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

The minerals of the pyrochlore group exhibit a large range of chemical composition. The general structural formula can be written as A2–mB2X6–wY1–n•pH2O. 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 Fe3+ and assigned to the B site. The present 57Fe Mössbauer study focuses on altered pyrochlore-group minerals from two Nb ore deposits in laterite: Lueshe in the Democratic Republic of Congo, and Araxá in Brazil. In contrast to the common assumption that all Fe is present as Fe3+, 47 ± 2% of the total Fe is in the Fe2+ oxidation state in the kalipyrochlore sample from Lueshe. Spectra of both samples are consistent with the presence of all the Fe3+ at the B site, but with two different coordination spheres. Fe3+ coordinated by 6 O2– is observed in both cases. In the Lueshe sample, it is also coordinated by 5 O2– and 1 OH–, whereas in bariopyrochlore from Araxá, the unusually large value of the quadrupole splitting, 3 mm/s, affecting ~19 ± 2% of the Fe3+, suggests that this Fe3+ is coordinated by only 5 O2– owing to the presence of anion vacancies at the X site.