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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

Takashi Mikouchi, Michael Zolensky, Marina Ivanova, Osamu Tachikawa, Mutsumi Komatsu, Loan Le and Matthieu Gounelle
Dmitryivanovite; a new high-pressure calcium aluminum oxide from the Northwest Africa 470 CH3 chondrite characterized using electron backscatter diffraction analysis
American Mineralogist (May 2009) 94 (5-6): 746-750

Abstract

Dmitryivanovite (CaAl (sub 2) O (sub 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 (sub 2) O (sub 4) [Ca (sub 1.000) (Al (sub 1.993) Si (sub 0.003) Ti (sub 0.002) ) (sub 1.998) O (sub 4) ], was found in one of these CAIs. It occurs as approximately 10 mu m subhedral grains intergrown with grossite (CaAl (sub 4) O (sub 7) ), perovskite, and melilite. Electron backscatter diffraction (EBSD) analysis revealed that dmitryivanovite is a high-pressure polymorph of CaAl (sub 2) O (sub 4) (a = 7.95, b = 8.62, c = 10.25 Aa, beta = 93.1 degrees , space group P2 (sub 1) /c, and Z = 12). Dmitryivanovite is the third phase to be described from nature in the binary system of CaO-Al (sub 2) O (sub 3) , the other two being hibonite (CaAl (sub 12) O (sub 19) ) and grossite (CaAl (sub 4) O (sub 7) )--all are found in CAIs. The presence of CaAl (sub 2) O (sub 4) in NWA470 suggests a local elevated dust/gas ratio in the solar nebula. The phase diagram of CaAl (sub 2) O (sub 4) shows that approximately 2 GPa is required to stabilize the high-pressure CaAl (sub 2) O (sub 4) polymorph at 1327 degrees C, above which CaAl (sub 2) O (sub 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 (sub 2) O (sub 4) -bearing CAI was incorporated into the NWA470 parent asteroid.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 94
Serial Issue: 5-6
Title: Dmitryivanovite; a new high-pressure calcium aluminum oxide from the Northwest Africa 470 CH3 chondrite characterized using electron backscatter diffraction analysis
Affiliation: University of Tokyo, Department of Earth and Planetary Science, Tokyo, Japan
Pages: 746-750
Published: 200905
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 33
Accession Number: 2009-054240
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
Secondary Affiliation: NASA, Johnson Space Center, USA, United StatesVernadsky Institute of Geochemistry, RUS, Russian FederationJacobs Sverdrup Company, USA, United StatesMuseum National d'Histoire Naturelle, FRA, France
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200930
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