RUTHERFORD BACKSCATTERING SPECTROSCOPY STUDY OF THE KINETICS OF OXIDATION OF (MG,FE)2SIO4

Single crystals of olivine, (Mg//0//. //9Fe//0//. //1)//2SiO//4, have been oxidized in air at temperatures between 700 degree and 1100 degree 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 Fe-depleted matrix of olivine. Rutherford backscattering spectrometry (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 Fe**2** plus and Mg**2** plus ions toward the surface with Si**4** plus and O**2** minus 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.