Structure and properties of grain and phase boundaries in rocks
Final Report Abstract
In this project, we have used atomic scale modeling to develop structural models for symmetric tilt grain boundaries of forsterite, which is a representative of olivine, the major phase of the Earth’s upper mantle. Whereas most of the previous computational studies in this field were concerned with either metals or simple oxides (many of them with cubic lattice), the orthorhombic crystal structure of olivine with at least two different cations constituted a new level of complexity. When building an interface between two crystal grains, both the surface termination and the relative displacement of the two grains in the grain boundary plane have to be considered. Our simulations using an advanced ionic interaction model suggest that low energy grain boundary structures preserve the main building units of the crystal structure, i.e. isolated SiO4−4 tetrahedra separated by Mg2+ ions. For tilt angles below about 22°, the grain boundary energy is well described by the Read-Shockley dislocation model. From this model, an effective dislocation core radius can be derived, which for the system studied here is in reasonable agreement with the corresponding value obtained from natural olivine samples derived from dihedral angles and somewhat larger than the value from multiscale modeling using ab initio simulations combined with the Peierls-Nabarro model. For tilt angles larger than 22°, the dislocation cores overlap, which may be interpreted as the transition from low- to high-angle grain boundaries. The structural models from the simulations may help to interpret high resolution images from transmission electron microscopy (TEM). Image simulations using our model structures as input yield good agreement with TEM images of synthetic forsterite bicrystals. The computed structural models of grain boundaries combined with the respective grain boundary energies provide a useful set of parameters for continuum scale modeling of polycrystalline olivine aggregates and will be a starting point for future investigations of transport processes along grain and phase boundaries in olivine containing rocks.
