Interpretation of the Shape of Mineral Grains in Metamorphic Rocks

Grain boundary angles at junctions of three scapolite grains in a scapolite-pyroxene-sphene rock measure 120 degrees with a standard deviation of 7. 5 degrees. These data are comparable to those obtained from annealed metals, and indicate a close approach to static equilibrium of interfacial tensions. An examination of hornblende-hornblende and biotite-biotite interfaces in gneisses has shown that for certain angles of misorientation, the interface lies parallel to a plane of low indices in one of the adjacent grains. These interfaces are considered to possess lower free energy than those of different orientations. An examination of inclusions of quartz in grains of hornblende, biotite, and garnet has revealed a tendency for hornblende to impose its 110 form, biotite its 001 form, and garnet its 110 form on the inclusions, regardless of the crystallographic orientation of the inclusions relative to the host. The faces of these forms, when in contact with quartz, are considered to be interface of relatively low specific interfacial free energy, and the particular forms are considered to be present in the equilibrium shapes of the corresponding minerals. The shape and dimensions of phlogopite and pyroxene grains in marble have been examined and measured. Although ratios of dimensions are not constant, as demanded by the Wulff theorem, the presence of a degree of regularity in the shape of the phlogopite and pyroxene grains is taken to indicate that interfacial energy is relatively low. Consideration is given to the forms displayed by various metamorphic minerals, when grains of these are embedded in a quartz-feldspar grain aggregate. An attempt is made to assess the effect of temperature, adsorption, and composition on interfacial energy in metamorphic rocks. The Becke crystalloblastic series of minerals can be refined by application of the principles of interfacial energy. It is concluded that several aspects of the shape of mineral grains in metamorphic rocks can be attributed to a local reduction or minimization of interfacial free energy.