THE PARAGONITE-MUSCOVITE SOLVUS .1. P-T-X LIMITS DERIVED FROM THE NA-K COMPOSITIONS OF NATURAL, QUASI-BINARY 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 less-than-or-equal-to 6.2 and/or SIGMA(Mg + Fe2+ + Fe3+) less-than-or-equal-to 0.35 for Ms) have been plotted and analyzed to determine the P - T - X limits of the bi nary Pg-Ms solvus. There is considerable scatter in the data, but the T-X morphology of the solvus is clearly discernible. Scatter in the da ta can be attributed to numerous potential sources of error, including (1) inaccuracies in determinations of the Na-K compositions of the co existing micas, (2) H3O+ --> 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 compositi ons of both Pg and Ms are available for twenty-eight of the Pg-Ms pair s examined in this report. This subset of data was used to develop par ametric 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 l imb has a shallower slope than the Pg limb. However, calculated solvus critical constants (critical temperature = 796.5-degrees-C; critical composition = 34.1 mol% muscovite) are significantly different from co rresponding values obtained from published thermodynamic models for bi nary Pg-Ms micas.