Polarized absorption spectra of oriented fayalite single crystals, Fe2SiO4, have been measured in-situ at high temperatures as a function of oxygen activity. Electronic defects associated with crystal non-stoichiometry give rise to defect-induced optical absorption. The oxygen activity-dependent spectral intensities are discussed in respect to the microscopic nature of the defects. Their kinetics are studied in isothermal relaxation experiments using sudden changes in oxygen activity within the stability field of fayalite. Oxidation and reduction processes differ in their reaction kinetics. For reduction, the equilibration is essentially controlled by diffusion processes in the bulk, whereas oxidation is also influenced by the kinetics of the fayalite/gas interface. Defect diffusion is found to be anisotropic: it is fastest in c-direction and slowest in a-direction.