Stability and phase equilibria of chloride and carbonate bearing scapolites at 750°C and 4000 bar

At 750°C and 4000 bar scapolite is stable relative to plagioclase + calcite over the range of plagioclase compositions An₅₃–An₈₃. The assemblage plagioclase + scapolite + calcite is stable relative to plagioclase + calcite over the ranges of plagioclase composition An₄₈-An₅₃ and An₈₃–An_91.5. When NaCl is present in the coexisting fluid the range of scapolite compositions stable relative to plagioclase increases. High mole fractions of NaCl in the fluid stabilize scapolite relative to plagioclases from An₂₅ to An₈₇ in the presence of excess calcite. Determination of the $\text{Cl}\text{(Cl + CO}{}_3\text{)}$ ratios of the synthetic scapolites shows that the range of stable scapolite compositions is significantly larger than heretofore proposed, and that even the chloride and carbonate bearing scapolites must be considered a four component solid solution. The K_D for the exchange of NaCl and CaCo₃ between coexisting scapolite, fluid and carbonate is given by the equation In $\text{K}{}_{\mathrm{D}}\text{= (?0.0028)}\text{Al}\text{(Al + Si)}\text{]}{}^{\mathrm{?5.5580}}$. This equation implies that Cl-poor natural scapolites coexisted with fluids low in NaCl, and that regional occurrences of Cl-rich scapolites are likely to represent metamorphosed evaporite sequences.