Ba-, Si- and vacancy-rich phlogopites from the talc-bearing sulfide ore deposit of La Creuse, Beaujolais, France. (2nd October 2018)

• Record Type: Journal Article
• Authors: Pascal, Marie-Lola; Fonteilles, Michel; Tournis, Véronique; Baptiste, Benoît; Robert, Jean-Louis; Boulliard, Jean-Claude
• Is Part Of: Mineralogical magazine. Volume 82:Number 5(2018)
• Page Start: 1187
• Page End: 1210
• Publication Date: 2018-10-02
• Subjects: barian phlogopite, -- kinoshitalite, -- Chessy, -- Beaujolais, -- Brévenne, -- sulfide ore, -- talc, -- baryte, -- Mg-metasomatism, -- interlayer vacancy; Mineralogy -- Periodicals; Mineralogy -- Great Britain -- Periodicals; 549.05
• DOI: 10.1180/mgm.2018.124
• Languages: English
• ISSNs: 0026-461X

Abstract

ABSTRACT: Ba-rich and Si-rich phlogopites occur in the talc-bearing rocks of the La Creuse sulfide ore deposit in Beaujolais, France. They form a group of compositions completely separated from the common Al-rich phlogopites that occur in the surrounding talc-free metasiltites and metarhyolites, with higher Ba and Mg and lower Al contents. The Ba-rich phlogopites have a relatively narrow compositional range (0.24 to 0.80 Ba per formula unit, for 44 valencies) with high and constant Si (5.8 atoms per formula unit, apfu) and Mg + Fe (5.6 apfu), probably buffered by the presence of talc. Compared to low-Al phlogopites from talc-free rocks, the excess charge introduced by the BaK–1 substitution is compensated by interlayer vacancies. Such a high level of interlayer vacancy (0.56 pfu), related to the talc-producing metasomatic conditions, is essential for the stability of this special group of Ba-rich and Si-rich phlogopites. Single crystal X-ray diffraction analyses were performed. Ba-rich and Si-rich phlogopite is monoclinic, space group C 2 /m, ( R = 5.31%) with a = 5.3185(5), b = 9.2136(9), c = 10.1349(11) Å and β = 100.131(11)°. The occupancies of Mg/Fe and K/Ba were refined exploring different vacancies. The solutions giving the best R factor (4.77%) and goodness-of-fit (1.06) are obtained with 15% < vacancy < 40% at the interlayer site.