THE HYDROTHERMAL RECRYSTALLIZATION OF METAMICT ALLANITE-(Ce)

Samples of metamict allanite-(Ce) originating from granite pegmatites were recrystallized by heating in air and under hydrothermal conditions. Recrystallization in air resulted in partial recovery of the crystal structure at 650 degrees C. Heating at higher temperatures resulted in decomposition of the allanite structure to a multiphase assemblage containing hematite, cerianite, feldspar, britholite and thorianite. To our knowledge, this is the first report of thorianite as a breakdown product of allanite in literature. The collapse of the structure is mainly related to the removal of structural H(2)O. Recrystallization under hydrothermal conditions yielded a more rapid and complete recovery of the structure at lower temperatures, without the appearance of additional phases. The stability of the allanite structure during hydrothermal recrystallization is related to the retention of OH-groups, available because of the aqueous environment of recrystallization. Also, H(2)O promotes a more rapid and efficient recrystallization due to the enhanced diffusion of cations. Crystallite size and strain calculation also record evidence of the recovery of the structure of damaged allanite.