Interpretation of the Shape of Mineral Grains in Metamorphic Rocks

Grain boundary angles at junctions of three scapolite grainsin a scapolite-pyroxene-sphene rock measure 120 degrees witha standard deviation of 7.5 degrees. These data are comparableto those obtained from annealed metals, and indicate a closeapproach to static equilibrium of interfacial tensions. An examination of hornblende-hornblende and biotite-biotiteinterfaces in gneisses has shown that for certain angles ofmisorientation, the interface lies parallel to a plane of lowindices in one of the adjacent grains. These interfaces areconsidered to possess lower free energy than those of differentorientations. An examination of inclusions of quartz in grains of hornblende,biotite, and garnet has revealed a tendency for hornblende toimpose its {110} form, biotite its {001} form, and garnet its{110} form on the inclusions, regardless of the crystallographicorientation of the inclusions relative to the host. The facesof these forms, when in contact with quartz, are consideredto be interface of relatively low specific interfacial freeenergy, and the particular forms are considered to be presentin the equilibrium shapes of the corresponding minerals. The shape and dimensions of phlogopite and pyroxene grains inmarble have been examined and measured. Although ratios of dimensionsare not constant, as demanded by the Wulff theorem, the presenceof a degree of regularity in the shape of the phlogopite andpyroxene grains is taken to indicate that interfacial energyis relatively low. Consideration is given to the forms displayed by various metamorphicminerals, when grains of these are embedded in a quartz-feldspargrain aggregate. An attempt is made to assess the effect oftemperature, adsorption, and composition on interfacial energyin metamorphic rocks. The Becke crystalloblastic series of mineralscan be refined by application of the principles of interfacialenergy. It is concluded that several aspects of the shape of mineralgrains in metamorphic rocks can be attributed to a local reductionor minimization of interfacial free energy.