Rutherford Backscattering Spectroscopy Study of the Kinetics of Oxidation of (Mg, Fe)2SiO4

Single crystals of oilivine, (Mg_0.9Fe_0.1)₂SiO₄, have been oxidized in air at temperatures between 700° and 1100°C for times from 0.5 to 100 h. Both an internal and an external oxidation layer developed. Transmission and analytical electron microscopy observations reveal that the internal oxidation layer is composed of precipitates of magnetite plus amorphous silica, which nucleated heterogeneously on dislocations and grew in an Fedepleted matrix of olivine. Rutherford backscattering spec-trometry (RBS) demonstrates that the thin external oxidation layer is free of Si; that is, it is made up of Mg-Fe oxide phases. Thus, the oxidation process is primarily controlled by diffusion of Fe²⁺ and Mg²⁺ ions toward the surface with Si⁴⁺ and O^2- remaining largely immobile. The kinetics of oxidation, as determined from RBS analyses of the external oxidation layer, are parabolic with an activation energy of 140 kJ/mol. Although this activation energy is lower than that reported for self-diffusion of Mg in Mg₂SiO₄, the diffusivity calculated from the reaction rate constant is in good agreement with published values for lattice diffusion of Mg in the limited temperature range in which data overlap. However, the rate of accumulation of Fe in the external layer is more rapid than expected for lattice diffusion, indicating that the transport of Fe is dominated by short-circuit diffusion along the precipitate complexes which decorate dislocations.