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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,Alrich 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 Å, β = 93.1°, space group P2 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 O7)-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.

american mineralogist

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