Interpretation of metamorphic assemblages containing fibrolitic sillimanite

Microstructural criteria distinguish two types of fibrolitic sillimanite-bearing aggregates, namely: (1) those with fibrolitic sillimanite in clear microstructural equilibrium with the other minerals, and (2) those with fibrolitic sillimanite that appears to have grown without evident modification of pre-existing grain boundaries of the other minerals. However, microstructural criteria rarely permit unambiguous inferences of the relative "timing" of metamorphic reactions producing fibrolitic sillimanite.