Grain boundary energies in olivine derived from natural microstructures

Abstract.

An absolute value for the interfacial free energy of high-angle grain boundaries in olivine/fo₉₀ has been derived from a microstructurally equilibrated coarse-grained natural peridotite. The three-dimensional configuration of high and low-angle grain boundaries and the disorientation were determined simultaneously in transmitted light with a universal stage on a polarizing microscope. For the low-angle grain boundaries, the interfacial free energy was calculated for the dislocation configuration indicated by the disorientation, using the model of Read and Shockley (Phys Rev 78(3):275–289, 1950). A dislocation core radius of 1/2b (b=magnitude of the Burgers vector) is suggested by fitting the theoretical function to the plot of relative grain boundary energy versus disorientation. The relative grain boundary energy is obtained from the dihedral angle at grain edges. Torque forces can be neglected because high-angle grain boundaries are not facetted and low-angle grain boundaries are unilaterally rational tilt boundaries parallel to (100) with a rotation axis parallel [001]. The relation between sub-grain disorientation and dihedral angles at grain edges yields an absolute value for the energy of high-angle grain boundaries of ~1.4 J m^–2. The advantages of using natural materials are that (1) experimental efforts are minimal, (2) the material is coarse grained and thus three-dimensional grain boundary configuration and crystallographic orientation can both be studied simultaneously in transmitted light, (3) the low energy grain boundary configuration was adjusted over geological time scales in the solid state, and (4) effects of impurities on grain boundary energies and mobilities are those relevant to natural conditions.