A model system for mineral facies in pelitic schists

The system Na₂O-K₂O-Al₂O₂-SiO₂-H₂O contains many mineral phases of major importance in the diagenesis and metamorphism of shales and sandstones, as well as in felsic igneous rocks and their metamorphic derivatives. It is thus a useful model-system containing many of the key equilibria of concern in the genesis of such rocks. It is also a system for which extensive experimental and thermodynamic data are available.The discontinuous reactions among the phases quartz, albite, potassic feldspar, muscovite, paragonite, pyrophyllite, kaolinite, kyanite, andalusite, sillimanite, jadeite and analcime have therefore been used to construct a model system for mineral facies in pelitic rocks. There appear to be fiftynine possible facies types, separated by forty-one discontinuities, only thirtysix of which are readily observed in the field. The continuous reactions, involving rotations of tie-line and displacements of three-phase triangles in the NaAlO₂-KAlO₂-Al₂O₃ projection from SiO₂-H₂O, may be formulated using either an Na or K end-member reaction together with Na-K exchange reactions between coexisting white micas, alkali feldspars and analcimes. The general stoichiometric coefficients for all likely discontinuous reactions have been evaluated in terms of mol fractions of end-members. Available experimental data have been used to calibrate the discontinuous equilibria for the limiting conditions of = 0 and = 1. The (Na-K) facies types may be correlated with (Fe-Mg) facies types in pelitic rocks or with assemblages in mafic rocks.This paper is a revised and extended version of a paper entitled Mineral Facies in Pelitic Schists by J. B. Thompson, Jr., published in 1961 as part of a testimonial volume in honor of D. S. Korzhinskii (originally published in Russian with an English summary)