The nature and formation of silica minerals in eucrite meteorites: Insight into the protoplanetary magma crystallizationOPEN ACCESS 

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Rei Kanemaru, Akira Yamaguchi, Naoya Imae, Ryoga Maeda, Makoto Kimura, Kenji Horie, Takashi Mikouchi, Masaaki Miyahara, Masahiro Yasutake, Atsushi Takenouchi, Haruka Ono, Aiko Nakato, Junko Isa, Tomohiro Usui, Hirotsugu Nishido

MAPS, Version of Record online: 06 September 2025

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“Silica polymorphs in meteorites provide critical constraints on crystallization processes associated with thermal activity in the early solar system. A detailed investigation of silica polymorphs in eucrites (the largest group of achondrites) using cathodoluminescence imaging and laser-Raman spectroscopy revealed significant variations in the relative abundance of silica polymorphs. Based on these variations, the eucrites were divided into four “Si-groups” according to their dominant silica phase: Si-0 (cristobalite-dominant eucrites), Si-I (quartz-dominant eucrites), Si-II (quartz and tridymite-dominant eucrites), and Si-III (tridymite-dominant eucrites). In studied eucrites, tridymite and cristobalite form lathy euhedral shapes, while quartz is anhedral, coexistent with opaques and phosphates, suggesting that silica polymorphs were crystallized from different stages and formation processes. We propose a new model that explains the formation pathways of silica minerals in eucrites and accounts for the distinct formation histories represented by each Si-group: tridymite crystallizes from alkali-rich immiscible melts (starting at ≥ ~1060°C), cristobalite crystallizes from quenched melts (~1060°C), and quartz crystallizes from extremely differentiated melts and/or by solid-state transformation from tridymite and cristobalite through interactions with sulfur-rich vapor below ~1025°C. This model explains the occurrences of silica polymorphs in eucrites without requiring secondary heating or shock processes.”

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