The energy of grain boundaries and interfaces in high-grade metamorphic rocks: inferences from the dihedral angle method

Abstract

A compilation of equilibrium dihedral angles measured in texturally equilibrated high-grade metamorphic rocks demonstrates that, for the great majority of mineral pairs, the energy of the interface between grains of the two phases is lower than those of both grain boundaries. This situation is the case for most two-phase metallic systems, but is not seen in experimental charges formed of alkali halides or ceramics: for these materials, the energy of the interface between the two phases is intermediate to that of the two grain boundaries. Comparison of published values of dihedral angles for the olivine-pyroxene pair in experimental charges with those measured in natural peridotites suggests that an extended history of deformation, reaction and recrystallisation is critical in determining the relative energies of grain and interphase boundaries, with implications for the relevance of studies of deformation behaviour and grain growth rates using synthetic materials. The data-set permits the ranking of grain boundary energies in metamorphic rocks: grain boundary energy correlates with mineral density and also with the crystalloblastic series. The latter is argued to be the result of a complex mix of incomplete textural equilibration, anisotropy of interfacial energy, and a tendency for high-density minerals to grow by step-wise addition to planar faces.