High-temperature Fe-mg Cation Partitioning In Olivine - In-situ Single-crystal Neutron-diffraction Study

The intracrystalline partitioning of Fe and Mg in the octahedral sites of olivine is known to be dependent on temperature, pressure, and composition. Interpretations of the temperature effect on the partitioning have been mostly based on heating and quenching experiments, which seem to indicate that Fe2+ preferentially orders into the M1 site with increasing temperature. The present single-crystal neutron diffraction experiments yield the first in-situ high-temperature structure refinements above 900 degrees C and clearly indicate an ordering reversal above this temperature. Three data sets collected at 960, 1030, and 1060 degrees C show a remarkable progressive decrease in the K-D parameter with temperature, whereas the data at 880 degrees C are consistent with a slight preference of Fe2+ for M1, as reported in the literature. The effect is tentatively interpreted on the basis of competing contributions of configurational and vibrational entropy at high temperature, and it is expected to have profound implications for the thermodynamic modeling of olivine in the Earth's mantle and in planetary processes.