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Sb-rich titanite in the manganese concentrations at St. Marcel-Praborna, Aosta Valley, Italy: petrography and crystal-chemistry

Published online by Cambridge University Press:  05 July 2018

Elena-Adriana Perseil  and
David C. Smith
Elena-Adriana Perseil
Affiliation:
Muséum National d'Histoire Naturelle, Laboratoire de Minéralogie, France CNRS, Unité de Recherche Associée no 736, 61 Rue Buffon, 75005 Paris, France
David C. Smith
Affiliation:
Muséum National d'Histoire Naturelle, Laboratoire de Minéralogie, France
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Abstract

Titanites rich in antimony (up to 12.59 wt.% Sb2O5; equal to 0.165 Sb5+ per Si4+ = 1.000) arc described for the first time. They coexist with various greenschist-facies minerals in three different petrographical environments in the manganese concentrations at St. Marcel-Praborna in the Aosta Valley, Italy. These titanites are chemically zoned, the Sb-richest parts generally occurring alongside grain boundaries, microfractures, pores or inclusions. Moderately-good positive correlations exist between the Sb, Al and Fe contents, with negative correlations of each of these elements with Ti. Cr has perturbed the Al and Fe contents since a better negative correlation with Ti is obtained with [Al + Cr + Mn + Fe]. The Sr, Ba, Mn and F contents are rather small and vary irregularly. Several crystal-chemical hypotheses are presented concerning the method of structural formula calculation and the valencies and site distributions of Sb and the companion additional minor elements. The favoured interpretation is to place: Sr2+, Ba2+, Mn2+, Fe2+ and possibly also Sb3+ in the viiCa2+ site; Sb5+, Cr3+, Al3+, Sb3+, Fe3+ and possibly also Mn3+ in the viTi4+ site; and Al3+ in the ivSi4+ site; this indicates the existence of a natural silicate containing not only both trivalent and pentavalent Sb, but also both valencies in the same site. The principal ionic substitution is believed to be: 2 viTi4+ = vi[Al3+ + Cr3+ + Mn3+ + Fe3+] + viSb5+ accompanied by a substantial minor proportion of: 2 viTi4+ = viSb3+ + viSb5, or possibly = 2 viSb4+ The data indicate a very late stage penetration of pre-existing titanites by mobilized Sb, Al, Cr, Mn, Fe, F and presumably also (OH) at this locality, whereas Sr and Ba were mobile at an earlier stage. Manganese, which existed in the sedimentary protoliths, did not enter the titanite structure in very significant quantities.

Keywords

antimonytitanitecrystal-chemistrypetrographySt. Marcel-PrabornaItaly
Type
Mineralogy
Information
Mineralogical Magazine , Volume 59 , Issue 397 , December 1995 , pp. 717 - 734
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1995

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