Calcium Tschermak’s pyroxene, CaAlAlSiO₆, 

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, ß =106.06º, V = 421.35 ų and Z = 4. Its EBSD pattern is an excellent match to that of synthetic CaAlAlSiO₆ with the C2/c structure. MicroRaman is also carried out to confirm the crystal structure. The Allende CaTs, with 46.00 wt% Al₂O₃ and 97 mol% Al in the M1 site, has the formula Ca_1.02(Al_0.97Fe_0.01Mg_0.01)_∑0.99(Si_1.00Al_1.00)_∑2.00O₆. It occurs as micro-sized crystals along with melilite, hibonite, perovskite, spinel, corundum, Ti³⁺-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_0.81Mg_0.16Ti⁴⁺_0.04)_∑1.01(Si_1.11Al_0.89)_∑2.00O₆, 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.