Octahedral cation ordering in olivine at high temperature. I: in situ neutron single-crystal diffraction studies on natural mantle olivines (Fa12 and Fa10)

Crystallographic determinations on natural olivine single crystals of mantle composition and origin, carried out by in situ neutron diffraction at high temperature, show that the octahedrally co-ordinated Fe2+ and Mg cations undergo two successive trends of cation ordering with increasing temperature. An initial slight preference of Fe2+ for site M1, up to a temperature of about 850 degreesC, is followed by a reverse-ordering reaction with a site preference exchange between the two cations. The cross-over between the two regimes of ordering, corresponding to a situation of complete disorder, occurs at about 900 degreesC. Above this temperature Fe2+ progressively and strongly segregates into site M2 up to 1300 degreesC, the practical limit of the experimental setup utilized in the experiments. Care was taken to ensure that no chemical changes occurred in the crystals (i.e. oxidation), as testified by Mossbauer spectroscopy determinations carried out before and after the heat treatment. The cation-ordering behaviour is reflected in temperature-dependent changes of geometrical and atomic displacement parameters occurring in the octahedral sites M1 and M2. A thermodynamical explanation of this behaviour is proposed in terms of a prevailing vibrational contribution to entropy.