Discovery of coesite from the martian shergottite Northwest Africa 8657. (1st October 2020)

• Record Type: Journal Article
• Authors: Hu, Sen; Li, Yang; Gu, Lixin; Tang, Xu; Zhang, Ting; Yamaguchi, Akira; Lin, Yangting; Changela, Hitesh
• Is Part Of: Geochimica et cosmochimica acta. Volume 286(2020)
• Page Start: 404
• Page End: 417
• Publication Date: 2020-10-01
• Subjects: Mars -- Shock metamorphism -- Coesite -- Plagioclase; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2020.07.021
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: We report occurrences of coesite in a martian meteorite, expanding previously-reported silica polymorphs such as stishovite (El Goresy et al., 2000), seifertite (Sharp et al., 1999; Goresy et al., 2008), 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.