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An experimental study of the external reduction of olivine single crystals

¹ Laboratoire de Minéralogie et Cristallographie de Paris, case 115, 4 place Jussieu, 75252 Paris, France
² Département des géomatériaux, Institut de Physique du Globe de Paris, 4 place Jussieu, 75252 Paris cedex 05, France
³ Laboratoire Structure et Propriétés de l’Etat Solide, (ESA 8008), Université Sciences et Technologies de Lille, 59655 Villeneuve d’Ascq-Cedex, France
⁴ Centre de Recherches Pétrographiques et Géochimiques (CRPG), Vandoeuvre-lès-Nancy, France

Single crystals of San Carlos olivine in contact with graphite were annealed at P = 1 bar, T = 1373 K, for studying the reaction of extraction of (Fe, Ni) metal. Scanning electron microscopy and transmission electron microscopy were performed on samples recovered after the experiments. Precipitates of (Fe, Ni) and thin amorphous layer of silica were identified, exclusively on the surface of the single crystals. Mass balance indicates that volatilization of Fe, Mg, and Si is negligible under these conditions. The reaction can be summarized as:

which occurs at the crystal surface without affecting the interior of the crystal, except for an Fe²⁺ and Mg²⁺ compositional profile in the olivine matrix. These chemical profiles are consistent with measured values of Fe²⁺-Mg²⁺ interdiffusion coefficients, in agreement with the fact that Si and O are relatively immobile in olivine under such conditions.

This study shows that annealing at relatively moderate temperature under reducing conditions can cause surface modifications and thus probably can strongly influence the surface evolution of planetary objects exposed directly to space environments (regoliths, surfaces of asteroids, or interplanetary dust particles).