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 (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.