Silica polymorphs in the Millbillillie eucrite: Implications for their formation conditions

Abstract

We discuss the relationship between the variation of silica polymorphs and thermal metamorphism in non-cumulate eucrites using Millbillillie (type 5 or 6). Both monoclinic tridymite and quartz are present in basaltic clasts. Their occurrences suggest transformation from cristobalite by thermal metamorphism. The presence of monoclinic tridymite also shows slowly cooling below 400℃. Therefore, this study demonstrated that Millbillillie experienced high thermal metamorphism at the depth of crust