Davisite, ideally CaScAlSiO6, is a new member of the Ca clinopyroxene group, where Sc3+ is dominant trivalent cation in the M1 site. It occurs as micro-sized crystals along with perovskite and spinel in an ultra-refractory inclusion from the Allende meteorite. The mean chemical composition determined by electron microprobe analysis is (wt%) SiO2 26.24, CaO 23.55, Al2O3 21.05, Sc2O3 14.70, TiO2 (total) 8.66, MgO 2.82, ZrO2 2.00, Y2O3 0.56, V2O3 0.55, FeO 0.30, Dy2O3 0.27, Gd2O3 0.13, Er2O3 0.08, sum 100.91. Its empirical formula calculated on the basis of 6 O atoms is Ca0.99(Sc0.50Ti3+0.16Mg0.16Ti4+0.10Zr0.04V3+0.02Fe2+0.01Y0.01)Σ1.00(Si1.03Al0.97)Σ2.00O6. Davisite is monoclinic, C2/c; a = 9.884 Å, b = 8.988 Å, c = 5.446 Å, β =105.86°, V = 465.39 Å3, and Z = 4. Its electron back-scattered diffraction pattern is an excellent match to that of synthetic CaScAlSiO6 with the C2/c structure. The strongest calculated X-ray powder diffraction lines are [d spacing in Å (I) (hkl)]: 3.039 (100) (221), 2.989 (31) (310), 2.943 (18) (311), 2.619 (40) (002), 2.600 (26) (131), 2.564 (47) (221), 2.159 (18) (331), 2.137 (15) (421), 1.676 (20) (223), and 1.444 (18) (531). The name is for Andrew M. Davis, a cosmochemist at the University of Chicago, Illinois.
