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.