_Auerlite_, _Britholite_, _Erikite_ and _Florencite_, phospho-silicates. _Cappelenite_, _Homilite_ and _Tritomite_, boro-silicates. CHAPTER III THE TITANO-SILICATES AND TITANATES (_a_) THE TITANO-SILICATES ~Yttrotitanite or Keilhauite.~--A titano-silicate of calcium, aluminium, iron and yttrium metals. The mineral is isomorphous with titanite, CaO,TiO₂,SiO₂ (_q.v._), and is itself probably an isomorphous mixture of titanite with the silicate (Y,Al,Fe)₂SiO₅, where Y = yttrium metals. Its composition will then be represented by the formula _m_ (Y,Al,Fe)₂(SiO₅) + _n_ CaTi(SiO₅). It is monoclinic, with axial ratios and angles very close to those of titanite. Usual forms--pinakoids _a_ {100} and _c_ {001}, hemi-prism _m_ {110}, hemi-pyramids _n_ {111}, _e_ {1̅11} and _l_ {1̅12}. Cleavage ∥ _n_ distinct. Birefringence weak, +ve. Colour brown to brownish-black. Hardness 6¹⁄₂; sp. gr. 3·52 to 3·77. The mineral is fusible before the blowpipe, and is decomposed by hydrochloric acid. It was named by Scheerer in 1844 from its composition, and by Ekeberg in the same year in honour of the Norwegian geologist Keilhau. ~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.