In the Sierra de San Luis, Argentina, rare-metal (Be, Li, Nb, Ta, Sn) granitic pegmatites represent the extremes of fractional crystallization of syn-tectonic, meta- to peraluminous S-type granites. Nb-Ta-enriched pegmatites outcrop in an eastern belt, while Sn-enriched pegmatites occur in a parallel belt in the west. The Nb-Ta pegmatites show a clear internal fractionation, whereas the smaller Sn pegmatites are homogeneous. Columbite-tantalite is dispersed mainly in the wall zones of the Nb-Ta-enriched pegmatites, and shows a large variation of the degree of order between 25% and 84% (XRPD) with predominant intermediate degree of order (53 %). Heating treatment caused highly ordered structures, resulting in higher a values and lower c values. Disorder percentages obtained from Mössbauer spectroscopy and estimated by infrared spectroscopy (FTIR) correlate with the cationic ordering obtained by XRPD. A compositional variation from ferrocolumbite in less fractionated pegmatites to manganotantalite in more fractionated pegmatites reveals an evolution in the Nb-Ta-enriched pegmatites. A direct relation between the composition and the degree of order could not be observed. Microscopic exsolved columbite-tantalite also appears along the borders of cassiterite from Sn-enriched pegmatites, and mainly corresponds to ferrocolumbite and manganotantalite with a distinct composition within single pegmatite bodies.

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