The Energy of Olivine Grain Boundaries Deduced from Grain Boundary Frequency Analyses

Abstract:

The properties of grain boundaries strongly differ from those of the crystal lattice, and there is growing evidence that the presence of grain boundaries influence detected geophysical signals such as electrical conductivity and seismic velocities especially in aggregates with a LPO that favours the alignment of specific grain boundaries. However, neither the anisotropic frequency or energy distribution of grain boundary networks are understood in olivine dominated aggregates, neither with nor without LPO.

We used electron backscatter diffraction, EBSD to detect the orientations of over 1.4x10⁴ grains corresponding to roughly 5000mm length of grain boundary separating them. Subsequently we used a stereological approach to determine the grain boundary character distribution, GBCD, defined as the relative areas of grain boundaries of different types, distinguished by their five degrees of freedom (Rohrer, 2007).

The grain boundary planes showed a preference for low index planes, which is in agreement with recent findings on other materials (e.g. MgO, TiO₂, SrTiO₃, MgAl₂O₄). However, our inferred surface energies are controversial with respect to previously simulated surface energies (Watson et al., 1997; de Leeuw et al., 2000; Gurmani et al., 2011). We find that the principal crystallographic planes have the lowest energies and that at 60° misorientation specific grain boundaries with common [100] axis of misorientation are favored compared to 60° misorientations about random axis of rotation. This seems to support the results of (Faul and Fitz Gerald, 1999), even though our data imply that 90°/[001] (100)(010) should be even less favorable for the propagation of melt films. These differences and similarities will be discussed with respect to the different methods and their limitations.

References:

Faul U. H. and Fitz Gerald J. D. (1999) Phys. Chem. Miner. 26, 187–197.

Gurmani S. F. et al. (2011). J. Geophys. Res. 116, B12209.

De Leeuw N. H. et al. (2000) Phys. Chem. Miner. 27, 332–341.

Rohrer G. S. et al. (2004) Z. Metallkd. 95, 1-18.

Watson G. W., Oliver P. M. and Parker S. C. (1997) Phys. Chem. Miner. 25, 70–78.