Grossmanite, CaTi3+AlSiO6, a new pyroxene from the Allende meteorite

Grossmanite, Ca(Ti³⁺,Mg,Ti⁴⁺)AlSiO₆ with an end-member formula CaTi³⁺AlSiO₆, is a new member of the Ca clinopyroxene group, where the trivalent cations are dominant in the M1 site with Ti³⁺ being the dominant trivalent cation. It occurs as micrometer-sized crystals along with spinel and perovskite in a melilite host in Ca-, Al-rich refractory inclusions from the Allende meteorite. The mean chemical composition determined by electron microprobe analysis of the type material is (wt%) SiO₂ 27.99, Al₂O₃ 24.71, CaO 24.58, Ti₂O₃ 10.91, TiO₂ 6.68, MgO 4.45, Sc₂O₃ 0.43, V₂O₃ 0.19, ZrO₂ 0.13, FeO 0.08, Cr₂O₃ 0.03, sum 100.20. Its empirical formula calculated on the basis of 6 O atoms is Ca_1.00[(Ti_0.35³⁺Al_0.18Sc_0.01V_0.01³⁺)_∑0.55Mg_0.25Ti_0.19⁴⁺]_∑1.00(Si_1.07Al_0.93)_∑2.00O₆. Grossmanite is monoclinic, C2/c; a = 9.80 Å, b = 8.85 Å, c = 5.36 Å, β = 105.62°, V = 447.70 ų, and Z = 4. Its electron back-scatter diffraction pattern is an excellent match to that of Ti³⁺-rich pyroxene with the C2/c structure. The five strongest calculated X-ray powder diffraction lines are [d spacing in Å, (I), hkl] 2.996 (100) (2̅21), 2.964 (31) (310), 2.581 (42) (002), 2.600 (28) (1̅31), 2.535 (47) (221). The name is for Lawrence Grossman, a cosmochemist at the University of Chicago.