A quaternary solution model for white micas based on natural coexisting phengite–paragonite pairs

A thermodynamic model for the quaternary white mica solid solution with end-members muscovite–Mg-celadonite–paragonite–Fe-celadonite (Ms–MgCel–Pg–FeCel) is presented. The interaction energies for the MgCel–Pg join, the FeCel–Pg join and the ternary interactions were obtained from natural coexisting phengite– paragonite pairs. Phengite–paragonite pairs were selected based on the criteria that their chemical compositions may be represented as a linear combination of the model end-member compositions and that the respective formation conditions (350–650C, 4–21 kbar) are accurately known. Previously published excess free energy expressions were used for the Ms–Pg, Ms–MgCel and Ms–FeCel binaries. The suggested mixing model was tested by calculating multicomponent equilibrium phase diagrams. This proved to be particularly well suited to reproduce compositional variations of white micas from amphibolite-facies metapelites. KEY WORDS: white mica; solution model; equilibrium phase diagram