Fe speciation in weathered pyrochlore-group minerals from the Lueshe and Araxa (Barreiro) carbonatites by Fe-57 Mossbauer spectroscopy

The minerals of the pyrochlore group exhibit a large range of chemical comp osition. The general structural formula can be written as A(2-m)B(2)X(6-w)Y (1-n). pH(2)O. The occupancy of the eight-fold-coordinated A site can refle ct the degree of alteration of the pyrochlore. In unaltered pyrochlore-grou p minerals, this site is completely filled by Na and Ca, whereas in the alt ered equivalents, the A site is only partly filled by cations as K, Ba, Sr and Cc. In the literature, the structural formula is systematically calcula ted assuming an ideal stoichiometry, i.e., a total of two six-fold-coordina ted B-site cations per formula unit, dominantly Nb, with subordinate Ti and Ta in certain rock-types, e.g., carbonatites. Fe is systematically conside red as Fell and assigned to the B site. The present Fe-57 Mossbauer study f ocuses on altered pyrochlore-group minerals from two Nb ore deposits in lat erite: Lueshe in the Democratic Republic of Congo, and Araxa 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 sampl e from Lueshe. Spectra of both samples are consistent with the presence of all the Fell at the B site, but with two different coordination spheres. Fe 3+ 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 Ar axa, the unusually large value of the quadrupole splitting, 3 mm/s, affecti ng similar to 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.