Microstructures of High-grade Metamorpbic Rocks at Broken Hill, Australia

Frequency distributions of true interfacial angles in high-grade, regional metamorphic rocks from Broken Hill indicate that interfacial tension varies with relative crystallographic orientation in common minerals. In some single-phase aggregates (e. g. quartz, feldspar, garnet, and calcite) the grains are equant and polygonal, anisotropy of interfacial tension having only a small effect on the appearance of such aggregates. This is also true of two-phase aggregates of these phases, but their appearance is complicated by the presence of inclusions, the relative proportions of ihe phases, and the characteristic dihedral angle formed by a grain of one phase in contact with two grains of the other. In phases for which interfacial tension is more orientation-dependent, crystal forms exert some control on the appearance of the microstructure. For example, in hornblende and pyroxene aggregates, some interfaces are parallel to planes of the form 110, although most are irrational. In still more anisotropic phases, low-energy forms (e. g. 001 of biotite and 110 of sillimanite) are very stable, giving rise to a predominance of planar, rational boundaries over curved, adjustment boundaries. Anisotropy of interfacial tension is expressed in the shapes of inclusions. However, even where strongly anisotropic minerals are involved, inclusions with partly planar boundaries occur alongside inclusions with completely curved boundaries.