Thermodynamic Models of Reactions Involving Garnet in a Sillimanite/Staurolite Schist

C. T. Foster Jr.
Department of Geology, University of Iowa, Iowa City, Iowa 52242, USA

Abstract: Textures produced by reactions involving garnet in a sillimanite-staurolite schist have been investigated using an irreversible thermodynamic model. The model predicts that the local production or consumption of garnet is strongly influenced by the proximity of garnet to growing sillimanite and dissolving staurolite. Garnets within sillimanite segregations should dissolve near the sillimanite-bearing centre of the segregation while garnets within or adjacent to staurolite poikilo-blasts should grow as the staurolite is replaced by a muscovite-rich pseudomorph. Garnets located in the matrix may grow, not react at all, or dissolve depending on the local configuration of nearby sillimanite, staurolite, and garnet. Textures predicted by the model are similar to those observed in thin section: many garnets are truncated within sillimanite segregations; garnets located in pseudo-morphs after staurolite are equant and have thick unpoikilitic rims that suggest growth; most matrix garnets also have thick inclusion-free rims that suggest growth but a few have thin rims, indicating little growth, or are irregularly shaped, suggesting dissolution.

Keywords: garnet textures • garnet-sillimanite-staurolite schist • irreversible thermodynamic models

Mineralogical Magazine; September 1986 v. 50; no. 357; p. 427-439; DOI: 10.1180/minmag.1986.050.357.07
© 1986, The Mineralogical Society
Mineralogical Society (