Davisite, CaScAlSiO6, a new pyroxene from the Allende meteorite

Davisite, ideally CaScAlSiO6, is a new member of the Ca clinopyroxene group, where Sc3+ is dominant in the MI 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, Se2O3 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 Ca-0.99(Sc0.50Ti0.163+Mg0.16Ti0.104+Zr 0.04V0.023+FeS0.012-Yi0.01) (Sigma 1.00)(Si1.03Al0.97)(Sigma 2.00)O-6. Davisite is monoclinic, C2/c; a = 9.884 angstrom, b = 8.988 angstrom, c = 5.446 angstrom, beta = 105.86 degrees, V = 465.39 angstrom(3), 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 angstrom (I) (hkl)]: 3.039 (100) ((2) over bar 21), 2.989 (31) (310). 2.943 (18) ((3) over bar 11). 2.619 (40) (002), 2.600 (26) ((1) over bar 31) 2.564 (47) (221), 2.159 (18) ((3) over bar 31), 2.137 (15) ((4) over bar 21). 1.676 (20) ((2) over bar 23), and 1.444 (18) (531). The name is for Andrew M. Davis, a cosmochemist at the University of Chicago, Illinois.