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

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 Fe3Fe–2R–1 typical of the ferrian ferrotapiolite strongly suggests the incorporation of Fe3+ at both the A and the B sites in ferrotapiolite, indicative of solid solution toward FeTaO4. The substitution Sb3 Fe–2R–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 2Sb(Ta,Nb)O4 + Fe(OH)2 + 2H2O = Fe(Ta,Nb)2O6 + 2Sb(OH)3. Such a significant Fe-enrichment (stibiotantalite versus newly formed ferrotapiolite), documented by abundant secondary ferrotapiolite at Laštovičky, is exceptional among products of secondary replacement after primary (Nb,Ta)-oxide minerals in granitic pegmatites. High Fe2O3 in ferrotapiolite indicates high but variable f(O2) during alteration. Values of Ta/(Ta + Nb) in primary stibiotantalite and secondary ferrotapiolite are virtually equal; they suggest very low mobility of both Nb and Ta. Absence of hydrothermal (fluid) fractionation of Nb from Ta in granitic pegmatites and low mobility of Ta and Nb documented here and in other studies support the importance of magmatic processes as a dominant factor in crustal fractionation of Ta from Nb.