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American Mineralogist; October 2009; v. 94; no. 10; p. 1491-1494; DOI: 10.2138/am.2009.3310
© 2009 Mineralogical Society of America
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Letter

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.

Correspondence: * E-mail: chi{at}gps.caltech.edu

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[(Ti0.353+Al0.18Sc0.01V0.013+){sum}0.55Mg0.25Ti0.194+]{sum}1.00(Si1.07Al0.93){sum}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 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) (Formula21), 2.964 (31) (310), 2.581 (42) (002), 2.600 (28) (Formula31), 2.535 (47) (221). The name is for Lawrence Grossman, a cosmochemist at the University of Chicago.

Key Words: Grossmanite • Ca(Ti3+,Mg,Ti4+)AlSiO6 • CaTi3+AlSiO6 • new mineral • Ti-rich pyroxene • refractory inclusion • Allende meteorite • carbonaceous chondrite






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