An experimental study of the external reduction of olivine single crystals

Single crystals of San Carlos olivine in contact with graphite were anneale d at P = 1 bar, T= 1373 K, for studying the reaction of extraction of (Fe, Ni) metal. Scanning electron microscopy and transmission electron microscop y were performed on samples recovered after the experiments. Precipitates o f (Fe, Ni) and thin amorphous layer of silica were identified, exclusively on the surface of the single crystals. Mass balance indicates that volatili zation of Fe, Mg, and Si is negligible under these conditions. The reaction can be summarized as: Fe2SiO4in olivine + 2(graphite) =2Fe(in metal) + SiO2 amorphous + 2CO(in ga s) which occurs at the crystal surface without affecting the interior of the c rystal, except for an Fe2+ and Mg2+ compositional profile in the olivine ma trix. These chemical profiles are consistent with measured values of Fe2+-M g2+ interdiffusion coefficients, in agreement with the fact that Si and O a re relatively immobile in olivine under such conditions. This study shows that annealing at relatively moderate temperature under re ducing conditions can cause surface modifications and thus probably can str ongly influence the surface evolution of planetary objects exposed directly to space environments (regoliths, surfaces of asteroids, or interplanetary dust particles).