At a total pressure of 5 kb, calcic, Cl-free scapolite (Me83) is stable relative to plagioclase-bearing assemblages at T ??? 625??C, XCO2 ??? 0.12. With decreasing temperature, scapolite breaks down to plagioclase + calcite. Scapolite is replaced by plagioclase + grossular + cancrinite + CO2 in the presence of H2O-rich fluids. The stable coexistence of scapolite and calcite, an assemblage typical of most natural occurrences of calcic scapolite, is limited by the reaction: scapolite + calcite ??? grossular + cancrinite + CO2, which occurs at 750??C, XCO2 = 0.46; 700??C, XCO2 = 0.33; 650??C, XCO2 = 0.18, for the chosen bulk composition. Generalization of the experimental results to encompass the complete range of fully carbonated scapolite compositions indicates that mizzonite (Me75) has the largest T-XCO2 stability field. For scapolite more calcic than mizzonite, stable growth is restricted to conditions of increasingly higher temperature and XCO2. The experimental results are consistent with various petrologic features of scapolite-bearing rocks, particularly scapolite-clinopyroxene granulites, and indicate that such rocks were formed in the presence of CO2-rich fluids. ?? 1983.
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T-XCO2 stability relations and phase equilibria of a calcic carbonate scapolite