Dmitryivanovite: A new high-pressure calcium aluminum oxide from the Northwest Africa 470 CH3 chondrite characterized using electron backscatter diffraction analysis

Abstract

Dmitryivanovite (CaAl₂O₄) is a newly described, calcium aluminum oxide from the Northwest Africa 470 (NWA470) CH3 chondrite (Ivanova et al. 2002). NWA470 contains abundant small Ca,Alrich inclusions (CAIs), and dmitryivanovite, whose composition is close to stoichiometric CaAl₂O₄ [Ca_1.000(Al_1.993Si_0.003Ti_0.002)_1.998O₄], was found in one of these CAIs. It occurs as ~10 μm subhedral grains intergrown with grossite (CaAl₄O₇), perovskite, and melilite. Electron backscatter diffraction (EBSD) analysis revealed that dmitryivanovite is a high-pressure polymorph of CaAl₂O₄ (a = 7.95, b = 8.62, c = 10.25 Å, β = 93.1°, space group P2₁/c, and Z = 12). Dmitryivanovite is the third phase to be described from nature in the binary system of CaO-Al₂O₃, the other two being hibonite (CaAl₁₂O₁₉) and grossite (CaAl₄O7)-all are found in CAIs. The presence of CaAl₂O₄ in NWA470 suggests a local elevated dust/gas ratio in the solar nebula. The phase diagram of CaAl₂O₄ shows that ~2 GPa is required to stabilize the high-pressure CaAl₂O₄ polymorph at 1327 °C, above which CaAl₂O₄ condenses from the solar nebula. Because it is unlikely that the solar nebula ever had such a high total gas pressure, it appears more probable that condensation of the low-pressure polymorph occurred in the solar nebula with an enhanced dust-to-gas ratio and that subsequently the high-pressure polymorph was produced by shock metamorphism, most likely after the CaAl₂O₄-bearing CAI was incorporated into the NWA470 parent asteroid.