An absolute value for the interfacial free energy of high-angle grain boundaries in olivine was derived from a microstructurally equilibrated coarse-grained natural peridotite. The three-dimensional configuration of high- and low-angle grain boundaries and the misorientation were determined simultaneously in transmitted light using a polarizing microscope. For low-angle grain boundaries, the interfacial free energy was calculated for the dislocation configuration indicated by the disorientation by using the Reed-Shockley model. A dislocation core radius of half of the magnitude of the Burgers vector was suggested by fitting the theoretical function to the plot of relative grain-boundary energy versus misorientation. The relative grain-boundary energy was obtained from the dihedral angle at grain edges. Torque forces could be neglected because high-angle grain boundaries were not faceted and low-angle grain boundaries were rational tilt boundaries parallel to (100), with a rotation axis parallel to [001]. The relationship between sub-grain disorientation and dihedral angles at grain edges yielded an absolute value of about 1.4J/m2 for the energy of high-angle grain boundaries. Among the advantages of using natural material were that a low-energy grain-boundary configuration was adjusted over geological time-scales in the solid state, and that the effects of impurities on the grain-boundary energy and mobility were those relevant to natural conditions.

Grain Boundary Energies in Olivine Derived from Natural Microstructures. J.Duyster, B.Stöckhert: Contributions to Mineralogy and Petrology, 2001, 140[5], 567-76