An analysis of existing data and models for point defects in pure (Fe,Mg)-olivine showed that it was necessary to consider thermodynamic non-ideality of mixing in order to describe adequately the concentrations of point defects over the range of measurement. In spite of various sources of uncertainty, the concentrations of vacancies in the octahedral sites in (Fe,Mg)-olivine were of the order of 10−4 per atomic formula unit at 1000 to 1200C. The first explicit plots of vacancy concentrations in olivine were presented as a function of temperature and O fugacity, according to the 2 models. It was found that, in contrast to the absolute concentrations at 1100C and the dependence upon fO2, there was considerable uncertainty in the temperature-dependence of the vacancy concentrations. The defect models in pure (Fe,Mg)-olivine had to be extended by considering aliovalent impurities such as Al and Cr in order to describe the behavior of natural olivine. Defect formation enthalpies in the TaMED regime (distinct from intrinsic defect formation) lay between −66 and 15kJ/mol. The migration energies of octahedral cations were probably 260kJ/mol.

Fe–Mg Diffusion in Olivine II - Point Defect Chemistry, Change of Diffusion Mechanisms and a Model for Calculation of Diffusion Coefficients in Natural Olivine. R.Dohmen, S.Chakraborty: Physics and Chemistry of Minerals, 2007, 34[6], 409-30