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—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.