Читать книгу The Rare Earths: Their Occurrence, Chemistry, and Technology - Stanley Isaac Levy - Страница 172

—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 (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.

Crystal system—monoclinic; a: b: c = 0·7547: 1: 0·8543. β = 60° 17´.

Common forms (Des Cloizeaux’s orientation)—the pinakoids a {100} and c {001}, with m {110}, s {021}, x {102}, n {111}, and many others.

(100) ∧ (110) = 38° 14¹⁄₂´; (001) ∧ (1̅01) = 65° 57´; (001) ∧ (011) = 36° 34´.

The habit is very varied, the commonest being the wedge form, elongated ∥ c. Twinning is fairly common, especially on the law—Twin plane ∥ a, which gives both contact and interpenetrant twins. Cleavage ∥ m, fairly distinct. Hardness 5 to 5¹⁄₂; sp. gr. 3·40 to 3·56. Lustre adamantine to resinous. The colour varies very much, doubtless with the content of iron and manganese; it is commonly yellow, green, or brown. Pleochroism is very distinct. The refraction and dispersion are very high, giving the facetted stone a ‘fire’ inferior only to that of diamond. Birefringence positive, strong; the axial angles vary very widely in different specimens.

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). 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.