completely; certain minerals, also, are known to be rich in elements of one or another subgroup. An approximate knowledge of the relative[148] proportions of the cerium, terbium, and yttrium groups will be afforded by a rough double sulphate separation; thorium, zirconium, and scandium come down with the cerium earths. For approximate separation, Urbain[185] proposes the use of the ethylsulphates. The yttrium elements can be quickly separated in an approximate manner by fractional precipitation of the hydroxides with magnesia. The successive fractions obtained by these methods are examined spectroscopically; from the results, the composition of each, and so of the original mixture, may be roughly deduced.

The Spectrum Examination

In no department of chemistry have the methods of spectrum analysis proved of more value than in the field of the rare earths. They provide the chemist with a means of following and controlling his processes of separation which is far more delicate and decisive than the older method of determining the equivalent weight. Whilst the examination of emission spectra, and especially of arc spectra, is of decisive value in every case, it has the disadvantage of requiring delicate and complicated apparatus and great experimental skill; wherever possible, therefore, the examination of the absorption spectra is preferred, though this is useful only for a few of the elements, and varies considerably with the conditions employed.

The Absorption Spectra.

—Absorption in the visible region of the spectrum is observed only with those rare earth compounds which are coloured, and is of value, therefore, chiefly for identification in the case of praseodymium and neodymium among the cerium elements, and of erbium among the yttrium metals; these give characteristic absorption bands, even in dilute solution. The absorption spectra of the rare earth compounds are highly characteristic, the bands being well defined and sharply bounded, whereas coloured compounds of the common elements show general absorption, or at best diffuse bands, under the same conditions.

In observing an absorption spectrum, the light from a[149] Nernst lamp, or incandescent burner, is passed through a layer of a suitable solution of the coloured compound, of known concentration and thickness, and after collimation is analysed by a suitable prism; the spectrum is observed by a telescope in the ordinary way. Where accurate readings are not required, as, for example, in testing for the presence or absence of a particular element, the position of the bands may be read to a sufficient degree of accuracy by means of a scale, the image of which is adjusted to coincide with the spectrum as seen through the eyepiece; but in mapping a spectrum accurately, more refined methods must of course be used. The photographic method, in which a photograph of the spectrum is taken on a plate which bears, for purposes of measurement, a comparison spectrum of known lines, is very convenient for examining the absorption in the violet and ultraviolet regions.

The intensity, and to some extent also the position, of bands in an absorption spectrum may vary considerably, according to the conditions employed. Of the various factors which must be considered, the concentration of the solution, the thickness of the layer used, the nature of the solvent, and of the acid radicle, and the presence of other earths are the most important. The concentration of the solution, and the thickness of the layer, which together constitute the Optical Density, must be so adjusted that the absorption is neither too strong nor too weak; in the first case the sharp bands tend to merge into broad diffusion areas, and details are obscured, whilst in the second case the presence of coloured compounds which do not show strong absorption bands may be overlooked.

The nature of the acid radicle has considerable influence on the position of the absorption maxima, the general rule being that the bands are shifted towards the red end of the spectrum as the molecular weight of the compound used increases. Naturally, also, the nature of the solvent has an important effect, all the usual phenomena which must be considered in the measurement of the physical properties of substances in solution coming into play; electrolytic dissociation,[150] hydration, dissociation and the formation of complexes, for example, are all important factors. The presence of colourless earths has also been found to cause important differences. It follows, therefore, that for the chemist, the absorption spectra can be considered as a valuable aid only in detecting the presence or absence of the three elements which give the strongest and most characteristic absorption bands, viz. praseodymium, neodymium, and erbium, and that conclusions regarding the quantitative composition of mixtures must be drawn with the utmost caution.

The Emission Spectra: Spark Spectra.

—The factors which tend to limit the value of the absorption spectra for analytical purposes, for the most part disappear when the emission spectra are employed. In the case of the spark spectra, indeed, great differences are observed according to the conditions and method of experiment; but the arc spectra are practically invariable under all conditions, and hence they constitute the ultimate test in all cases. The spark spectra are observed when one terminal—the cathode—of an induction coil is embedded in the oxides to be examined, and the discharge then passed. The discharge is also frequently passed between platinum poles partly immersed in a strong solution of a salt of the element under examination; a form of apparatus very suitable for this method of observation has been described by Sir W. Crookes.[186] The spectra so obtained are in a high degree characteristic, but they vary very considerably with the form and dimensions of the coil, the length and cross-section of the wires, the potential difference employed, and so on. An entirely new spectrum also is obtained in many cases by mere reversal of the current; under these conditions, a phosphorescent appearance is observed, the spectrum of which—reversed spark spectrum of de Boisbaudran—has been found in many cases to resemble the cathode luminescence spectra of Crookes.

The Arc Spectra.

—The final criterion of purity in the examination of a rare earth element is in almost all cases the[151] arc spectrum. Since for some of the elements, especially in the yttrium group, the entire spectrum has not yet been accurately mapped out, spectra are generally observed frequently throughout the course of a fractionation; by this means, the separation can be followed by the disappearance of some lines, and the appearing or strengthening of others, and such examinations have led occasionally to the discovery of new elements (see, for example, under Separation of ytterbium earths, p. 205). Such determinations, however, require much time and extensive and complicated apparatus.

Carbon electrodes are generally employed, and it is immaterial in this case which is the anode, and which the cathode. The lower carbon is hollowed out, and the space filled with the oxide or sulphate of the element or mixture to be examined; or the electrode may be impregnated with a concentrated solution of a salt. The light is examined by means of a diffraction grating, and the spectrum photographed on a plate which bears a comparison spectrum for measurement. The lines are most numerous in the violet and ultraviolet regions, and the most characteristic spectra are given by the colourless earths. The method is naturally more delicate for some elements than for others; the great persistency of the scandium line 3613·984, for example, was found very valuable by Crookes and by Eberhard in the examination of various rocks and minerals for that element, whilst other intense and persistent lines have served for the detection of various rare earth elements in the sun and many stars.

The Cathode Luminescence Spectra.

—The phenomenon of cathode luminescence, which was observed and very fully investigated by Sir William Crookes, and which led that author to his theory of Meta-elements, is one of the greatest scientific interest. Crookes observed that certain of the rare earths, when subjected to the action of cathode rays in a vacuum tube, exhibit a brilliant phosphorescence, which, when examined by the spectroscope, show characteristic spectra, which differ greatly for fractions of apparently identical chemical composition, and are otherwise distinguishable by physical properties. The researches of Lecoq de[152] Boisbaudran, and the more recent work of Baur and Marc,[187] have shown that this luminescence is observed when a small quantity of a coloured earth is present with a very large quantity of a colourless earth, the maximum phosphorescence being produced by about 1 per cent. of the coloured earth, or ‘phosphorogen.’ The question has recently been very fully examined by Urbain.[188] He shows that the sensitiveness of the phenomenon is so great that it cannot be employed for the ordinary purposes of chemical analysis, one part in a million of the phosphorogen being sufficient to cause a clearly perceptible luminescence in a pure colourless oxide.

The Magnetic Susceptibility.

—The fact that the rare earths differ very considerably from one another in their magnetic properties has been known for several years,[189] and has recently been employed by Urbain and Jantsch[190] as a means of identification, and a test of purity, and for following processes of fractionation. The magnetic susceptibility reaches a minimum at samarium, and rises very sharply on either side of that element, so that the presence of the closely related elements, neodymium on the one side, and europium and gadolinium on the other, which differ only very slightly from samarium in atomic weight and solubility, can easily be detected by this means. The property is highly additive, and can be used, therefore, to estimate the relative proportions of two oxides in a mixture; the determinations are said to be easily and quickly carried out.

When the elements are considered in order of atomic weight, the coefficient reaches a maximum at neodymium in the cerium group, and again at dysprosium (or holmium) in the yttrium group:—[191]

[153]

Erbium, thulium, ytterbium, and lutecium appear in descending order at the end of the series, but no figures are given.

The most interesting application of the property has been Urbain’s discovery of the new element Celtium (see p. 207).

The Equivalent Weight Determination

The determination of the mean equivalent weight, which was for the earlier chemists the only reliable method of controlling their fractionations, is still of considerable importance for this purpose, especially in the yttrium group, in which the differences in atomic weights are more considerable than among the cerium metals. Great importance, moreover, still attaches to these determinations, since they serve to fix the atomic weights; save that the methods used in an atomic weight determination are somewhat more elaborate and refined than those used when it is desired merely to test a fractionation, the same processes apply in both cases.

The methods which have been most commonly used are those based on a determination of the ratio R₂O₃ : R₂(SO₄)₃, and these are of two kinds, the synthetic and the analytical. The first, in which a known weight of the oxide is converted into the sulphate, has been most used for the most strongly basic oxides, since with these it is difficult to remove the last traces of sulphuric anhydride from the oxide by heat. The oxides are best obtained from the oxalates, which are precipitated from an acid solution of the nitrates, washed thoroughly with water, alcohol and ether in succession, dried, and ignited[154] in a tarred platinum crucible. The oxide is best dissolved in dilute hydrochloric or nitric acid on the waterbath, a slight excess of sulphuric acid being added only when a clear solution has been obtained; the liquid is then heated gradually to 300°, and finally in the electric furnace at 450°-550° until constant in weight. If sulphuric acid be added directly to the weighed oxide, particles of the latter may become completely coated with the insoluble sulphate, and so escape the action of the acid.

In the analytical method, a known weight of sulphate is ignited to the oxide, and weighed as such. This method is most suitable for the less basic members of the yttria earths, of which the sulphates can be completely decomposed without difficulty at a red heat. By the use of the microbalance, a sufficiently accurate determination can be carried out by either of these methods in little more than half an hour, as the chemical changes are exceedingly rapid where only small quantities are employed, and no time is required to allow the vessels and solids to cool. Using the microbalance, Brill[192] has carried out a series of experiments to determine the limits of temperature within which the various steps of the process should be carried out. He finds that a temperature of 400°-550° is required to decompose the last traces of acid sulphate, and give the pure neutral sulphate. Between the temperatures of 850° and 950°, basic salts are formed, from which the last trace of sulphuric anhydride is expelled at 900°-1150°; the precise temperature required in each case depends, of course, on the basic strength of the oxide in question.

The determination of equivalents by means of the ratio R₂O₃ : R₂(C₂O₄)₃, has been brought to a high degree of accuracy by Brauner.[193] A weighed quantity of the carefully prepared oxalate is ignited, with suitable precautions, to the oxide, in a tarred platinum crucible. A second weighed specimen of the same oxalate preparation is dissolved in dilute sulphuric acid, and titrated at 60° with permanganate, which is standardised against pure ammonium oxalate.

[155]

Of the methods of volumetric analysis which have been proposed, that put forward by Feit and Przibylla appears to be the most suitable. A convenient quantity of oxide, which has been ignited until constant in weight, is dissolved by gently heating with a known excess of N 2 sulphuric acid, in a conical flask of Jena glass. The excess of acid is titrated with N10 sodium hydroxide, using methyl orange as indicator. This method, which has the advantages of ease and quickness, is very reliable, if suitable precautions are taken, in the case of the more strongly basic oxides; but with the least strongly basic members of the yttria group, the erbia and ytterbia oxides, the end point is not very sharp, whilst with the weakly basic scandia, the method breaks down entirely.[194]

[156]

CHAPTER XI
THE CERIUM GROUP—CERIUM

The extraction of the rare earth elements from minerals, by which they are obtained in the form of the oxalates, and the methods of bringing these into solution, have already been described. From the solution, before any separation of the rare earths is attempted, thorium should be removed; for this purpose, any of the methods described under estimation of thorium (see p. 286) may be used, the most convenient being the peroxide precipitation of Wyrouboff and Verneuil.

The solution is then treated with potassium sulphate until the absorption bands of didymium (praseodymium and neodymium) can no longer be observed, or appear only very faintly, when a layer of the solution is examined with a spectroscope; the precipitate then consists of the potassium double sulphates of the cerium with some of the terbium elements. If the mixture is very rich in the cerium elements, and correspondingly poor in the yttrium elements—as, for example, the mixture of earths obtained from monazite—Drossbach[195] recommends a preliminary separation by means of the double carbonates; the double sulphate method may then be employed to remove the last of the yttrium and most of the terbium elements. The sparingly soluble double sulphates of the cerium metals may be transformed into the hydroxides by digestion with potassium hydroxide, and these taken into solution, after washing, by hydrochloric or nitric acid.

Cerium, Ce = 140·25

Of all the rare earth elements, cerium, by virtue of its property of forming ceric salts corresponding to the dioxide[157] CeO₂, is the one most easily separated and obtained in the pure state. In those compounds in which it is tetravalent, cerium functions as a much less strongly electropositive element than in the cerous compounds, and all the methods of separation are based on this fact. Mosander, who first demonstrated that the old ‘ceria’ was a mixture, separated the element by treating a suspension of the hydroxides in potassium hydroxide with chlorine; yellow ceric hydroxide remains undissolved, whilst the other elements go into solution as the chlorides and hypochlorites. This method was extensively used until quite recently; it has the advantage of separating the cerium completely, but the product is very impure, and several repetitions are required to give good results. The basic nitrate method, which is now used on the commercial scale in extracting cerium from monazite (see p. 284), is also due to Mosander, though it has been employed subsequently by many workers.

Several methods take advantage of the ease with which the ceric salts, as compared with salts of the trivalent elements, may be hydrolysed. Brauner[196] dissolves the oxides in nitric acid, and after removal of excess of acid, boils with a large volume of water—basic ceric nitrate is thrown down, the other elements remaining in solution as nitrates. The precipitate is redissolved, and the process repeated until the cerium is found spectroscopically to be free from didymium. The hydrolysis of the ceric salt may be effected more quickly and completely by the addition of ammonium sulphate or magnesium acetate.[197] James[198] boils the solution of the nitrates with potassium bromate, keeping the whole neutral by addition of powdered marble; the cerium is completely and very quickly precipitated as basic nitrate.

An interesting method is due to Koppel[199]; the oxides are dissolved in a solution of hydrogen chloride in methyl alcohol, and treated with pyridine, when the sparingly soluble double[158] chloride, (C₅H₅NH)₂CeCl₆, separates, and may be obtained pure by recrystallisation from alcohol and ether. The permanganate method of Drossbach, which is used on the commercial scale, is described on p. 285.

The cerium compounds obtained by these methods are purified by transformation into the anhydrous sulphate, which is dissolved in ice-water; when this solution is allowed to come slowly to room temperature, the pure octohydrate separates. Pure cerium salts should show no trace of absorption when concentrated solutions are examined spectroscopically; on ignition, the oxide obtained should be almost colourless, having at most a faint yellow tinge. A reddish or brownish-red shade indicates the presence of praseodymium. An arc spectrum examination will generally show the presence of lanthanum, which occurs in traces even in the most carefully purified cerium preparations.

The preparation and properties of metallic cerium have already been described (see p. 114); for an account of the pyrophoric alloys, see p. 314.

The Cerous Compounds

The salts of trivalent cerium are very similar to those of the other rare earth elements, and a detailed description of them is therefore unnecessary. The sesquioxide, Ce₂O₃, cannot be obtained by ignition of the oxalate, nitrate, or other similar salt, since these decompose at high temperatures with formation of the dioxide, CeO₂. It has been prepared by the reduction of the dioxide with calcium;[200] it has a great affinity for oxygen, and readily absorbs the gas when exposed to moist air. Cerous hydroxide, Ce(OH)₃, obtained by addition of alkali to solutions of cerous salts, has also strong reducing properties,[201] and can only be prepared and preserved when oxygen is carefully excluded. It has been obtained as a perfectly white solid by the action of water on the carbide;[202] when dried in an inert atmosphere, it yields a perfectly white oxide. In presence of[159] air, it darkens, assuming a reddish-violet colour, which passes into yellow as the oxidation becomes complete. The oxidation proceeds more quickly in presence of potash or soda, ceric hydroxide, Ce(OH)₄, being formed; in presence of potassium carbonate, however, a dark-coloured peroxyhydrate is formed by autoxidation. The colour so produced disappears on shaking if an ‘acceptor’ is present, ceric hydroxide being left; if the acceptor cannot reduce this, the solution after shaking loses the power of re-forming the dark peroxide, but if the acceptor can reduce the ceric compound to cerous hydroxide, the solution after shaking regains the power of forming the peroxide which is a property of the lower hydroxide.

Cerous nitride, CeN, has been prepared by Moissan[203] by the action of ammonia on the heated carbide; it can also be obtained by heating the hydride in a stream of nitrogen.[204] Muthmann and Kraft also state[205] that it can be prepared by heating metallic cerium in the gas, the metal burning with the liberation of much energy in the form of heat and light; but Dafert and Miklanz[206] deny that it can be obtained in this way. Cerium nitride is a lustrous, brass yellow to bronze coloured solid, stable in dry air, but at once attacked by moist air, with evolution of ammonia, and formation of the dioxide. When moistened in air with a few drops of water, the substance reacts violently, becoming heated to redness. Alkalies and acids decompose it, with formation of cerous compounds.

The sulphide, Ce₂S₃, has been prepared by Biltz[207] by heating the sulphate to a red heat in a current of sulphuretted hydrogen; he describes it as a red powder. The chloride, CeCl₃, combines with ammonia with evolution of heat even at a temperature of -80°. Five additive compounds are described;[208] they are white powders, decomposed by water.

The solubility curve of the various sulphate hydrates has already been given (see p. 125). Various double sulphates with ammonium sulphate, and the sulphates of sodium, potassium,[160] thallium and cadmium are known. The cadmium double compound has the composition Ce₂(SO₄)₃,CdSO₄,6H₂O, and is prepared by mixing solutions of the simple salts in presence of sulphuric acid. Many double nitrates have been prepared; these are for the most part stable, highly crystalline compounds, easily soluble in water and alcohol. With the nitrates of the common divalent metals, cerous nitrate forms a series of double salts of the general formula 2Ce(NO₃)₃,3R(NO₃)₂,24H₂O, where R = Mg, Mn, Co, Ni, or Zn; these form an isomorphous series, crystallising in the hexagonal system. The acetylacetone compound melts at 131°-132°.

In the presence of hydrogen peroxide in the cold, ammonia throws down from solutions of cerous salts a reddish-brown peroxyhydrate, Ce(OOH)(OH)₃,[209] which on heating loses oxygen, and yields ceric hydroxide. The reaction is very delicate, and may be used as a test for cerium. If the precipitate be treated with acids in the cold, ceric salts are first obtained, but these are at once reduced, in the acid solution, by the hydrogen peroxide formed, so that cerous salts remain; ceric salts may be obtained by first boiling the suspension of the peroxyhydrate and treating the ceric hydroxide so obtained with acids.

The Ceric Compounds

The ceric salts are much more readily hydrolysed than the cerous salts, and show a great tendency, in dilute solution, to pass over into the latter. So great is this tendency that a solution of a ceric salt acts as if it were supersaturated with oxygen; ceric sulphate, for example, in dilute solution slowly evolves oxygen, whilst the chloride evolves chlorine. In consequence of this behaviour, ceric compounds have a very powerful oxidising action. The ceric salts are yellow to red in colour; their solutions are strongly acid, owing to the ease with which the salts hydrolyse, and on boiling deposit insoluble basic salts.

Beside the methods which have already been mentioned, ceric compounds may be prepared from cerous by oxidation[161] with sodium peroxide, bismuth tetroxide, ammonium persulphate, etc. In electrolysis of cerous salts, also, ceric compounds are obtained at the anode.

Ceric hydroxide, Ce(OH)₄, is obtained as a gelatinous yellow precipitate on the addition of alkali to a solution of a ceric salt, or by the oxidation of cerous hydroxide. The freshly prepared precipitate dissolves in nitric acid with a reddish colour; hydrochloric acid reduces it, with evolution of chlorine, and formation of cerous chloride, whilst sulphuric acid dissolves it with partial reduction, oxygen being evolved. If a solution of a ceric compound be dialysed for some days, a clear neutral solution is obtained, which contains the hydroxide in the colloidal condition; by evaporation of the solution, a gummy mass is obtained, which dissolves again in water to a clear solution. Electrolytes rapidly cause coagulation.

Cerium dioxide, CeO₂, is obtained by the ignition of any salt of cerium with a volatile acid, or by burning the element in oxygen; the latter reaction produces a very intense and blinding light, on account of which cerium compounds are often suggested for use in flashlight powders (see p. 319). The pure oxide should be almost white, or at most a very faint yellow, but the exact shade and appearance vary according to the method and temperature employed in preparation, doubtless by reason of the possibility of different degrees of polymerisation.[210] The oxide can act as an oxygen carrier towards other substances, notably towards other oxides of the rare earth group,[211] but the phenomena have not been fully elucidated. In virtue of this property, the dioxide has been proposed as a substitute for platinised asbestos in Dennstedt’s method for the combustion of organic bodies.[212]

The ignited oxide is soluble in nitric or hydrochloric acid only in presence of a reducing agent. Concentrated sulphuric acid converts it into ceric sulphate; fused bisulphate attacks it more readily. In the crystalline form, obtained by fusing[162] the amorphous form with borax, or a suitable salt,[213] it is extremely resistant to acids and to alkalies.

By heating the dioxide in a stream of hydrogen, care being taken to exclude air, a dark blue oxide, of which the composition corresponds approximately to that required by the formula Ce₄O₇, is obtained.[214] This substance has strong reducing properties; when warmed in air, it glows, forming the dioxide, and reduces carbon dioxide when heated in a current of that gas. This intermediate oxide is said to correspond in composition to the violet hydroxide which is obtained as an intermediate product in the oxidation of cerous to ceric hydroxide, and which is said to yield the blue oxide, Ce₄O₇, when dried in vacuo.

The disulphide, CeS₂, has been obtained by Biltz[215] by prolonged heating of anhydrous cerous sulphate in a current of sulphuretted hydrogen at a dull red heat; it is a dark, yellowish-brown, crystalline solid, which on treatment with hydrochloric acid yields hydrogen persulphide.

Halogen salts.—No halogen compounds are known in the free state, except the fluoride, CeF₄,H₂O, which was obtained by Brauner as a yellowish-brown mass, by the action of hydrofluoric acid on the hydroxide. A double fluoride, 2CeF₄,3KF,2H₂O, was prepared by the same author by dissolving the hydroxide in potassium hydrogen fluoride; it is insoluble in water. By dissolving a ceric salt in concentrated hydrochloric acid, a dark red solution is obtained, which is believed to contain the unstable complex acid, H₂CeCl₆; this decomposes slowly in the cold, more quickly on warming, with evolution of chlorine, and formation of cerous chloride. Several double compounds of ceric chloride with hydrochlorides of organic bases have, however, been obtained.

Ceric sulphate, Ce(SO₄)₂, is obtained by the action of concentrated sulphuric acid on the dioxide. It is a deep yellow crystalline powder, dissolving readily in water to a brown solution, which has a strongly acid reaction; on warming or[163] diluting, a basic sulphate separates. The solution slowly evolves oxygen, and therefore always contains cerous compounds. On evaporation, a cero-ceric acid sulphate of the formula HCeⁱⁱⁱCe^iv(SO₄)₄,12(13 ?)H₂O first separates; the hydrated sulphate Ce(SO₄)₂,4H₂O, being more soluble, separates on further concentration.[216] The relative amounts of the two compounds obtained depends on the temperature and the concentration of acid in the solution; if both these factors are kept low, the almost pure hydrated sulphate can be at once obtained. This separates in yellow crystals belonging to the rhombic system; it is readily soluble in water. The mixed acid salt is less soluble, and forms orange prisms and needles, which cling tenaciously to sulphuric acid. Other complex and double salts have also been obtained. When, for example, silver nitrate is added to a warm solution of the sulphate in concentrated sulphuric acid, a bright orange-yellow precipitate of the salt 10Ce(SO₄)₂,6Ag₂SO₄ is obtained.[217]

Neutral ceric nitrate is unknown. A basic nitrate, Ce(NO₃)₃OH,3H₂O, is obtained in red crystals by evaporation of a solution of ceric hydroxide in strong nitric acid. The solid is readily soluble in water, forming a yellow, acid solution, which becomes paler by hydrolysis, on warming or on standing. The course of the hydrolysis is also indicated by the action towards acids, and towards hydrogen peroxide.[218] A freshly prepared ceric salt, on addition of acid, becomes immediately much darker in colour, whereas the colour change is very slow, if considerable hydrolysis has occurred. Similarly, hydrogen peroxide at once reduces a freshly prepared solution, forming colourless cerous salts, whilst if much hydrolysis has occurred, deeply coloured higher oxidation products are at first formed, and these lose their colour only slowly.

The double ceric nitrates[219] are a large and very important class of compounds; they are the most stable of the ceric salts. With nitrates of the monovalent metals, ceric nitrate[164] forms double nitrates of the type R₂Ce(NO₃)₆; these are deep red hygroscopic substances, crystallising in the monoclinic system, readily soluble in water and alcohol, but dissolving only sparingly in nitric acid. The ammonium salt is important for the separation of cerium. A series of double nitrates with the nitrates of manganese, magnesium, zinc, nickel, and cobalt has the general formula RCe(NO₃)₆,8H₂O, but these are much less stable in solution than the alkali double salts.

Atomic Weight of Cerium

No less than twenty-eight separate determinations of the atomic weight of cerium have been carried out. The earlier determinations are rendered unreliable by the almost certain presence of other elements, and Brauner[220] has shown that some of the methods employed in later work give erroneous results.

A very careful determination was made by Robinson in 1884.[221] Cerium oxalate was heated in a stream of dry hydrogen chloride, mixed with carbon dioxide, and the anhydrous chloride freed from traces of acid in a vacuum over chalk. The weighed chloride was then dissolved in water, and titrated with silver nitrate. He obtained the value 140·26; recalculation from his data with the modern values for silver and chlorine give 140·19. Brauner points out that this result is too low, since no account was taken of the solubility of silver chloride in water. In the following year, Brauner[222] determined the ratio Ce₂(SO₄)₃ : 2CeO₂, and obtained the atomic weight 140·22. Wyrouboff and Verneuil[223] in 1897 disputed Brauner’s work, and as a result of several determinations gave the values 139·21, 139·43, and 139·50; their determinations, however, varied very considerably, and the work has been severely criticised by Brauner. In 1903, the latter author and Batěk[224] obtained the values 140·21 and 140·27 by the sulphate and oxalate methods respectively; whilst in the[165] same year, using the same methods, Brauner[225] obtained from three independent series of determinations the values 140·25, 140·24, and 140·25.

The International Atomic Weight Committee have accepted the value 140·25 since 1904.

Detection and Estimation of Cerium

The detection of cerium in a mixture of earths is a comparatively simple matter, as it has several distinctive reactions. The brown colour of the peroxy-compounds has been suggested as a convenient test by several authors. This may be observed when ammonia is added to a cerous salt in presence of hydrogen peroxide. In the presence of a large excess of foreign earths, very dilute ammonia should be added, drop by drop, with continuous shaking, until a small permanent precipitate remains; this will be rich in the weakly basic ceric hydroxide, and on addition of the peroxide solution will show the colour clearly.[226] For very small quantities of cerium, the neutral solution is added to warm concentrated potassium carbonate solution, and one or two drops of dilute hydrogen peroxide added to the clear liquid; the yellow colour is then very characteristic.[227]

Biltz and Zimmerman[228] employ the reducing powers of cerous hydroxide; ammoniacal silver nitrate is added to the neutral solution of the cerous salt, and the mixture warmed. Dilute solutions (1-2 mgms. per litre) give a brown colour, concentrated solutions a black precipitate. The oxidation of an ammoniacal solution of the tartrate by air or hydrogen peroxide, by which an intense yellowish brown colour is developed, has been recently suggested by Wirth[229] as a very delicate test for the element.

Spectrum analysis.—Cerous salts show no absorption, ceric salts general absorption of the violet end of the spectrum. Arc spectrum—see Exner and Haschek,[230] Eder and Valenta,[231][166] and Cooper.[232] The emission spectrum of cerium is especially rich in lines; for identification, the following may be used:

The estimation of cerium cannot be carried out accurately by gravimetric methods in the presence of other earths; volumetric methods, however, will give reasonably accurate results, if the necessary precautions are taken. In Bunsen’s method the ignited oxides are treated with hydrochloric acid in presence of potassium iodide, the iodine set free from the hydriodic acid by reduction of the cerium dioxide being estimated by means of sodium thiosulphate, in the usual way. This method gives very inaccurate results, since in the presence of cerium dioxide, other oxides of the group can be converted into higher oxides which will also liberate iodine under these conditions.

The most reliable method is that of v. Knorre.[233] The solution to be estimated is acidified with sulphuric acid, and oxidised by means of ammonium persulphate. The excess of the oxidising agent having been destroyed by boiling, the cooled solution is treated with a slight excess of hydrogen peroxide, which reduces the ceric salt according to the equation:

2Ce(SO₄)₂ + H₂O₂ = Ce₂(SO₄)₃ + H₂SO₄ + O₂

The excess of hydrogen peroxide is then estimated by means of a dilute permanganate solution. Permanganate is itself reduced by the cerous salt formed, but the action is so slow in acid solution at the ordinary temperature that the excess of peroxide can be accurately determined without unduly hurrying the titration. In this form the method is generally employed for the estimation of cerium in monazite sands, and in the incandescent mantle industry. The greatest difficulty is the adjustment of the concentration of the sulphuric acid required. If this be too low, basic[167] ceric sulphate separates on boiling, and the estimation fails; if it be too high, oxidation to the ceric salt is hindered, and may even be inhibited. This difficulty disappears in the modified method of Waegner and Muller,[234] in which the oxidation to the ceric condition is effected by means of bismuth tetroxide in nitric acid solution. A similar method, in which reduction to the cerous state is effected by a ferrous salt, in place of hydrogen peroxide, has been employed by Metzger.[235]

Many attempts have been made to estimate cerium compounds by means of permanganate, which in alkaline solution oxidises cerous salts to the ceric condition, but the autoxidation of cerous hydroxide in the air introduces errors, unless suitable precautions are taken. Meyer and Schweitzer[236] show that if the solution of the cerous salt be added, with constant shaking, to a known volume of a standard permanganate solution, in presence of excess of magnesia, the liquid being kept warm, this difficulty is overcome; the results are usually a little high, however, probably by reason of the oxidising action of the cerium dioxide on the other oxides present.

Good results have also been obtained by the use of potassium ferricyanide in alkaline solution,[237] oxidation taking place according to the equation:

Ce₂O₃ + 2K₃Fe(CN)₆ + 2KOH = 2K₄Fe(CN)₆ + 2CeO₂ + H₂O

The ceric hydroxide is filtered off, and the ferrocyanide formed estimated by means of permanganate in acid solution.

[168]

CHAPTER XII
CERIUM GROUP (continued)

Lanthanum, Praseodymium, Neodymium, and Samarium

In his examination of the ceria earths in 1839, Mosander discovered a new constituent, which he called Lanthana; the new oxide was removed in solution when the ignited mixture was extracted with dilute nitric acid, which leaves cerium dioxide undissolved. On examination, the new oxide was found to be heterogeneous; by fractional precipitation with ammonia, and subsequent recrystallisation of the sulphates, he obtained two oxides, which he called respectively Lanthana (λανθανειν, to be hidden), from the absence of colour and specific reactions, and Didymia, (διδυμοι, twins) from their similarity and the occurrence of the two together.

Samaria was isolated by Lecoq de Boisbaudran, in 1879, from a specimen of didymia extracted from the mineral samarskite. Two years previously, Delafontaine had shown that the didymia separated from this mineral was not spectroscopically identical with the oxide obtained from other sources, and in 1878 had isolated an oxide which he called Decipia; this was shown later, however, to be a mixture of which samaria was one component. The samaria obtained by de Boisbaudran was by no means pure, being associated with terbia earths; several investigators claimed to have separated from it new oxides, most of these being proved afterwards to have been more or less impure specimens of Europia.

In 1885, Auer von Welsbach[238] employed for the first time the method which has now become of paramount importance for the separation of the cerium group, viz. the fractional crystallisation[169] of the double nitrates. By this method he succeeded in resolving Mosander’s didymia into two new oxides, for which he proposed the names Praseodidymia (πρασινος, leek-green), from the colour of the salts, and Neodidymia respectively; the shorter names praseodymia and neodymia are, however, now generally adopted.

GROUP A
Mixed Double Nitrates.

2R(NO₃)₃,3Mg(NO₃)₂,24H₂O.

1 La, Pr Compounds. Fractionate as R(NO₃)₃,​2NH₄NO₃,​4H₂O.
2 Pr, Nd Compounds. Fractionate as 2R(NO₃)₃,​3Mn(NO₃)₂,​24H₂O.
3 Crude Nd Compounds. Continue the Separation.
4 Mother-liquors. Sa, Eu, Gd, etc. Crystallise with Bismuth magnesium nitrate.

Terbium elements.

5 Pure La Compound. Refine by Sulphate crystallisation.
6 Pr with La. Continue.
7 Impure Pr Compound. Continue.
8 Pure Nd Compound. Refine by Sulphate crystallisation.
9 Pure Sa Compound.

Mixture of Pr, La.
Pr. Refine by Sulphate crystallisation.

Fig. 8.—Separation of the Cerium Elements

Separation

The modern methods for the separation of these elements are based almost entirely on the differences in solubility of the various double nitrates.[239] The mixed double sulphates separated[170] by saturation of a solution of the chlorides with sodium sulphate, which contain the cerium and most of the terbium elements, are transformed into nitrates, and the neutral solution boiled with potassium bromate, in presence of powdered marble, till all the cerium is precipitated as basic ceric nitrate. From the filtered solution the other elements are thrown down as oxalates, transformed into the magnesium double nitrates (A in Fig. 8), and fractionated from nitric acid solution[240] until a rough separation has been effected (fractions 1, 2, 3, and 4). The separation, which is somewhat long and tedious, is followed by means of the absorption spectra, and by the colour changes of the fractions. Fraction 1, containing lanthanum and some praseodymium, should be faint green to colourless; fraction 2 is colourless by the complementary action of the coloured salts of neodymium and praseodymium; fraction 3, which should contain the crude neodymium salt, is amethyst; and fraction 4, the mother-liquor, is yellow from the presence of the samarium compound.

Fraction 1 is now converted to the double ammonium nitrates, which allow of a readier separation at this stage; two fractions are obtained, of which the less soluble, fraction 5, is the fairly pure lanthanum compound, whilst the more soluble, fraction 6, contains the praseodymium with a little lanthanum. The lanthanum ammonium nitrate, fraction 5, is converted into the anhydrous sulphate, which is dissolved in ice-water; when the solution is gradually warmed, the enneahydrate, La₂(SO₄)₃,9H₂O, separates, and may be obtained perfectly pure by recrystallisation. It is of interest that the radioactive element actinium is chemically very similar to lanthanum, and follows it closely through the process of separation.

The mixed praseodymium and neodymium magnesium nitrates which constitute fraction 2 are transformed into the double manganese nitrates, and the crystallisation from nitric acid continued.[241] The less soluble part, fraction 7, is fairly free from neodymium, and the separation is continued with that of[171] fraction 6, until both lanthanum and neodymium have been completely removed. The more soluble part, fraction 8, yields the pure neodymium compound, as does also the crude neodymium magnesium nitrate which constitutes fraction 3, if the crystallisation be continued.

The mother-liquors, fraction 4, are treated with bismuth magnesium nitrate,[242] which is intermediate in solubility between the analogous compounds of samarium and europium, and the crystallisation continued. The less soluble fraction contains the samarium compound, in which bismuth is the only impurity; this is easily removed by treatment with sulphuretted hydrogen. The remaining fractions are used as a source of the terbium elements (see p. 186).

The double carbonate method[243] is very suitable for the preparation of pure lanthanum compounds after the removal of cerium. The mixture of salts is added to a warm 50% solution of potassium carbonate, and to the clear liquid, water is added gradually, with constant stirring. The double carbonates of the most positive elements are the least soluble, and are first thrown down, so that the precipitate is rich in lanthanum; it is collected and washed with a 25% potassium carbonate solution, and the process repeated. A few repetitions suffice to separate lanthanum completely from the other members of the group. The method may also be used for the purification of praseodymium salts.

Lanthanum, La = 139·0

As the most electropositive element of the rare earth group, lanthanum is the most similar in its chemical properties to the metals of the alkaline earths. The metal itself (see p. 115) oxidises even in dry air, and in moist air rapidly becomes coated with a white layer of hydroxide; it attacks water, and burns vigorously when heated in the air. An alloy with aluminium, of the formula LaAl₄, has been prepared by Muthmann and Beck[244]; it forms lustrous white crystals, very stable in the air and very resistant towards acids.

[172]

The hydroxide is of interest from the fact that, if precipitated under suitable conditions, it has the power of taking up solid iodine to form a deep blue adsorption compound[245]; colloidal solutions of basic lanthanum acetate are also coloured blue by addition of a few drops of iodine solution. If precipitation with alkali be carried out in presence of hydrogen peroxide, an hydrated peroxide of the composition La₂O₅,nH₂O is obtained.[246] This compound partially decomposes with evolution of oxygen at ordinary temperatures; towards carbon dioxide and acids it acts as a true peroxide, with formation of hydrogen peroxide.

The oxide is colourless, and forms colourless salts with those acids in which the anion is not coloured. The oxide is distinguished from the other rare earth oxides in that it turns moistened litmus paper blue; it resembles lime, in hissing when slaked, absorbing carbon dioxide from the air, and liberating ammonia from ammonium salts. By fusion with alkali carbonates, and by digestion with concentrated alkali hydroxides, Baskerville and Catlett[247] claim to have obtained lanthanates and metalanthanates, but their work has not yet been confirmed.

The sulphate, La₂(SO₄)₃,9H₂O, is the least soluble of all the rare earth sulphates. The enneahydrate is the only form stable at ordinary temperatures,[248] though under special conditions, hydrates with 6 and with 16 molecules of water of crystallisation have been obtained. It separates in needles belonging to the hexagonal system; 100 parts of water dissolve at 0°, 3·01, and at 100°, 0·69 parts of the salt. The acetylacetone compound melts at 185°.

A large number of other lanthanum compounds have been prepared, but these are so typical of the rare earth salts generally that no detailed treatment is required; for a full account of them, the reader is referred to Abegg’s classical handbook.

[173]

Atomic Weight.

—A large number of determinations of this constant have been made, but the results even of recent investigations do not agree so closely as might be desired. The value adopted by the International Committee, 139·0, is based on the work of Brauner and Pavliček,[249] carried out in 1902. These authors give an account of all the determinations made up to that date, with critical discussion of the methods employed and the possible sources of error. The more important investigations have been based on the ratio La₂O₃ : La₂(SO₄)₃, for the determination of which the most stringent precautions must be taken. The synthetic method has generally been employed, on account of the tenacity with which the oxide clings to traces of sulphuric anhydride. In this method, the total decomposition of the acid sulphate, and the protection of the very hygroscopic sulphate, La₂(SO₄)₃, from atmospheric moisture, constitute the chief difficulties. By this method, H. C. Jones[250] in 1902 obtained a result (138·76) considerably lower than the value found by Brauner and Pavliček (loc. cit.) A later research by Brill,[251] who carried out a synthetic sulphate determination on a minute scale, using a Nernst microbalance, gave the value 139·5, which, whilst considerably higher than either of the other figures, shows that Brauner and Pavliček’s number can hardly be too high.

Praseodymium, Pr = 140·6

This element occurs only in small quantities in the commoner rare earth minerals, and its separation in the pure state is in[174] consequence a matter of very great difficulty. The salts and their solutions have a characteristic green colour. The salts are derived from the sesquioxide, Pr₂O₃, but a dioxide, PrO₂, and an intermediate oxide of uncertain composition are known. The absorption spectrum has five absorption bands, one of which coincides with a band in the absorption spectrum of neodymium; this fact has been interpreted as an indication of the non-elementary nature of both metals.[253] Difference in the absorption spectra have been put forward by several workers as indicating the complex nature of praseodymium, but an exhaustive examination by Stahl[254] in 1909 showed that there is no reason to doubt that the metal is really an element.

The metal is prepared by electrolysis of the fused chloride; in order to attain the temperature required to fuse the element, a very thin cathode is employed; if too powerful a current be used, the dioxide is formed. The metal is purified by remelting it in crucibles of magnesia, under a layer of anhydrous barium chloride. It has a yellowish shade, and is more stable in the air than lanthanum and cerium. For physical properties, see p. 115. No alloys have been prepared.

The hydroxide is thrown down by alkalies as a gelatinous green precipitate; in the presence of hydrogen peroxide, an hydrated peroxide, which closely resembles the corresponding lanthanum compound, is thrown down.

The Oxides.—By ignition of salts of volatile acids, Auer von Welsbach[255] obtained an oxide to which he assigned the formula Pr₄O₇. More recent work[256] has shown that the composition of the oxide obtained depends upon the conditions under which the various salts are decomposed. By fusing the nitrate in presence of potassium nitrate at 400-450°C., Meyer obtained the dioxide, PrO₂; at higher temperatures this decomposes, giving the intermediate oxides. The formation of the dioxide is greatly influenced by the presence of other oxides,[257]—ceric[175] oxide, acting as an oxygen carrier, favouring whilst the other oxides hinder. The pure dioxide is a brownish-black powder, which resembles manganese dioxide, but is less stable. It liberates halogens from the halogen acids, and oxidises manganese salts to permanganates, but does not completely oxidise ferrous or stannous salts, losing instead a part of its oxygen in the gaseous form. The dioxide cannot be obtained in the wet way.

When heated in a stream of hydrogen, the dioxide yields the sesquioxide, Pr₂O₃, as a greenish-yellow powder,which readily absorbs oxygen from the air, becoming brown, with formation of the intermediate oxide.

The chloride, PrCl₃,7H₂O, forms large green prisms, very readily soluble in water; 100 parts of the solvent at 13° take up 334·2 parts of the hydrated salt, the solution having the specific gravity 1·687. The anhydrous chloride is a pale green deliquescent powder, which melts at a red heat to a clear green liquid; ebullioscopic measurements show that in alcoholic solution it has the simple molecular formula PrCl₃.

The Bromate, Pr(BrO₃)₃,9H₂O, has been obtained by James and Langelier[258] by dissolving the oxide in aqueous bromic acid, and also by double decomposition. It forms greenish hexagonal prisms, melting at 56·5°, and is readily soluble; 100 parts of water dissolve 190 parts of this salt at 25°. At 100° it loses five molecules of water, forming the tetrahydrate Pr(BrO₃)₃,4H₂O, which loses all its water at 130°. The anhydrous salt begins to decompose at 150°.

The sulphate crystallises with 8 molecules of water of crystallisation at ordinary temperatures, but hydrates with 15¹⁄₂, 12, and 5 molecules of water respectively have been described. The octohydrate is considerably more soluble than lanthanum sulphate enneahydrate. The anhydrous salt is a bright green powder.

Praseodymium acetylacetone melts at 146°.

Atomic Weight.

—The value 140·6, adopted by the International Committee, is based on the work of Jones, v. Scheele,[176] Auer von Welsbach, and Feit and Przibylla; the work of Brauner, however, points consistently to a higher atomic weight. Most of these investigators have used the sulphate method. The first determinations of von Welsbach for the newly discovered element[259] gave the value 140·8 (see p. 179); another series of determinations published in 1903[260] gave the mean value 140·57. Jones[261] obtained the sesquioxide for the synthetic sulphate operation by reduction of the peroxide in a current of hydrogen; according to Brauner, this method gives an oxide which is not perfectly pure, probably by absorption of water vapour and carbon dioxide from the air. Jones’ mean value was 140·466. v. Scheele[262] used the same method, as well as a combined oxalate-sulphate method; his figures vary considerably, the mean value being 140·55. Feit and Przibylla,[263] using their volumetric method, obtained the value 140·54.

Brauner’s earlier work,[264] carried out in 1898, gave the value 140·95. In 1901 this author[265] carried out an extensive research on the atomic weight of praseodymium, employing four different methods with spectroscopically pure material; the mean value of his very concordant results was 140·97, almost the value he obtained in 1901. A further investigation into the value of this constant appears desirable.

Neodymium, Nd = 144·3.

Neodymium is, after cerium, the commonest constituent of the cerium group in the more important rare earth minerals, and its separation is therefore by no means so difficult as that of praseodymium. The compounds of the element obtained by von Welsbach in 1885 were not pure, being admixed with samarium compounds which had not been completely separated. Neodymium salts were first prepared free from samarium by Demarçay[268] in 1898; they are of a violet-rose colour, and show in solution a well-marked and characteristic absorption spectrum, the bands being very numerous and sharply defined, and extending over the whole optical region. In chemical as well as in physical and crystallographic properties, they show an extremely close resemblance to the compounds of praseodymium.

On account of the high melting-point, the preparation of the metal presents the same difficulties as that of praseodymium. A current of 90-100 ampères is employed at a potential difference of 15-22 volts; this suffices to raise the thin carbon cathode to a bright white heat, and to fuse the liberated metal. For the properties of the element, see p. 115.

The sesquioxide, Nd₂O₃, when perfectly pure, has a light blue or lilac colour, with a faint reddish fluorescence; the shade varies somewhat according to the method of and temperature employed for the preparation. A bluish or violet-red fluorescence is highly characteristic of the salts, and is particularly noticeable if the powdered recrystallised oxalate be viewed in a good light. The greyish or brownish colour of the oxide observed by some authors is probably due to traces of impurity.[269] The existence of higher oxides of the formulæ Nd₂O₄ and Nd₂O[178]₅ respectively, which Brauner[270] put forward, has been disputed by other writers, though it is found[271] that in the presence of ceria and praseodymia, the sesquioxide can take up more oxygen. Waegner[272] claimed to have obtained the compound Nd₄O₇ by heating the oxalate in a stream of oxygen, though his material, as well as that of Brauner, contained praseodymia. More recently, Joye and Garnier[273] have shown that the spectrum attributed by Waegner to the hypothetical Nd₄O₇ was in reality that of an hydrated oxide, 2Nd₂O₃,2H₂O; these authors have also prepared a second hydrated oxide of the formula 2Nd₂O₃,3H₂O.

The chloride, NdCl₃,6H₂O, is obtained by crystallisation from aqueous solutions; it is also precipitated by addition of water to an alcoholic solution. It forms large deliquescent rose-coloured crystals; 100 parts of water at 13° dissolve 246·2 parts of the salt, the saturated solution having the density 1·741; at 100°, 511·6 parts are dissolved. The solution resembles those of the other chlorides of the group in that it readily dissolves the rare earth oxalates. When heated in a current of hydrogen chloride of 130°, the hexahydrate yields a monohydrate, NdCl₃,H₂O; at 160° the anhydrous chloride is obtained as a very deliquescent rose-coloured powder, which melts at a red heat to a clear red liquid. The anhydrous chloride forms an additive compound NdCl₃,12NH₃, when exposed to the action of ammonia at low temperatures;[274] by gradually heating this, a large number of other additive compounds are formed, containing smaller quantities of ammonia.

The anhydrous iodide, NdI₃, has been obtained[275] by passing hydrogen iodide over the heated anhydrous chloride, and also by heating the carbide in iodine vapour. It fuses to a black liquid, which at a higher temperature suddenly becomes transparent.

[179]

The bromate, Nd(BrO₃)₃,9H₂O, which is exactly similar to the analogous compound of praseodymium, forms rose-coloured hexagonal prisms, melting at 66·7°.

The sulphate, Nd₂(SO₄)₃,8H₂O, is isomorphous with the corresponding salt of praseodymium, but is considerably less soluble. Only the one hydrate is known.

The nitrates show an interesting case of isomorphism with the corresponding bismuth nitrate hydrates.[276] The stable form of the neodymium salt is the hexahydrate, Nd(NO₃)₃,6H₂O, whilst the pentahydrate, Nd(NO₃)₃,5H₂O, is labile. Of the bismuth salts, on the other hand, the pentahydrate is stable whilst the hexahydrate is labile; but mixed crystals of both pairs may be obtained, the stable neodymium hexahydrate with the unstable bismuth compound, and the stable bismuth pentahydrate with the labile neodymium salt.

Many double carbonates are obtained by dissolving the normal carbonate in excess of the precipitant. The absorption spectra of these solutions, which have a blue colour, are abnormal and very intense, and have been suggested as a basis of quantitative estimation.[277]

The acetylacetone derivative forms violet crystals, melting at 144°-145°.

A large number of organic salts of neodymium have been prepared by James, Hoben, and Robinson.[278]

Atomic Weight.

—The earlier determinations of this constant were carried out by the sulphate method, the synthetic process being usually employed. Auer von Welsbach, at the time of the discovery of praseodymium and neodymium,[279] gave the values 143·6 and 140·8 respectively for their atomic weights. Brauner, who carried out a determination in 1898,[280] showed that these numbers should be interchanged, and gave the value 143·63 for neodymium. Boudouard,[281] employing the analytical sulphate method, obtained the value 143·05, whilst[180] in the same year Jones[282] gave the value 143·6. A second determination by Brauner[283] gave the value 143·89. All these values are undoubtedly too low, the material being probably contaminated with other earths.

In his second determination in 1908, Auer von Welsbach[284] gave the value 144·54 as the mean of three determinations. Feit and Przibylla,[285] using their volumetric method, gave the value 144·52, whilst Holmberg,[286] using material which he considered to have been the purest obtained up to that time, obtained the figure 144·11. More recently, Baxter and Chapin[287] have made determinations by treating the chloride with pure silver nitrate, and weighing the precipitated silver chloride, as well as by titration. The mean value obtained by the first method—ratio NdCl₃  : 3AgCl—was 144·272 (extremes 144·250 and 144·298), and by the second method—ratio NdCl₃  : 3Ag—was 144·268 (extremes 144·249 and 144·283), giving the mean value for the whole series of 144·270.

The value adopted by the International Committee is 144·3.

Samarium, Sa = 150·4

The samarium of the earlier chemists (see p. 168) contained a large proportion of the terbium elements, from which a fairly complete separation was first effected by Demarçay in 1900.[291] By the fractional crystallisation of the double magnesium nitrate in presence of bismuth magnesium nitrate, Urbain and Lacombe[292] succeeded in preparing samarium compounds, which were shown by spectroscopic examination[293] to be free from other earths. The element is intermediate in electropositive character and in the solubility relations of its salts between neodymium and the terbium earths; its salts are topaz-yellow in colour, and in concentrated solutions show absorption in the blue and violet regions. The oxide is almost white in colour, with only a faint yellow tinge. A systematic investigation of samarium compounds was carried out by Cleve,[294] but his work was vitiated by the fact that his material was very impure. More recently, the pure salts have been examined by Matignon and his pupils.

The melting-point of the metal lies between 1300° and 1400°C., so that its preparation by the electrolytic method is a matter of great difficulty. A mixture of the chloride with one-third of its weight of barium chloride is electrolysed by means of a current of 100 ampères, using a cathode of[182] only 2·5 mm. thickness; the metal so obtained is greyish white in colour, and is the hardest of the cerium elements.

The chloride separates from aqueous solution as the hexahydrate, SaCl₃,H₂O, in large tabular yellow crystals. The anhydrous chloride is white, but fuses to a chocolate-brown liquid; it forms a large number of additive compounds with ammonia. When heated in an atmosphere of dry hydrogen or ammonia, air and moisture being carefully excluded, it yields the subchloride,[295] SaCl₂, as a dark brown crystalline solid, insoluble in alcohol and all organic solvents. Samarous chloride dissolves in water, forming a deep brownish-red solution, which rapidly becomes colourless, with evolution of hydrogen, and precipitation of the oxide and oxychloride. Samarous iodide, SaI₂, may be obtained by a similar process, and closely resembles the chloride.

The bromate, Sa(BrO₃)₃,9H₂O, melts at 75°, and closely resembles the corresponding compounds of the didymium metals. The sulphate crystallises with 8, and the nitrate with 6 molecules of water. The carbonate, Sa₂(CO₃)₃,3H₂O, can be obtained only by passing carbon dioxide through an aqueous suspension of the hydroxide; addition of alkali carbonate to a solution of a samarium salt precipitates hydrated double carbonates.

The acetylacetone compound melts at 146°-147°C.

Many organic salts have been prepared by James, Hoben, and Robinson (loc. cit.).

Atomic Weight.

—The earlier determinations of this constant were carried out with material not entirely free from europium. Demarçay[296] carried out a synthetic sulphate operation with the material which he obtained free from europium in 1900, and found values between the limits 147·2 and 148·0. The International Committee has adopted the value 150·4, which is based on the work of Urbain and Lacombe[297] in 1904. These authors made determinations of three series of ratios, obtained by (a) conversion of sulphate octohydrate to anhydrous sulphate, (b) conversion of anhydrous sulphate to oxide, and (c)[183] conversion of sulphate octohydrate to oxide; these gave the values 150·314, 150·533, and 150·484 respectively, from which the mean atomic weight is 150·44.[298]

[184]

CHAPTER XIII
THE TERBIUM GROUP

In his examination of the yttria earths in 1842, Mosander described two new oxides isolated from the old yttria. To one of these, an orange-yellow earth which yielded colourless salts, he gave the name Erbia; the second earth, which was colourless and gave rose-coloured salts, he called Terbia. Bahr and Bunsen examined the yttria oxides in 1866, and obtained only the latter earth, which gave rose-coloured salts; to this they applied Mosander’s name Erbia, and stated that the earth to which Mosander had given that name had no existence. Delafontaine, however, confirmed Mosander’s work, showing that the orange-yellow earth which yielded colourless salts (Mosander’s Erbia) had been fractionated out of their material by Bahr and Bunsen in the double sulphate separation of the cerium group; to avoid further confusion, however, he proposed to give to this oxide (Mosander’s Erbia) the name Terbia, leaving for the colourless oxide, which forms rose-coloured salts (which Mosander had called Terbia) the name Erbia applied to it by Bahr and Bunsen. This reversed nomenclature has been generally accepted.

Delafontaine,[300] continuing his work on the earths from samarskite (see p. 168) announced in 1878 the discovery of a new oxide, Philippia, intermediate between terbia and yttria; but this was subsequently shown to be a mixture of yttria and terbia (see p. 133). In the same year, Lawrence Smith[301] announced the discovery of another oxide, Mosandria, from the samarskite earths; this was afterwards shown by Lecoq de Boisbaudran to be a mixture of terbia with gadolinia.[302] In 1880[185] Marignac[303] announced the discovery of two more new oxides, Y_α and Y_β from the same mineral; Y_β was afterwards found to be identical with samaria, whilst Y_α was subsequently separated from the old terbia earths by Lecoq de Boisbaudran, who proposed, with the assent of Marignac, the name Gadolinium.[304] The terbia left after removal of the erbia earths and gadolinia was believed by that author to be still a mixture, a conclusion supported by the work of Hofmann and Kruss in 1893.[305]

In 1886 Demarçay[306] isolated from samaria a new oxide, which he designated S₁. From his work on this oxide in 1892-1893, de Boisbaudran[307] concluded that samaria consisted of at least three oxides, samaria proper, and two new oxides Z_ξ and Z_ε. In 1896, Demarçay[308] separated an earth Σ, which showed the spark-spectrum of Z_ε and the reversal spectrum of Z_ξ, and finally in 1901[309] he obtained the new oxide in a fairly pure condition, and gave it the name Europia.

The complicated history of the terbium group has been entirely cleared up by the work of Urbain and his co-workers during the early years of the present century, and processes have been devised by which the separation of the three members of the group from one another, and from the related elements of the erbium group on the one side, and samarium on the other, can be satisfactorily accomplished. The chemistry of this group, therefore, may be regarded as satisfactorily settled, though relatively little is known of the properties of the elements and their compounds.

In their general chemical relations, elements of the terbium group occupy an intermediate position between the cerium group and the elements of the yttrium group in the narrower sense. In the solubility relations of the double salts, they are bounded on the one side by samarium and the less soluble cerium group, on the other by dysprosium and holmium and the more soluble yttrium group. They show only very slight[186] differences in electropositive character, and methods based on differences in basic strength of the oxides, therefore, are of very little use for separating them from one another. Fractional precipitation with ammonia separates them in the order terbium, samarium, gadolinium, and europium—samaria being less strongly basic than the oxides of gadolinium and europium; this constitutes an exception to the general rule regarding the solubilities of the double nitrates and sulphates with increasing electropositive character.[310] The difficulties of separation are greatly increased by the very small proportions in which the elements are usually found in rare earth minerals. Gadolinium usually occurs in the largest quantities; in consequence of this, there is little doubt that most of the material described by the earlier workers as terbia consisted very largely of gadolinia.

The group is not characterised by well-marked absorption spectra; europium and terbium show weak absorption in the blue region. Terbium, of which the salts are colourless, forms a very strongly coloured peroxide, analogous to that of praseodymium; small quantities of this give to the mixed oxides obtained by ignition the characteristic yellow colour, whilst mixtures richer in the peroxide become correspondingly darker and darker.

Separation

In the double sulphate separation of the yttrium and cerium groups, the terbium elements divide themselves between the soluble and the insoluble portions; if the separation is made as complete as possible by addition of a large excess of alkali sulphate under suitable conditions, the larger part of the compounds of the group will be precipitated with the cerium elements. In the separation of the cerium elements the terbium elements collect in the most soluble fractions, and the mother-liquors of the double nitrate crystallisations therefore form a very convenient source of these elements. A considerable proportion, however, will usually remain in solution with the double sulphates of the yttrium group; in[187] the bromate separation of these (see p. 198), the terbium elements collect in the least soluble fractions. By careful fractionation under suitable conditions, the double sulphate method may be used to separate the terbium group completely from the cerium and yttrium elements. A very convenient method of separating the terbium group from a rare earth mixture is the ethylsulphate process of Urbain. By fractional crystallisation of these salts from alcohol or water, the separation into three groups can be satisfactorily accomplished.

For the separation of the terbium elements from one another, the nitrate and double nitrate methods are most suitable. Samarium can readily be separated by crystallisation of the double magnesium nitrates in presence of bismuth magnesium nitrate; by continuing the fractionation, europium magnesium nitrate can be separated in a pure state, as there is a considerable difference between the solubility of this salt and the corresponding compound of gadolinium;[311] the process, however, is somewhat long and tedious. For the separation of gadolinium and terbium, the double nitrates are converted into the simple nitrates, and these fractionated from nitric acid in presence of bismuth nitrate. The gadolinium nitrate separates before the bismuth nitrate, and may be obtained fairly pure in this way, though the process is extremely tedious, and several thousand recrystallisations are required.[312] Terbium nitrate has almost the same solubility as bismuth nitrate, and the two separate together in the middle fractions. The more soluble nitrates of the erbia earths collect in the mother-liquors.

Europium, Eu = 152·0

This element is one of the rarest of the whole group, and occurs only in extremely small quantities. Monazite sand is said to contain about 0·002 per cent. of the oxide, though on account of the remarkable intensity of some of the stronger lines in the arc spectrum, Eberhard[313] was able to detect europium[188] with ease in a mixture of rare earth oxides from that mineral, after the separation of cerium. The oxide has a pale rose colour; the salts are also faintly coloured, and in solution show weak absorption bands.

Europium sulphate, Eu₂(SO₄)₃,8H₂O, separates in pink crystals, which are completely dehydrated at 375°; europic chloride, EuCl₃, in the anhydrous state forms fine yellow needles; europium oxychloride, EuOCl, prepared by heating europic chloride in dry air to 600°, is a white solid, insoluble in water, but soluble in strong acids; europous chloride, EuCl₂, prepared by reduction of the higher chloride in hydrogen, is a white amorphous solid, soluble in water to a neutral solution, which on boiling throws down the oxide, Eu₂O₃.[314] Several organic salts have been prepared by James and Robinson.[315]

Atomic Weight.

—Using the material isolated from samaria, Demarçay[316] in 1900, by the synthetic sulphate method, found the atomic weight of europium to be about 151. Urbain and Lacombe[317] determined the value in 1904, with material free from gadolinium and samarium, using the three ratios which they employed in the case of the latter element (see p. 182); their values, corrected by Brauner, were 152·00, 151·93 and 151·94 respectively. Another series of determinations was carried out by Jantsch[318] in 1908, the same method being employed; he obtained the mean value 152·03, with an error of ±·02. The International Committee have adopted the value 152·0.

Gadolinium, Gd = 157·3.

Gadolinia is the commonest of the terbia oxides, and occurs in considerable quantities in some of the rare earth minerals, notably in samarskite and gadolinite; its separation from the neighbouring oxides, europia and terbia, is, however, exceedingly difficult, and has only been satisfactorily accomplished in recent times. The gadolinium compounds prepared and examined by the earlier workers, as appears from the atomic weight determinations, must have been associated with earths of lower atomic weight, and undoubtedly also with small quantities of terbium. After the isolation of Marignac’s Y_α, and the examination of the element by Lecoq de Boisbaudran, to whom the name gadolinium is due, further investigations were carried out by Bettendorff[323] and by Benedicts.[324] Pure gadolinia was probably first obtained by Demarçay,[325] by fractional crystallisation of the magnesium double nitrate; the oxide obtained by Urbain and Lacombe[326] by crystallisation of the nitrates in presence of bismuth nitrate, was proved to be spectroscopically pure by Eberhard.[327]

[190]

The gadolinia obtained by ignition of the salts of volatile acids should be perfectly white; presence of terbia causes it to assume a yellow colour.[328] The salts are colourless, and their solutions show no absorption in the visible region, though Urbain[329] has shown that there are four strong bands in the ultraviolet.

The hydroxide, Gd(OH)₃, is a gelatinous precipitate with strongly basic properties, rapidly absorbing carbon dioxide from the air. The oxide, Gd₂O₃, also absorbs carbonic anhydride from the air, and is easily soluble in acids, even after strong ignition. The element is therefore strongly electropositive. Its position among the yttrium elements, however, is justified by the properties of the platinocyanide, 2Gd(CN)₃,3Pt(CN)₂,18H₂O, which forms long, pointed red crystals, with a green metallic lustre, belonging to the rhombic system, and isomorphous with the corresponding yttrium and erbium salts; the cerium elements, on the other hand, give yellow platinocyanides, with a blue metallic lustre, which crystallise in the monoclinic system.

The nitrate, Gd(NO₃)₃,6H₂O, separates from aqueous solutions at the ordinary temperatures in large crystals belonging to the anorthic system, and is isomorphous with the corresponding compounds of praseodymium and neodymium.[330] From solutions in strong nitric acid, a pentahydrate is obtained, which melts at 92°; the hexahydrate melts at 91°. The sulphate separates from aqueous solution as the octohydrate, Gd₂(SO₄)₃,8H₂O, isomorphous with the corresponding salts of both groups. The anhydrous sulphate is much less soluble in water at 0° than the corresponding compounds of the cerium elements. The selenate forms hydrates with 10 and 8 molecules of water of crystallisation respectively; these are isomorphous with the corresponding selenates of yttrium and the erbium metals.

Atomic Weight.

—The determinations of this constant made by the earlier workers were all carried out with impure[191] material and gave results which were considerably too low. The International Committee have adopted the value 157·3, which is based on the work of Urbain.[331] In employing the analytical sulphate method, that author observed that the anhydrous sulphate did not remain constant in weight when allowed to remain in a desiccator, and that it could not be accurately weighed. He therefore determined the ratio Gd₂(SO₄)₃,8H₂O : Gd₂O₃, by converting the octohydrate directly to oxide, and obtained the mean value 157·24.

Terbium, Tb = 159·2

Terbia occurs among the rare earth oxides in exceedingly small quantities, and its separation has in consequence presented such great difficulties that only within the last few years have terbium compounds been completely freed from gadolinium and neighbouring elements. In 1886 Lecoq de Boisbaudran,[338] by fractional precipitation of the hydroxides with ammonia, and subsequent fractional crystallisation of the double sulphates, obtained an oxide much richer in terbia[192] than any specimen previously prepared; it was dark yellow in colour. In 1902 Marc[339] obtained from monazite a very dark oxide containing about 15 per cent. of terbia, whilst Feit[340] in 1905 obtained a dark brown oxide consisting of gadolinia with about 13 per cent. of terbia. Pure terbium compounds were obtained by Urbain in 1904,[341] by fractional crystallisation of the nitrate from nitric acid, in presence of bismuth nitrate, and by crystallisation of the double nickel nitrates, and precipitation with ammonia; he showed that the element was identical with the Z_δ and Z_β of de Boisbaudran,[342] with the Γ of Demarçay,[343] and with the G_β and possibly the G_ζ of Crookes[344] (see p. 193).

The element gives the white sesquioxide, Tb₂O₃, and colourless salts.[345] The peroxide, of which the composition corresponds approximately to the formula Tb₄O₇, is obtained as a brownish-black powder by ignition of suitable salts. Its presence, even in small quantities, gives so deep a colouration to the other earths that some kind of salt formation seems probable. It is insoluble in cold acids; it dissolves in hot nitric acid with evolution of oxygen, forming a solution from which the nitrate, Tb(NO₃)₃,6H₂O, melting at 89·3°, separates on cooling. In hot hydrochloric acid, the peroxide dissolves with evolution of chlorine, forming solutions from which the chloride, TbCl₃,6H₂O, can be isolated with difficulty; this salt is extremely deliquescent, and easily forms supersaturated solutions. The sulphate, Tb₂(SO₄)₃,8H₂O, can be precipitated from a sulphuric acid solution of the oxide by addition of considerable quantities of alcohol; it is isomorphous with the other sulphate octohydrates, and is completely dehydrated at 360°.

Atomic Weight.

—The value adopted by the International Committee is 159·2, which was obtained by Urbain in 1905[193] (loc. cit.) from the ratio Tb₂(SO₄)₃,8H₂O : Tb₂(SO₄)₃. This is the only determination on which reliance can be placed, as the material of the earlier workers was seldom even approximately pure.

[194]

CHAPTER XIV
THE ERBIUM AND YTTERBIUM GROUPS—YTTRIUM AND SCANDIUM

In his examination of the ‘Yttria’ of Gadolin and Ekeberg, during the years 1839 to 1843, Mosander, by methods based on differences in strength of the oxides as bases, separated the earth into three new oxides, yttria proper, the most strongly basic, terbia, intermediate in strength, and erbia,[348] the least basic. No further separation was effected until 1878, when Marignac, by fractional decomposition of the nitrates, separated from erbia a new oxide, for which he proposed the name Ytterbia; the new oxide was the least basic of the erbia earths. In the following year, Nilson[349] isolated from ytterbia a still less basic oxide, by the same method; he proposed the name Scandia, to recall the fact that it occurred in gadolinite and euxenite, which up to that time had been found only in Scandinavia. In 1879 also, Soret[350] announced the discovery of a new element X, evidence for the existence of which he had obtained during a spectroscopic examination of a mixture of erbia and terbia earths; the oxide of X was isolated in the same year by Cleve[351] from the old erbia, by fractional decomposition of the nitrates, and the name Holmium, from the town of Stockholm, was proposed for the new element. The same investigation led to the discovery of Thulium, which derives its name from Thule, an old name for Scandinavia.

Lecoq de Boisbaudran[352] in 1886 showed Cleve’s Holmia to be a mixture of at least two oxides; he retained the name Holmium for the element which gave the most characteristic[195] absorption bands of the old holmium, and proposed the name Dysprosium (from δυσπροσιτος, difficult of access) for the second element. The name Erbia was retained for the oxide remaining after the removal of holmia, thulia, and dysprosia from the old erbia; the homogeneity of this erbia has been called in question, but is now fairly firmly established. The individuality of dysprosium[353] and holmium[354] may also be regarded as definitely established; that of thulium remains doubtful (see p. 204).

The homogeneity of ytterbia was questioned by Auer von Welsbach[355] in 1906; by fractionation of the ammonium double oxalates, that author isolated the oxides of two new elements, for which he proposed the names Aldebaranium and Cassiopeium. By fractionation of the nitrates from nitric acid solution, Urbain[356] arrived at the same result, and proposed the names Ytterbium (Neoytterbium) and Lutecium, which have been adopted by the International Committee. The latter author, employing the same method in the fractionation of the gadolinite earths, has recently obtained very strong evidence of the existence in this group of another element, for which he proposes the name Celtium;[357] the discovery, however, awaits confirmation.

Separation

In the separation of the yttrium elements, methods based on differences in electropositive character are of much greater importance than in the separation of the cerium and terbium groups, and the method of nitrate fusion has been very largely employed even in comparatively recent work. This method, which was introduced by Berlin in 1860, has been of great value in the separation of yttrium and the ytterbium elements from the erbium group; it was employed in the isolation of ytterbium by Marignac, and of scandium by Nilson.

[196]

If a concentrated solution of the nitrates be evaporated down, and the syrupy residue subjected to gradually increasing temperature, the nitrates of the ytterbium elements and scandium are converted first into the basic nitrates; at somewhat higher temperatures the erbium salts are decomposed, whilst yttrium nitrate and the nitrates of any cerium elements present are the last to break up. If the mixture of basic and neutral nitrates be dissolved in boiling water, the former, being less soluble, crystallise out on cooling, and may be separated by this means, the process being repeated with the filtrate containing the unchanged nitrates. In this way, the weakly basic scandia and ytterbia quickly collect in the first fractions, whilst the oxides of the erbia group are easily separated from the more strongly basic yttria. The presence of the intermediate terbium group renders the process much less easily workable.

The process may be modified by raising the temperature to such an extent that the soluble basic nitrates are converted into insoluble superbasic nitrates, the temperatures at which this change occurs increasing from element to element as the positive character becomes more marked; the mixture of basic and superbasic salts is then extracted with dilute nitric acid which leaves that latter undissolved and removes the more positive elements in solution.

Fractional precipitation of the hydroxides by means of ammonia, alkalies, or alkaline earths has also been frequently employed. A modification of this process is the precipitation with aniline, carried out by Kruss;[358] in this method, the solution of the chloride in warm dilute alcohol is treated with an alcoholic solution of the organic base. Another modification is the ‘Oxide process’ employed by Auer von Welsbach[359] for the separation of the cerium elements, and by Drossbach[360] in the yttrium group. The concentrated solution of the mixed salts is thoroughly digested with the oxides obtained by precipitating a fraction of the earths; the more strongly basic oxides tend to displace the less basic, so that these accumulate in the insoluble part. The solution is filtered from the undissolved oxides, another fraction precipitated, and the oxides obtained from the precipitate digested with the concentrated solution as before.

[197]

GROUP B
Yttrium Double Suplhates.

Fractionate as Bromates.

 1 Gd, Tb, Dy. For separation of Terbium group.
 2 Tb, Dy, Ho, Yt. Transform to Ethylsulphates.
 3 Dy, Ho, Er, Yt. Fractionate by Nitrate Fusion.
 4 Yt, Er, and Sc? Fractionate by Nitrate Fusion.
 5 Tm, Yb, Lu, etc. Continue.

 Terbium Group.

 6 Tb, Dy. Continue Ethylsulphate Fractionation.
 7 Dy. Ethylsulphate.
 8 Ho, Yt. Fractionate by Nitrate Fusion.
 9 Yt. Nitrate.
10 Yt, Er. Continue.
11 Er. Nitrate.
12 Tm. Bromate.
13 Yb. Bromate.
14 Lu. Bromate.

Yb, Lu. Bromates.
Ct? Bromate.

Ho. Basic Nitrate.
Ho, Yt.

Fig. 9.—Separation of the Yttrium Elements

[198]

The more modern methods of separation combine the above processes with the methods of fractional crystallisation, for which the bromates and alkylsulphates of these elements are well adapted. The procedure[361] which experience shows will lead to a fairly rapid separation is roughly represented in Fig 9. The double sulphates (B), left in solution after removal of the cerium and part of the terbium group, are transformed into the bromates, which are separated by fractional crystallisation into five main fractions. The least soluble portion, fraction 1, contains the terbium elements with some dysprosium; in the fractionation of the terbium group by means of the nitrates and double nitrates, the dysprosium, with some terbium, collects in the final fractions (fraction 6). Fraction 2 contains terbium, dysprosium, holmium, and yttrium as the bromates; these are converted into the anhydrous chlorides, from which, by treatment with sodium ethylsulphate in alcoholic solution, the ethylsulphates are obtained. By fractional crystallisation, dysprosium may be obtained in a fairly pure condition (fraction 7), the least soluble part (fraction 6) containing the terbium with some dysprosium. Holmium and yttrium collect in the most soluble part (fraction 8), from which pure holmium can be obtained by the method of nitrate fusion. Fraction 3 contains yttrium and erbium, with small quantities of dysprosium and holmium; the latter are readily separated by the nitrate fusion, which will also allow of a fairly complete separation of yttrium (fraction 9). Fraction 4 contains yttrium and erbium; scandium if present will also collect here. Erbium can be obtained pure by the nitrate fusion; the second fraction from this process contains both yttrium and erbium, and may be further worked up with the fraction of similar composition (fraction 10) from fraction 3.

The mother-liquors from the bromate separation (fraction 5)[199] contain thulium and the ytterbium elements; the crystallisation is continued, and allows of complete separation of thulium and ytterbium, and probably of lutecium, though the most soluble fractions do not seem to have been fully separated.

The Erbium Group

The oxides of this group, as contrasted with the ytterbia oxides, give rise to coloured salts, which in solution show definite absorption spectra in the optical region; the spectrum of erbium salts is particularly definite and characteristic. Erbium has among the yttrium elements the place of neodymium among the cerium elements; after yttria, erbia is the commonest oxide of the yttria group, though on account of the difficulties of separation the chemistry of erbium is by no means so complete and definite as that of neodymium. The oxides in order of decreasing basicity, as shown by the order in which they are thrown down by ammonia, are: dysprosia, holmia, erbia, thulia; the electropositive character becomes weaker, therefore—as generally in the rare earth series—as the atomic weight of the elements increases.

Dysprosium, Dy = 162·5

Compounds of this element were probably prepared in the pure state for the first time by Urbain[362] in 1906, by the fractional crystallisation of the ethylsulphate. He showed that after fourteen recrystallisations, the absorption spectrum of the salts and the mean atomic weight of the element remain unaltered, and that after removal of terbium by the very efficient ethylsulphate method, all remaining traces of yttrium could be rapidly removed by crystallisation of the nitrate. The salts have generally a more or less pronounced yellow colour.

The oxide, Dy₂O₃, is a white powder which does not alter in composition when strongly heated in reducing or oxidising atmospheres. It is remarkable in that it is the most strongly paramagnetic oxide known, having a coefficient of susceptibility[200] much greater than that of ferric oxide.[363] The chloride crystallises with 6, the sulphate with 8, and the nitrate with 5 molecules of water of crystallisation. The bromate, Dy(BrO₃)₃,9H₂O,[364] obtained by double decomposition, melts at 78°. The platinocyanide, Dy₂[Pt(CN)₄]₃,21H₂O, forms bright red cubic crystals, with greenish fluorescence.

Several other salts are described by Urbain, and by Jantsch and Ohl (loc. cit.).

Atomic Weight.

—Urbain and Demenitroux[365] determined this constant from the ratio Dy₂(SO₄)₃,8H₂O : Dy₂O₃. The mean value of six determinations carried out with material obtained by fractional crystallisation of the nitrate was 162·52; with material purified by the ethylsulphate crystallisation, the mean of six determinations gave the value 162·54. The International Atomic Weight is 162·5.