Composition of barium-rich white micas from the Berisal complex, Simplon Region, Switzerland

The chemistry of barian muscovite and ganterite in metamorphosed basement rocks from the Berisal Complex, Simplon Region, Switzerland, is described. Analysis of white micas from several Ba-enriched localities in this complex established an extensive solid-solution between ganterite and muscovite. Thus, the material is excellent for studying the crystal-chemical consequences of introducing Ba into the dioctahedral muscovite structure. The Ba-rich white micas are found in two lithologic units with whole-rock chemical compositions dominated by SiO2 + Al2O3 + BaO + K2O ± Na2O and SiO2 + Al2O3 + BaO + CaO. The first is a white-mica schist that consists of white mica + quartz + epidote-group minerals, and occurs as lenses or bands interlayered in garnet-bearing two-mica augen gneiss, zoisite–biotite gneiss or zoisite–celsian gneiss. The second host-rock type is a zoisite–celsian gneiss that is characterized by the assemblage white mica + zoisite + celsian + quartz + margarite. In both lithologies, the Ba-rich white micas are synkinematic, and petrographic observations suggest that Ba was present in the rocks prior to, or introduced during the peak of Alpine metamorphism. Concentrations of Ba in the white micas range from 3.6 to 18.2 wt.% BaO. Higher Ba content is always associated with higher Al, and lower K and Si concentrations. The highest Ba contents are also coupled to increases in Na, but there are no similar correlations observed for Fe or Mg. Accommodation of Ba2+ ions in micas is generally achieved by exchange vectors (1) [Ba][[IV]Al][K]−1[Si]−1 or (2) [Ba][Fe,Mg][K]−1[Al]−1. The data presented here show that vector (2) plays no part in the solid solution between muscovite and ganterite. Furthermore, vector (1) operates only if K >0.53 apfu, i.e., Ba < 0.32. At higher Ba concentrations, a second, distinct exchange-vector [Na][K]−1 also operates in the interlayer site. It appears that at higher Ba concentrations in dioctahedral white micas, there is extensive solid-solution involving Na+, K+, and Ba2+. In minerals where exchange between alkali and alkaline-earth elements is observed (e.g., feldspar system), a miscibility gap generally exists. Such a miscibility gap, however, was not observed in the system ganterite – muscovite – paragonite, making the system possibly unique. The reasons for such exchanges being possible are not fully understood, but it is shown that whole-rock composition does not play a significant role.