CONICET | Buscador de Institutos y Recursos Humanos

The variation in chemical composition (results of electron-microprobe analyses for 19 elements) and in unit-cell parameters (40 datasets on natural and heated samples each) of columbite-group minerals (CGM) from 29 rare-element-class, LCT-family granitic pegmatites of the Pampean pegmatite province in Argentina are evaluated. In general, the CGM range from ferrocolumbite to manganotantalite; the data plotted in the FeNb2O5-FeTa2O5-MnNb2O5-MnTa2O5 quadrilateral show oblique, upward, bifurcated or ill-defined fractionation trends. In beryl-type pegmatites, CGM have very variable compositions from ferrocolumbite to manganocolumbite or locally manganotantalite, but the range of variation is rather limited. The CGMs from the spodumene-subtype pegmatites from Salta to San Luis show a broad variation and diversified evolutionary trends among different pegmatites. The petalite-subtype pegmatites of the El Quemado pegmatite field carry more evolved CGM, from manganocolumbite to manganotantalite, as do those from the lepidolite-enriched pegmatites from the Altautina and La Estanzuela fields. The albite-type pegmatites, known to date from the El Totoral pegmatite field, contain CGM with oscillatorilly zoning, with intermediate compositions in terms of both the Nb/Ta and Fe/Mn values. The composition of the CGM in the post-orogenic beryl-type pegmatites from the Punilla pegmatite field corresponds to ferrocolumbite slightly enriched in high-field-strengh trace elements. The structural state of natural CGMs is variable, from moderately cation-disordered to moderately cation-ordered, but almost all samples undergo virtually total cation ordering upon heating. Key words: columbite-group minerals, niobium, tantalum, geochemistry, structural state, rare-element pegmatites, Pampean Ranges, Argentina.

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