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

Dmitryivanovite (CaAl 2 O 4 ) is a newly described, calcium aluminum oxide from the Northwest Africa 470 (NWA470) CH3 chondrite (Ivanova et al. 2002). NWA470 contains abundant small Ca,Al-rich inclusions (CAIs), and dmitryivanovite, whose composition is close to stoichiometric CaAl 2 O 4 [Ca 1.000 (Al 1.993 Si 0.003 Ti 0.002 ) 1.998 O 4 ], was found in one of these CAIs. It occurs as ~10 μm subhedral grains intergrown with grossite (CaAl 4 O 7 ), perovskite, and melilite. Electron backscatter diffraction (EBSD) analysis revealed that dmitryivanovite is a high-pressure polymorph of CaAl 2 O 4 ( a = 7.95, b = 8.62, c = 10.25 A, β = 93.1°, space group P 2 1 / c , and Z = 12). Dmitryivanovite is the third phase to be described from nature in the binary system of CaO–Al 2 O 3 , the other two being hibonite (CaAl 12 O 19 ) and grossite (CaAl 4 O 7 )—all are found in CAIs. The presence of CaAl 2 O 4 in NWA470 suggests a local elevated dust/gas ratio in the solar nebula. The phase diagram of CaAl 2 O 4 shows that ~2 GPa is required to stabilize the high-pressure CaAl 2 O 4 polymorph at 1327 °C, above which CaAl 2 O 4 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 CaAl 2 O 4 -bearing CAI was incorporated into the NWA470 parent asteroid.