engleski

Recrystallization of partly metamictized pyrochlore group minerals from Minas Gerais, Brazil

Eight mineral samples of pyrochlore group originating from granitic pegmatites of Minas Gerais, Brazil, were investigated by X-ray powder diffraction. Members of pyrochlore group minerals are cubic oxides with the space group Fd3m and general formula A2-mB2O6(O, OH, F)1-n  pH2O. Large cations like Na, Ca, K, U and REE occupy A-site, while B-site is occupied by Nb, Ta and Ti [D. D. Hogarth, Am. Mineral. 62 (1977) 403]. Occasionally, pyrochlore group minerals show different degree of metamictization. Metamictization is mainly assumed to be a result of radiation damage originating from radioactive elements present in crystal structure [R. C. Ewing, Nucl. Instr. Meth. B 91 (1994) 22]. X-ray powder diffraction patterns showed that investigated samples are partly metamictized. The mineral samples were heated in air in muffle furnace at 400, 500, 650, 800 and 1000°C for 24 hours in each case. The computer program « Unit Cell» [T. J .B. Holland and S. A. T. Redfern, Mineral. Mag. 61 (1997) 65] was used to refine cell parameters from powder diffraction data. The natural samples have unit cell parameters ranging from a=10.422(4) to a=10.455(5) Å . By heating at 400°C, an increase of a parameters was noticed for most of the samples. Only 3 of 8 samples show relatively small decrease in a parameter. After heating at 500°C, a slight decrease of unit cell volume was observed in most of the samples. With temperature increase the diffraction peaks should become narrower and more intense. However, the diffraction peaks are wider and less intense at 400°C comparing to those for the unheated samples. This is possibly a result of an increased number of defects within the crystal lattice and by heating induced dehydration and recrystallization. By further heating at 650°C, there is no systematic change of unit cell parameters. After heating at 800°C, a breakdown of early pyrochlore is followed by forming new phases, among them petscheckite and columbite were recognized coexisting with pyrochlore phase. The formation of the new phases is probably directly dependent on original chemical composition. The diffraction peaks of the additional phases become more intense at 1000°C, and their width decreases. The values of parameter a of the pyrochlore minerals heated at 1000°C are ranging from 10.439(2) to 10.462 (6) Å , showing a general increase in unit cell dimensions comparing to the unheated mineral samples.

pyrochlore; metamictization; recrystallization

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