Thermodynamic models of biotite pseudomorphs after staurolite

A local equilibrium, irreversible thermodynamic model was used to study biotite-rich pseudomorphs after staurolite in muscovite-bearing pelites near Rangeley, Maine. This type of pseudomorph is found in close proximity to sillimanite segregations which have grown during a staurolite breakdown reaction (Foster, 1977a). The local reactions and distribution of minerals expected to be produced by the growth of sillimanite near staurolite poikiloblasts in a matrix of muscovite, biotite, plagioclase, quartz and ilmenite were calculated using Gibbs-Duhem relations, conservation equations and relative thermodynamic diffusion coefficients in an SiO₂-fixed reference frame. Biotite-rich, plagioclase-rich, muscovite-free pseudomorphs are formed in the model when the muscovite-free mantle of a sillimanite segregation encloses a staurolite poikiloblast or grows over a muscovite-rich pseudomorph. The model produces biotite-rich, muscovite-free, plagioclase-free pseudomorphs when the muscovite-free, plagioclase-free mantle of a sillimanite segregation encloses a staurolite poikiloblast or grows over a muscovite-rich or plagioclase-rich pseudomorph. Biotite-rich pseudomorphs after staurolite in rocks from the lower sillimanite zone near Rangeley, Maine are similar to those calculated by the model. The proportion of muscovite-rich to biotite-rich pseudomorphs in the model is a function of the initial proportions of muscovite, plagioclase and staurolite, which govern the proportion of staurolite that is enclosed by sillimanite segregation mantles.