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Columbite-group minerals from rare-metal granitic pegmatites of the Sierra de San Luis, Argentina

Graciela M. Sosa, Marta S. Augsburger and Jose C. Pedregosa
Columbite-group minerals from rare-metal granitic pegmatites of the Sierra de San Luis, Argentina
European Journal of Mineralogy (June 2002) 14 (3): 627-636

Abstract

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 Mossbauer 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.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 14
Serial Issue: 3
Title: Columbite-group minerals from rare-metal granitic pegmatites of the Sierra de San Luis, Argentina
Affiliation: Universidad Nacional de San Luis, Departamento de Geologia, San Luis, Argentina
Pages: 627-636
Published: 200206
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 31
Accession Number: 2002-047665
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables, geol. sketch map
S36°30'00" - S32°00'00", W67°30'00" - W65°10'00"
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200216
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