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首页> 外文期刊>The American mineralogist >Dmitryivanovite: A new high-pressure calcium aluminum oxide from the Northwest Africa 470 CH3 chondrite characterized using electron backscatter diffraction analysis
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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

机译：Dmitryivanovite：一种来自西北非洲470 CH3球粒陨石的新型高压钙氧化铝，其特征在于电子背散射衍射分析

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摘要

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,Alrich inclusions (CAIs), and dmitryivanovite, whose composition is close to stoichiometric CaAl2O4 [Ca1.000(Al1.993Si0.003Ti0.002)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 A, 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.
机译：Dmitryivanovite（CaAl2O4）是一种新近描述的来自西北非洲470（NWA470）CH3球粒陨石的氧化铝钙（Ivanova et al。2002）。 NWA470包含大量的小型Ca，Alrich夹杂物（CAI）和绿铁矿，其成分接近化学计量的CaAl2O4 [Ca1.000（Al1.993Si0.003Ti0.002）1.998O4]。它以约10μm的亚面下晶粒与长石（CaAl4O7），钙钛矿和陨石共生而出现。电子背散射衍射（EBSD）分析表明，三水钙钛矿是CaAl2O4的高压多晶型物（a = 7.95，b = 8.62，c = 10.25 A，β= 93.1°，空间群P21 / c，Z = 12）。 Dmitryivanovite是在自然界中描述的CaO-Al2O3二元体系中的第三相，另外两个是菱铁矿（CaAl12O19）和格石（CaAl4O7），所有这些都存在于CAI中。 NWA470中CaAl2O4的存在表明太阳星云中局部尘埃/气体比升高。 CaAl2O4的相图表明，要使高压CaAl2O4多晶型物稳定在1327°C，需要〜2 GPa，高于该温度时CaAl2O4从太阳星云中冷凝。因为太阳星云不太可能具有如此高的总气压，所以似乎更有可能在太阳星云中发生低压多晶型物的凝结，且粉尘/气体比增加，随后高压多晶型物是由冲击变质作用产生的，最有可能是在将含CaAl2O4的CAI掺入NWA470母体小行星之后。

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