The role of biotite as a catalyst in reaction mechanisms that form sillimanite

Biotite can act as a mineral catalyst that accelerates sillimanite-forming reactions in many high-grade pelites. In this process, biotite is replaced by sillimanite in one area while it is precipitated by local reactions in other areas of a rock. This mechanism develops because of the constraints imposed on material transport to and from growing sillimanite by minerals in the surrounding matrix. Textural evidence for biotite replacement is generally easily identified, but that for biotite-producing reactions is usually subtle. Failure to recognize the biotite-producing reactions can result in erroneous conclusions regarding the role of metasomatism in the formation of sillimanite. Models based on irreversible thermodynamics show that this type of reacdon mechanism can develop where sillimanite grows under conditions that are isochemical at the hand-specimen scale.