Silicotitanate

Silicotitanate 

(also titanosilicate), any of the titanium-bearing minerals of the silicate class in which Ti⁴⁺, together with Si⁴⁺, forms a single anionic radical that is statistically not replaced and that retains its octahedral coordination. Silicotitanates are distinguished by the complexity of their composition, the presence of large cations (Na ⁺, K ⁺, Cs ⁺, Ca²⁺, Sr²⁺, Ba²⁺), and the presence of additional O^2–, (OH)^–, F^–, and Cl^– anions in the structure.

More than 20 silicotitanates are known, the most widely distributed of which are astrophyllite, with the formula (K, Na)₃(Mn, Fe)₇[Ti₂(Si₄O₁₂)₂]O₂(OH)₅; lamprophyllite, SrNa₃Ti[Ti₂(Si₂O₇)₂]O₂F; enigmatite, Na₂Fe₅[Ti(Si₂O₆)₃]O₂; ramsayite, Na₂[Ti₂(Si₂O₆)]O₃; benitoite, Ba[Ti(Si₃O₉)]; narsarsukite, Na₂[Ti(Si₄O₁₀)]O; and murmanite, Na[Ti(SiO₄)₂](OH) H₂O. They occur as fine grains, laminae, and scales and form large segregations in pegmatites. Fibrous aggregates are characteristic of astrophyllite and lamprophyllite. The color usually varies from brown and yellow-brown to almost black (enigmatite); astrophyllite is shot with bronze, benitoite with azure and dark blue, and murmanite with violet. Silicotitanates have a vitreous luster, a hardness on Mohs’ scale of 3–7, and a density of 2,900–3,500 kg/m³. They are typical rock-forming minerals of al-kalic and nepheline syenites and of associated pegmatites and metasomatites. Astrophyllite also occurs as an accessory mineral in alkalic granites and surrounding fenites.

A. I. GINZBURG