Mutsumi Komatsu

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 [Ca1.000(Al1.993Si0.003Ti0.002)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.