Journal of Petrology | Volume 17 | Number 4 | Pages 421-439 | 1976
© Oxford University Press 1976
research-article |
A Talc—Phengite Assemblage in Piemontite Schist from Brezovica, Serbia, Yugoslavia
Institut für Mineralogie, Ruhr-Universität Bochum W-Germany
Received July 28, 1975; Revised December 18, 1975; ABSTRACT
Talc occurs in direct contact with phengite in a manganiferous schist containing piemontite, spessartine, quartz, chlorite, hematite, braunite, and occasional phlogopite. The resulting talc-phengite tie line in the AKF plot is a novelty for both natural rocks and the synthetic model system K2O-MgO-Al2O3-SiO2-H2O which contains about 95 per cent of the components making up the four phyllosilicates present in the schist. The remaining components are CuO, MnO, and minor Fe2O3, CoO, NiO in the chlorite, talc, and phlogopite, as well as Fe2O3 in the phengite.
Two possibilities for the origin of the talc-phengite assemblage are discussed:
1. The presence of the components CuO etc. in the phyllosilicates provides additional degrees of freedom or causes shifts of reaction curves in the synthetic model system thus creating a hypothetical new invariant point at intermediate pressures allowing a talc-phengite field.
2. The rock was formed under very high water pressures which permit the coexistence of talc and phengite even in the pure synthetic system according to a theoretical prediction of phase relations from limited experimental data available.
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