Calcic micas in the allende meteorite: evidence for hydration reactions in the early solar nebula.

Two calcic micas, clintonite and margarite, have been identified in alteration products in a calcium- and aluminum-rich inclusion (CAI) in the Allende meteorite. Clintonite replaces grossular in alteration veins, and margarite occurs as lamellae in anorthite. Their occurrence suggests that, in addition to undergoing high-temperature alteration by a volatile and iron-rich vapor that produced the grossular and anorthite, some CAIs underwent alteration at moderate temperatures (</=400 kelvin). Petrographic evidence suggests that the calcic micas formed before accretion but after the formation of the layered rim sequences that surround the CAL. These calcic micas provide strong evidence that, contrary to theoretical calculations, some hydration reactions occurred in the early solar nebula.