Hydrothermal origin of hexagonal CaAl2Si2O8 (dmisteinbergite) in a compact type A CAI from the Northwest Africa 2086 CV3 chondrite

We report an occurrence of hexagonal CaAl2Si2O8 (dmisteinbergite) in a compact type A calcium-aluminum-rich inclusion (CAI) from the CV3 (Vigarano-like) carbonaceous chondrite Northwest Africa 2086. Dmisteinbergite occurs as approximately 10 μm long and few micrometer-thick lath-shaped crystal aggregates in altered parts of the CAI, and is associated with secondary nepheline, sodalite, Ti-poor Al-diopside, grossular, and Fe-rich spinel. Spinel is the only primary CAI mineral that retained its original O-isotope composition (Δ17O ~ −24‰); Δ17O values of melilite, perovskite, and Al,Ti-diopside range from −3 to −11‰, suggesting postcrystallization isotope exchange. Dmisteinbergite, anorthite, Ti-poor Al-diopside, and ferroan olivine have 16O-poor compositions (Δ17O ~ −3‰). We infer that dmisteinbergite, together with the other secondary minerals, formed by replacement of melilite as a result of fluid-assisted thermal metamorphism experienced by the CV chondrite parent asteroid. Based on the textural appearance of dmisteinbergite in NWA 2086 and petrographic observations of altered CAIs from the Allende meteorite, we suggest that dmisteinbergite is a common secondary mineral in CAIs from the oxidized Allende-like CV3 chondrites that has been previously misidentified as a secondary anorthite.