Thermodynamic Mixing Properties of Muscovite-Paragonite Crystalline Solutions at High Temperatures and Pressures, and their Geological Applications

Reversed Na-K exchange data between mica and a 2 molal aqueous(Na,K)Cl fluid (Flux & Chatterjee, 1986) have been employedto model the thermodynamic mixing behaviour of muscovite-paragonitecrystalline solutions on the basis of the Redlich-Kister equation.For these binary micas, G^ex_m may be expressed as where A=11222+1.389 T+0.2359 P, B=–1134+6.806 T–0.0840 P, and C=–7305+9.043 T, with T in K, P in b, G^ex_m, A, B, and C in joules/mol. G_m^ex is well constrained between 450 and 620?C, and may be extrapolatedbeyond that range with caution. The calculated solvi are skewedtoward the paragonite end member. In the range up to 15 kb,the critical temperature, T_c and the critical composition, X_cmay be expressed as a function of P by the relations: and with P indicated in bars. Calculated phase relations of muscovite-paragonite crystallinesolutions have been depicted in terms of the system KAlSi₃O₈-NaAlSi₃O₈-Al₂O₃-SiO₂-H₂O.These data may be applied to appropriate assemblages involvingmica, alkali feldspar, an Al₂ polymorph, and quartz to estimateP, T and a_H2O conditions of their equilibration. In principle,the muscovite limb of the solvus may be used to obtain geothermometricdata for coexisting muscovite-paragonite pairs, provided theequilibrium pressure is independently known. However, such applicationmust be restricted for the present to micas on the ideal muscovite-paragonitejoin. Mica-alkali feldspar-Al₂SiO₅-quartz or mica-plagioclase-Al₂SiO₅-quartzassemblages may be used to deduce a_H2O in the coexisting fluid,if P, and T of equilibrium are independently known. Examplesof such geological applications are given.