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.

[15] The symbol (Ca,Fe) here indicates that the iron and calcium occur in variable proportions, the variation however occurring in such a way that the equivalent of the two taken together is always the same, _i.e._ the iron can replace the calcium, or _vice versa_, atom by atom. The recognition of this possibility of ‘Vicarious Replacement’ between similar elements first brought order into the confused field of mineral chemistry, and allowed a systematic classification of minerals according to chemical composition to be made. Iron and calcium, or, according to the more convenient nomenclature of the mineralogists, lime and ferrous oxide, are here vicarious constituents.

The symbol Ce here stands for elements of the cerium group, which are never found singly.

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

Crystals, orthorhombic, holosymmetric; _a_ : _b_ : _c_ = 0·9988 : 1 : 0·8127. Usual forms--the Pinakoids _a_, _b_, and _c_ {100}, {010} and {001}, prisms _m_ {110} and _q_ {130}, domes _u_ {101}, _t_ {301} and _n_ {011}, and some pyramids {hkl}.

Angles, _a_ ∧ _m_ = 44° 58´, _u_ ∧ _c_ = 39° 8´, _n_ ∧ _c_ = 39° 6´.

The crystals usually occur as short prisms. No cleavage. Optical constants unknown. In flakes the absorption spectrum of didymium can be observed.

The mineral is brittle; hardness 5 to 6 on Mohs’ scale; sp. gr. varies a little about 4·9. Fracture splintery; lustre dull, resinous. Colour brown to red and greyish-red, streak greyish-white. The mineral is almost opaque.

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.

[16] _Pogg. Ann._, 1859, ~107~, 631.

[17] _Monats._, 1884, ~5~, 512.

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.

[18] This mineral, which Scheele knew as Tungstein, is now called Scheelite.

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.

[19] _Vide_ Hintze, _Handbuch der Mineralogie_, Leipzig, 1897, ~ii.~, 1329.

[20] Hermann, _J. pr. Chem._ 1843, ~30~, 194, and 1861, ~82~, 406.

[21] The announcement of the discovery of Lanthanum by Mosander was made in 1839.

[22] Lanthanite (see list) is an hydrated carbonate, R₂O₃,3CO₂,9H₂O, where R = cerium metals, chiefly Lanthanum.

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.

[23] G. Tschernik, _J. Russ. Phys. Chem. Soc._ 1896, ~28~, 345; 1897, ~29~, 291. Abstracts in _Zeitsch. Kryst. Min._ 1899, ~31~, 513 and 514.

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.

The crystals are monoclinic; _a_ : _b_ : _c_ = 0·6273 : 1 : 1·3215; β = 89° 26¹⁄₂´.

Common forms are--Ortho-, clino-, and basal pinakoids, _a_ {100}, _b_ {010}, and _c_ {001}, hemi-prisms _m_ {110}, _v_ {120}, clino-prisms _w_ {012}, _q_ {011}, and many others; and various hemi-pyramids {hkl} and {h̅kl}.

Angles _a_ ∧ _m_ = 32° 6´, _c_ ∧ _q_ = 52° 53´, _c_ ∧ (101) = 64° 9´.

Crystals commonly prismatic, terminated by _c_. Faces rough and coarse; lustre vitreous to greasy, seen only on freshly-broken surfaces. Brittle. No cleavage. Fracture conchoidal to splintery. Hardness 6¹⁄₂-7; sp. gr. 4·0-4·5.

Colour black, greenish- and brownish-black; green and transparent in flakes. The crystalline variety has strong positive birefringence, with the plane of the optic axes parallel to (_b_), the plane of symmetry; the amorphous variety is of course isotropic. The brown variety shows very distinct pleochroism, _i.e._ the colour as seen by transmitted light varies with the direction in which the light traverses the crystal; the green kinds have much weaker pleochroism.

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]

[24] _Vide_ Geijer, _Crell’s Chemische Annalen_, 1788, ~1~, 229.

[25] See _U.S. Geol. Survey_ (_Minerals_), 1904, 1213.

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).

[26] _Schweigg. J._, 1816, ~16~, 405.

[27] Berzelius (a letter to Pelouze), _Pogg. Ann._, 1839, ~46~, 648.

[28] _Berz. Jahres._, ~23~, 145; ~24~, 105.

[29] _Pogg. Ann._, 1842, ~56~, 483.

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.

[30] _Ber._, 1903, ~36~, 3095.

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.

[31] _Schweigg. J._, 1816, ~16~, 405.

[32] _Pogg. Ann._, 1840, ~51~, 493.

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.

Then vol. in arrangement A : vol. in arr. B ∷ H : _h_

_i.e._ ∷ √3/2 : √²⁄₃

∷ 1 : 0·943.

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.

[33] _J. pr. Chem._ 1858, ~73~, 391.

[34] _Ber._ 1907, ~40~, 3118.

[35] _Zeitsch. physikal. Chem._ 1898, ~25~, 568.

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.

[36] _Pogg. Ann._ 1841, ~53~, 249.

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.

Crystals--Monoclinic, holosymmetric; _a_ : _b_ : _c_ = 1·5509 : 1 : 1·7691, β = 64° 59´.

Common forms--Ortho- and basal pinakoids _a_ {100} and _c_ {001}; _m_ {110} and other prisms, _e_ {101} and other hemi-ortho-prisms, _o_ {011}, _d_ {111} and other hemi-pyramids.

Angles, (100) ∧ (110) = 54° 34´; (001) ∧ (101) = 63° 24´; (001) ∧ (011) = 58° 3´.

Tabular, parallel to _a_, or long and slender by elongation parallel to axis _b_.

Birefringence weak, variable. Refraction strong. Colour brown to brownish-black; almost opaque. In flakes very strongly pleochroic, the colours for light parallel to the three vibration directions ~c~, ~b~ and ~a~ being brownish-yellow, reddish-brown, and greenish-brown respectively.

Brittle. Hardness 5¹⁄₂-6; sp. gr. 3·5-4·2.

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.

[37] _Vide_ Schilling, pp. 70-75 for analyses of this mineral.

[38] Fromme, _Tsch. Min. Mitt._ 1909, ~28~.

[39] Meyer, _Sitzungsber. königl. Akad. Wiss. Berlin_, 1911, 379.

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.

[40] _Centr. Min._ 1911, 4.

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.

[41] _Vide_ Schilling, pp. 75-76, where full references are given.

[42] See Kobell’s _Geschichte der Mineralogie_, 1864, p. 679.

~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,Th/2)--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.

[43] Brögger, _Zeitsch. Kryst. Min._ 1906, ~42~, 417.

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

Crystal system--Monoclinic, holosymmetric, _a_ : _b_ : _c_ = 2·0646 : 1 : 2·507. β = 109° 45´. Habit usually prismatic, with {100}, {010}, and several prisms {_hko_}, terminated by various pyramid forms.

Angles (100) ∧ (001) = 70° 32´; (100) ∧ (110) = 62° 22´; (010) ∧ (110) = 27° 14´; (110) ∧ (11̅0) = 125° 0´.

Twinned on (001), twin plane (001), forming knee-shaped twins. Hardness varies from 5¹⁄₂ in the least altered to 1 in the most altered specimens; sp. gr. 3·70 in least altered specimens, decreasing with hydration. Colour of fresh crystals, reddish-brown; on alteration they become brownish-black, yellow, or even white.

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.

[44] Benedicts, Abstract in _Zeitsch. Kryst. Min._ 1900, ~32~, 614.

Monoclinic; _a_ : _b_ : _c_ = 1·154 : 1 : 0·602. β = 80° 12´.

Common forms are the pinakoids {100} and {010}, hemi-prism {110}, hemi-pyramids {111} and {111̅}, and others, and the hemi-dome {021}.

Angles, (100) ∧ (010) = 91° 0´; (100) ∧ (110) = 48° 9´; (100) : (111) = 59° 4´.

Double refraction weak. No cleavage. Brittle. Hardness 6¹⁄₂. Colour, bright flesh-red; translucent, with greasy lustre; sp. gr. 4·227, increasing to 4·29 after ignition. A yellow variety has sp. gr. 4·11-4·16, and is transparent.

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.

[45] J. Schetelig, _Centr. Min._ 1911, 721.

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]

[46] _Phil. Trans._ 1908, A, ~209~, 15.

[47] According to Eberhard, some varieties of Wiikite have a much lower scandia content.

Thortveitite is orthorhombic; _a_ : _b_ : _c_ = 0·7456 : 1 : 1·4912; commonly combinations of pyramids _o_ {111} and _s_ {211} with prism _m_ {110}, in radial aggregates of crystals elongated parallel to the _c_ axis. Cleavage parallel to _m_, fair. Twin plane _m_ (110), twinning very common.

Refraction strong; birefringence strong, negative. Acute bisectrix perpendicular to (001), plane of the optic axes (010). Hardness, 6-7; sp. gr. 3·571. Extremely brittle; lustre brilliant, vitreous to adamantine. Colour, greyish-green, white to reddish-grey on alteration; in transmitted light yellowish-green, after ignition, reddish; the change being probably due to presence of oxides of iron.

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.

Crystals are tetragonal, holosymmetric; _c_ = 0·6402; _p_ ∧ _p_´ = 56° 40´.

Common forms are the prism _m_ {110} with the pyramids _p_ {111} and _z_ {311}.

Hardness 4¹⁄₂-5; sp. gr. 4·4 to 4·8 for thorite, 5·2 to 5·4 for orangite.

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.

[48] _Pogg. Ann._, 1829, ~16~, 385.

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 (_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--

Mol. Wt. Sp. Gr. Mol. Vol.

Cassiterite, SnO₂ 151 6·9 22 Rutile, TiO₂ 80 4·2 19 Zircon, ZrSiO₄ 182 4·7 39 Thorite, ThSiO₄ 325 5·4 (Orangite) 60 Xenotime, XPO₄ 184 4·5 41

[49] This isomorphous series has recently been extended by Zambonini, and also by Schaller, by the inclusion of minerals containing Columbium and Tantalum; see under Ilmenorutile and Strüverite, end of Ch. IV., p. 71.

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 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.

[50] Strutt, _Nature_, 1906, 102.

System tetragonal, holosymmetric sub-class. _c_ = 0·6404; (001) ∧ (101) = 32° 38´.

Usual forms--Prisms _a_ {100} and _m_ {110}; pyramids _e_ {101}, _p_ {111}, _u_ {221} and _x_ {311}, etc. The basal pinakoid _c_ {001} is rare. The usual combination is one or both of the prisms _a_, _m_, with one or two pyramids. Twinning is rare, the twin plane being _e_ (101), giving knee-shaped twins similar to those so characteristic of cassiterite and rutile. Cleavage ∥ _m_ imperfect, ∥ _p_ bad.

Brittle; conchoidal fracture. Hardness 7¹⁄₂; sp. gr. usually 4·68-4·70, but varying from 4·2 to 4·86. Adamantine lustre. Clear and colourless to yellow-, red- or greenish-brown. Transparent to opaque. Refraction and double refraction strong, double refraction positive (ω = 1·924, ε = 1·968, for sodium light); on heating it becomes biaxial, and occasionally is found biaxial in nature. By alteration it becomes isotropic.

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 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.

[51] _Schriften der Gesellschaft naturforschender Freunde in Berlin_, 1789, vol. 9.

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 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]

[52] _Beiträge zur Mineralogie von Japan_, 1906, ~2~, 23.

[53] An earlier analysis (_Abstr. Chem. Soc._ 1905, ~88~, [ii.], 177) gave over 20 per cent. of uranous oxide, UO₂; the greater part of this appears to have been zirconia, ZiO₂.

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.

The hardness is 7¹⁄₂, the sp. gr. 4·091. The colour is dark green or brown, becoming dull by weathering. The double refraction is extremely weak.

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.

(_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 crystals are rhombohedral, _a_ : _c_ = 1 : 2·1116.

Common forms are--the pinakoid _c_ {111}, prisms _a_ {101}, and _m_ {211}, and pyramids _r_ {100} and _e_ {110}. _c_ ∧ _r_ = 31° 22´. Habit tabular parallel to _c_, rhombohedral with _e_ prominent, or prismatic with _a_ prominent.

Cleavage ∥ _c_ very good, ∥ _a_ difficult.

The colour is brown or red to brownish- or bluish-red. Brittle. Hardness 5 to 5¹⁄₂; sp. gr. 2·92 for eudialyte, 3·0 to 3·1 for eucolyte.

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.

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.

[54] _Abstr. Chem. Soc._ 1905, ~88~, ii, 177.

The crystals appear to belong to the cubic system, occurring in cuboid grains, and in octahedra and dodecahedra. It is brown, and isotropic, with cubic cleavage. Sp. gr. = 4·15.

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.