Abstract We report occurrences of coesite in a martian meteorite, expending previously-reported silica polymorphs such as stishovite (El Goresy et al., 2000), seifertite (Goresy et al., 2008; Sharp et al., 1999), and post-stishovite (El Goresy et al., 2000). The coesite was found in the shock-induced melt regions of NWA 8657, usually coexisting with deformed quartz and silica glass. Three morphological types of coesite have been identified: (I) in a silica-maskelynite assemblage, (II) needle grains, and (III) granular grains embedded in maskelynite. Transmission Electron Microscopy (TEM) shows that all types of coesite appear distributed in silica glass and/or nano-phase maskelynite. The stishovite-like morphology of Type II coesite and the presence of deformed quartz suggest coesite to have inverted from stishovite during decompression. The impact-induced peak pressures and temperatures are estimated at ∼ 18-30 GPa and ∼ 2000 ℃ respectively, based on static high pressure experiments (Langenhorst and Deutsch, 2012; Zhang et al., 1996). The polymorphs aggregates of silica in NWA 8657 indicate that the shock-induced melts in this meteorite cooled slower than those in other stishovite-bearing martian meteorites, but fast enough to preserve coesite.

中文翻译：

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从火星西非西北部 8657 的火星 Shergottite 中发现柯石英

摘要我们报告了火星陨石中柯石英的出现，扩展了先前报道的二氧化硅多晶型物，例如 stishovite（El Goresy 等人，2000）、seifertite（Goresy 等人，2008 年；Sharp 等人，1999 年）和后stishovite（El Goresy 等人，2000 年）。柯石英发现于 NWA 8657 的冲击诱发熔化区，通常与变形石英和石英玻璃共存。已经确定了三种形态类型的柯石英：（I）在二氧化硅 - 菱镁矿组合中，（II）针状颗粒和（III）嵌入在菱镁矿中的颗粒。透射电子显微镜 (TEM) 显示所有类型的柯石英都分布在石英玻璃和/或纳米相掩膜石英中。II 型柯石英的类 stishovite 形态和变形石英的存在表明柯石英在减压过程中从 stishovite 倒置。根据静态高压实验（Langenhorst 和 Deutsch，2012 年；Zhang 等人，1996 年），冲击引起的峰值压力和温度分别估计为 ~ 18-30 GPa 和 ~ 2000 ℃。NWA 8657 中二氧化硅的多晶型聚集体表明，这颗陨石中冲击引起的熔体比其他含 stishovite 的火星陨石中的熔体冷却得更慢，但速度足以保存柯石英。