Читать книгу The Rare Earths: Their Occurrence, Chemistry, and Technology - Stanley Isaac Levy - Страница 158

—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.