Hydrothermal origin of hexagonal CaAl 2 Si 2 O 8 (dmisteinbergite) in a compact type A CAI from the Northwest Africa 2086 CV 3 chondrite

We report an occurrence of hexagonal CaAl 2 Si 2 O 8 (dmisteinbergite) in a compact type A calcium‐aluminum‐rich inclusion ( CAI ) from the CV 3 (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 (Δ 17 O ~ −24‰); Δ 17 O 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 16 O‐poor compositions (Δ 17 O ~ −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 CAI s from the Allende meteorite, we suggest that dmisteinbergite is a common secondary mineral in CAI s from the oxidized Allende‐like CV 3 chondrites that has been previously misidentified as a secondary anorthite.