Publication Date:
2018
Description:
〈div data-abstract-type="normal"〉
〈p〉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〈span〉–1〈/span〉 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.〈/p〉
〈p〉Single crystal X-ray diffraction analyses were performed. Ba-rich and Si-rich phlogopite is monoclinic, space group 〈span〉C〈span〉2〈/span〉/m〈/span〉, (〈span〉R〈/span〉 = 5.31%) with 〈span〉a〈/span〉 = 5.3185(5), 〈span〉b〈/span〉 = 9.2136(9), 〈span〉c〈/span〉 = 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 〈span〉R〈/span〉 factor (4.77%) and goodness-of-fit (1.06) are obtained with 15% 〈/p〉
〈/div〉
Print ISSN:
0026-461X
Electronic ISSN:
1471-8022
Topics:
Geosciences
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