Kinetics and diffusion of defects in fayalite, Fe2SiO4

Polarized absorption spectra of oriented fayalite single crystals, Fe2SiO4, have been measured in-situ at high temperatures as a function of oxygen ac tivity. Electronic defects associated with crystal non-stoichiometry give r ise to defect-induced optical absorption. The oxygen activity-dependent spe ctral intensities are discussed in respect to the microscopic nature of the defects. Their kinetics are studied in isothermal relaxation experiments u sing sudden changes in oxygen activity within the stability field of fayali te. Oxidation and reduction processes differ in their reaction kinetics. Fo r reduction, the equilibration is essentially controlled by diffusion proce sses in the bulk, whereas oxidation is also influenced by the kinetics of t he fayalite/gas interface. Defect diffusion is found to be anisotropic: it is fastest in c-direction and slowest in a-direction. (C) 2001 Elsevier Sci ence B.V, All rights reserved.