engleski

HRTEM study of recrystallization mechanisms in metamict allanite

Allanite is a sorosilicate with general formula CaREEAlAlFe2+[SiO4|Si2O7]O(OH) where REE = Ce, La, Nd, Y, but Th and U (in small amounts ~1 wt. %) can substitute for REE. Rather complex crystal structure and chemical composition along with the radioactive decay of, most notably, Th cause the allanite crystal structure to become metamict i.e. amorphous. In order to recrystallize allanite crystal structure, two methods are employed: annealing in air and under hydrothermal conditions. Two mechanisms of recrystallization are possible: epitaxial and nucleation growth. HRTEM investigation is employed in order to compare the recrystallization mechanisms with method of recrystallization. Two samples (ALF and 1569) of naturally metamict allanite were annealed in air and under hydrothermal conditions at 800°C for 24 h in order to recrystallize the crystal structure. SAED and HRTEM of natural and annealed samples were performed on JEOL JEM 3010 operated at 300kV (LaB6, resolution of 1.8 Å). Natural ALF sample shows pronounced metamictization, although crystalline fragments of original crystal structure are preserved, while natural 1569 sample shows almost complete metamictization with minor preserved crystalline fragments, insufficient to yield observable diffraction pattern. After annealing in air at 800°C for 24 h, crystal structure of sample ALF is completely restored yielding formation of large crystallites. For sample 1569, annealing at the same conditions induces recrystallization of small crystallites, but with amorphous domains still present. On the contrary, annealing under hydrothermal conditions at 800°C for 24 h yields complete recovery of crystal structure in both samples, resulting in formation of large crystallites. HRTEM investigation of thermally treated metamict allanite showed that in case of incomplete metamictization, dominant mechanism of recrystallization is epitaxial growth, while for completely metamict samples, both nucleation and epitaxial growth are possible. Which mechanism will occur depends on experimental treatment ; if completely metamict samples are annealed in air, dominant mechanism will be nucleation growth, whilst if annealed under hydrothermal conditions, dominant mechanism is epitaxial growth. These results prove water as the most important factor in recrystallization of metamict minerals containing crystal water i.e. OH- groups.

HRTEM; allanite; recrystallization mechanism

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