Polarized absorption spectra of oriented fayalite single crystals were measured in situ, at high temperatures, as a function of O activity. Electronic defects which were associated with crystal non-stoichiometry gave rise to defect-induced optical absorption. The O activity-dependent spectral intensities were considered in terms of the microscopic nature of the defects. Their kinetics were studied, during isothermal relaxation experiments, by using sudden changes in O activity within the stability field of the fayalite. The oxidation and reduction processes differed with respect to their reaction kinetics. In the case of reduction, equilibration was essentially controlled by diffusion in the bulk. However, oxidation was also affected by the kinetics of the fayalite/gas interface. The defect diffusion was anisotropic: being fastest in the c-direction and slowest in a-directions.

Kinetics and Diffusion of Defects in Fayalite, Fe2SiO4. K.Ullrich, K.D.Becker: Solid State Ionics, 2001, 141-142, 307-12