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

¹ Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Hongo, Bunkyo-Ku, Tokyo 113-0033, Japan
² Code KT, NASA Johnson Space Center, Houston, Texas 77058, U.S.A.
³ Vernadsky Institute of Geochemistry, Kosygin Street 19, Moscow 119991, Russia
⁴ Jacobs Sverdrup Co., Houston, Texas 77058, U.S.A.
⁵ Laboratoire d’ Étude, de la Matiere Extraterrestre, Muséum National d’Histoire Naturelle, 57 rue Cuvier, 75005 Paris cedex, France

Correspondence: ^* E-mail: mikouchi{at}eps.s.u-tokyo.ac.jp

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,Al-rich 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.

Key Words: Dmitryivanovite • CAI • electron backscatter diffraction • new minerals • CH chondrite

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