Mg2SiO4 Forsterite Grain Boundary Structures and Self-diffusion from Classical Molecular Dynamics Simulations

Abstract:

It is well understood that grain boundaries influence many key physicochemical properties of crystalline materials and earth materials are no exception to this. Grain boundaries in the mineral olivine have reshaped our understanding of geophysical processes in the earth’s mantle, e.g. in form of enhanced element transport through grain boundary diffusion. Investigations of the relation between transport rate, energy and geometry of individual grain boundaries is compulsory to understand transport in aggregates with a lattice preferred orientation (LPO) that favours the presence and or alignment of specific grain boundaries over random grain boundaries in an undeformed rock. In this contribution, we perform classical molecular dynamics simulations of a series of symmetric and one asymmetric tilt grain boundaries of Mg₂SiO₄ (forsterite), ranging from 9.58° to 90° in misorientation and varying surface termination (see ¹). Our emphasis lies on unravelling structural characteristics of high and low angle grain boundaries and how these influence grain boundary energy and self-diffusion processes. To obtain diffusion rates for different grain boundary geometries, we equilibrate the respective grain boundary systems at ambient pressure and temperatures from 1900-2200K and trace their evolution for run durations of more than100 ps. Subsequently, we track the mean square displacement of the different atomic species within the grain boundary layer over time to estimate self-diffusion constants for each grain boundary geometry and temperature. First results suggest that diffusion rates decrease with increasing grain boundary energy. We will discuss these results in the light of recent experimental data and show strength and limitations of the method applied.

1. Adjaoud, O., Marquardt, K., Jahn, S., Phys Chem Miner 39, 749-760 (2012)