(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 O2–, (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