Grossmanite, CaTi3+AlSiO6,
a new
pyroxene from the Allende meteorite
Chi Ma and George R. Rossman
Division of Geological and Planetary Sciences
California Institute of Technology, Pasadena, California 91125, U.S.A.
Abstract
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
Ca1.00[(Ti3+0.35Al0.18Sc0.01V3+0.01)Σ0.55Mg0.25Ti4+0.19]Σ1.00(Si1.07Al0.93)Σ2.00O6.
Grossmanite is monoclinic, C2/c; a
= 9.80 Å, b
= 8.85 Å, c
= 5.36 Å, β = 105.62°, V = 447.70 Å3,
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) (221), 2.964 (31) (310), 2.581 (42) (002), 2.600 (28) (131), 2.535. The mineral is named for Lawrence Grossman, a cosmochemist at the University of Chicago.
Optical Absorption Spectrum - Two extinction directions
Visible light photomicrograph of grossmanite from the Allende meteorite..