TRIPHYLITE-SARCOPSIDE MISCIBILITY GAP IN THE FeO-MnO-Li2O-P2O5-H2O SYSTEM: EXPERIMENTAL INVESTIGATION AND THERMOMETRIC APPLICATION TO GRANITIC PEGMATITES

In order to assess the stability of the primary triphylite + sarcopside assemblage, we performed hydrothermal experiments between 400 and 700 C-omicron (Ni/NiO oxygen fugacity buffer, P = 1 kbar), starting from the Li(Fe2.5-x2+Mnx2+)(PO4)(2) (x = 0.0, 0.5, 1.0) compositions, which represent the ideal compositions of triphylite + sarcopside assemblages in which both minerals occur in a 1: 1 molar ratio. The triphylite + sarcopside assemblage is observed in all experiments, associated with other phosphates like (Fe2+, Mn2+)(2)P2O7, (Fe2+, Mn2+) Fe-2(3+)(PO4)(2)(OH)(2) center dot nH(2)O, or Fe-4(3+)(Fe2+, Mn2+) (3)(PO4)(6). Electron-microprobe and SIMS analyses show a progressive decrease of the Li contents in the triphylites, balanced by an increase of their Fe2+-contents, when the temperature increases. These compositional changes are due to the increase of the triphylite-sarcopside miscibility along the Li-2(Fe2+, Mn2+)(2)(PO4)(2)-Fe2+(Fe2+, Mn2+)(2)(PO4)(2) solid solution; the experimental phase diagrams can consequently be used as a geothermometer to calculate the exsolution temperatures of the assemblages. A linear fit of the experimental data leads to the general equation: T(C-omicron) = (-142 * XFe) - (773 * Li pfu) + 1131, where XFe = Fe/(Fe + Mn). The uncertainty is around +/- 15 C-omicron, and the influence of pressure is assumed to be negligible. By using this equation, exsolution temperatures were calculated for nine triphylite-sarcopside assemblages from pegmatites; these temperatures do not represent the crystallization temperatures of the phosphate nodules, but correspond to the closing temperature of the triphylite-sarcopside element exchange. Nevertheless, these temperatures, between 276 and 397 C-omicron, are in fairly good agreement with those generally accepted for the crystallization of primary phosphate assemblages in granitic pegmatites.