FERROTAPIOLITE AS A PSEUDOMORPH OF STIBIOTANTALITE FROM THE LASTOVICKY LEPIDOLITE PEGMATITE, CZECH REPUBLIC; AN EXAMPLE OF HYDROTHERMAL ALTERATION AT CONSTANT Ta/(Ta + Nb)

Abstract

Dark brown porous ferrotapiolite is a dominant product of alteration in pseudomorphs after primary stibiotantalite from the lepidolite pegmatite at Laštovičky, western Moravia, Czech Republic. Two compositionally distinct varieties of ferrotapiolite were recognized, a volumetrically dominant Fe3+-rich phase and a very rare Sb-rich phase. Niobium-rich ferrotapiolite, with up to 17.51 wt. % Nb2O5 (0.61 apfu), is highly heterogeneous, with Ta/(Ta + Nb) of 0.69 to 0.84 and Mn/(Mn + Fe) of 0.01 to 0.05; Fe2O3calc. ≤ 3.09 wt. % (0.18 Fe3+ apfu, ~0.16 to 0.22 Fe3+ apfu from Mössbauer spectroscopy) in the ferrian variety, and Sb2O3 ≤ 9.87 wt. % (0.34 Sb3+ apfu) in antimonoan variety. The unit-cell dimensions of ferrian ferrotapiolite (4.746 < a < 4.757 Å, 9.198 < c < 9.244 Å) calculated from X-ray powder-diffraction data indicate highly variable degrees of structural order. The substitution NbTa –1 is dominant; the minor substitution Fe3+3Fe2+–2R5+–1 typical of the ferrian ferrotapiolite strongly suggests the incorpora-tion of Fe3+ at both the A and the B sites in ferrotapiolite, indicative of solid solution toward Fe3+TaO4. The substitution Sb3+3 Fe2+ –2R5+–1 in the antimonoan variety tends to an end-member composition of SbTaO4, corresponding to stibiotantalite; however, it very likely reflects microscopic relics of primary stibiotantalite in ferrotapiolite hidden below the surface of polished samples. Stibiotantalite is replaced by ferrotapiolite along irregularly distributed fractures according to the hypothetical reaction