The Rare Earths: Their Occurrence, Chemistry, and Technology

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 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 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.

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.

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.

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 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.

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 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.

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 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 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 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.

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:

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.