The mechanism of Mg diffusion in forsterite and the controls on its anisotropy

Abstract

Mg diffusion is important for explaining many properties of forsterite but its mechanism is unknown. This makes it hard to predict how it will behave in different circumstances. In this study we used Density Functional Theory (DFT) and a Kinetic Monte Carlo (KMC) method to calculate the diffusivity of Mg vacancies and interstitials in forsterite and thus the diffusion rate of Mg in forsterite. We predict vacancy diffusion to be highly anisotropic with considerably faster diffusion in the [001] direction while interstitial diffusion is predicted to be more isotropic. Thus we predict that a combination of interstitial and vacancy diffusion is required to reproduce experimentally derived anisotropies. Interstitial diffusion is predicted to be highly pressure dependant such that with increasing pressure the anisotropy of Mg diffusion decreases while temperature has little effect on this anisotropy. Impurities like Fe and water likely cause increases in Mg diffusion rate through the creation of extrinsic Mg vacancies and we predict that without modifications to the inherent mobility of Mg vacancies these cause small increases to diffusional anisotropy at 1300 and 1600 K but very large increases at 1000 K. The activation volume of Mg self diffusion is also predicted to decrease with increasing extrinsic vacancy concentration.