Fe SPECIATION IN WEATHERED PYROCHLORE-GROUP MINERALS FROM THE LUESHE AND ARAXA (BARREIRO) CARBONATITES BY 57Fe MOSSBAUER SPECTROSCOPY

Abstract

The minerals of the pyrochlore group exhibit a large range of chemical composition. The general structural formula can be written as A 2− m B 2 X 6− w Y 1− n · p H 2 O. The occupancy of the eight-fold-coordinated A site can reflect the degree of alteration of the pyrochlore. In unaltered pyrochlore-group minerals, this site is completely filled by Na and Ca, whereas in the altered equivalents, the A site is only partly filled by cations as K, Ba, Sr and Ce. In the literature, the structural formula is systematically calculated assuming an ideal stoichiometry, i.e. , a total of two six-fold-coordinated B -site cations per formula unit, dominantly Nb, with subordinate Ti and Ta in certain rock-types, e.g. , carbonatites. Fe is systematically considered as Fe 3+ and assigned to the B site. The present 57 Fe Mossbauer study focuses on altered pyrochlore-group minerals from two Nb ore deposits in laterite: Lueshe in the Democratic Republic of Congo, and Araxa in Brazil. In contrast to the common assumption that all Fe is present as Fe 3+ , 47 ± 2% of the total Fe is in the Fe 2+ oxidation state in the kalipyrochlore sample from Lueshe. Spectra of both samples are consistent with the presence of all the Fe 3+ at the B site, but with two different coordination spheres. Fe 3+ coordinated by 6 O 2− is observed in both cases. In the Lueshe sample, it is also coordinated by 5 O 2− and 1 OH − , whereas in bariopyrochlore from Araxa, the unusually large value of the quadrupole splitting, 3 mm/s, affecting ~19 ± 2% of the Fe 3+ , suggests that this Fe 3+ is coordinated by only 5 O 2− owing to the presence of anion vacancies at the X site.