The paragonite-muscovite solvus: I. P-T-X limits derived from the Na-K compositions of natural, quasibinary paragonite-muscovite pairs

Data on the Na-K compositions and final equilibration temperatures of natural, quasibinary paragonite-muscovite (Pg-Ms) pairs (mole fraction of margarite in Pg < 0.05; Si/formula unit ≤ 6.2 and/or Σ(Mg + Fe 2+ + Fe 3+) ≤ 0.35 for Ms) have been plotted and analyzed to determine the P- T - X limits of the binary Pg-Ms solvus. There is considerable scatter in the data, but the T- X morphology of the solvus is clearly discernible. Scatter in the data can be attributed to numerous potential sources of error, including (1) inaccuracies in determinations of the Na-K compositions of the coexisting micas, (2) H 3O + → K + and vacancy → K + substitution in Ms and/or Pg, and (3) crystallization and preservation of quasibinary Pg-Ms pairs with metastable Na-K compositions. Data on the Na-K compositions of both Pg and Ms are available for twenty-eight of the Pg-Ms pairs examined in this report. This subset of data was used to develop parametric equations for an average (pressure-independent), binary Pg-Ms solvus. In accordance with results obtained by previous investigators, the new solvus is strongly asymmetric toward paragonite, and the Ms limb has a shallower slope than the Pg limb. However, calculated solvus critical constants (critical temperature = 796.5°C; critical composition = 34.1 mol% muscovite) are significantly different from corresponding values obtained from published thermodynamic models for binary Pg-Ms micas.