• journal_title：American Mineralogist
• Contributor：Takashi Mikouchi ; Michael Zolensky ; Marina Ivanova ; Osamu Tachikawa ; Mutsumi Komatsu ; Loan Le ; Matthieu Gounelle
• Publisher：Mineralogical Society of America
• Date：2009-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2009.3080
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：94
• issue：5-6
• firstpage：746
• section：Articles

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.