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

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Marco E. Ciriotti
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Ba-, Si- and vacancy-rich phlogopites from the talc-bearing sulfide ore deposit of La Creuse (Beaujolais, France)

Messaggio da Marco E. Ciriotti » mer 04 lug, 2018 10:39

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Referenza:
▪ Pascal, M.-L., Fonteilles, M., Tournis, V., Baptiste, B., Robert, J.-L., Boulliard, J.-C. (2018): Ba-, Si- and vacancy-rich phlogopites from the talc-bearing sulfide ore deposit of La Creuse (Beaujolais, France). Mineralogical Magazine, 82, (in press).

Abstract:
Ba-rich and Si-rich phlogopites occur in the talc-bearing rocks of the La Creuse sulfide ore deposit. 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 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.
A single crystal X-ray diffraction analysis was performed. It leads to a monoclinic C2/m cell (R = 5.31%) with a = 5.3185(5) Å, b = 9.2136(9) Å, c = 10.1349(11) Å and beta = 100.131(11)°. The refinement of K occupancy leads to 109%, which confirmed the presence of a heavier atom within the interlayer space. The occupancies of Mg/Fe and K/Ba were refined exploring different vacancies. The solutions giving the best R factor (4.77%) and fit goodness (1.06) are obtained with 15%<vacancy<40% at the interlayer site.
Marco E. Ciriotti

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