Calcium Tschermak's pyroxene, CaAlAlSiO_6, from the Allende and Murray meteorites: EBSD and micro-Raman characterizations

Abstract

Calcium Tschermak's pyroxene (CaTs), CaAlAlSiO_6, is well known as an important component in pyroxene. It is a member of the Ca clinopyroxene group in which Al dominates in the M1 site. Pyroxenes with more than 80 mol% CaTs were observed previously in Ca-,Al-rich refractory inclusions (CAI) from five carbonaceous chondrites. This study re-investigated the near end-member CaTs in the Allende and Murray chondrites. Electron backscatter diffraction (EBSD) is used to establish that its crystal structure is monoclinic, C2/c; a = 9.609 Å, b = 8.652 Å, c = 5.274 Å, β =106.06°, V = 421.35 Å^3, and Z = 4. Its EBSD pattern is an excellent match to that of synthetic CaAlAlSiO_6 with the C2/c structure. MicroRaman is also carried out to confirm the crystal structure. The Allende CaTs, with 46.00 wt% Al_(2)O_3 and 97 mol% Al in the M1 site, has the formula Ca_(1.02)(Al_(0.97)Fe_(0.01)Mg_(0.01))0.99(Si_(1.00)Al_(1.00))_(Σ2.00)O_6. It occurs as micrometer-sized crystals along with melilite, hibonite, perovskite, spinel, corundum, Ti^(3+)-rich pyroxene, and grossular in a fluffy Type A CAI. It is probably a secondary phase resulting from the alteration of gehlenitic melilite. The CaTs in Murray, with a formula Ca_(0.98)(Al_(0.81)Mg_(0.16)Ti_(0.04)^(4+))_(Σ1.01)(Si_(1.11)Al_(0.89))_(Σ2.00)O_6, occurs with hibonite and Al-rich diopside in a glass-free refractory spherule. This sample formed by solidification of a once-molten droplet early in the history of the solar system.

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