Pressure-induced ordering in (Ni,Mg)₂SiO₄ olivine

An imaging-plate detector interfaced to a large-volume high-pressure device allows quantitative in situ powder X-ray diffraction measurements at the National Synchrotron Light Source (NSLS). High-quality diffraction patterns were recorded from an (Ni,Mg)₂SiO₄ olivine solid-solution sample at 4 GPa and 800 °C as a function of time using 3 min exposures at 40, 63, and 109 min. Refinement of the crystal structure indicated an increase in the ordering of Ni²⁺ and Mg²⁺ cations over the Ml and M2 sites at high pressure. The unit-cell volume was found to decrease with increasing ordering. Kinetic phenomena associated with the cation redistribution were observed on the time scale of tens of minutes. Extrapolation based on an exponential law of ordering relaxation toward equilibrium gave a distribution coefficient (K_D) of 10.7(1) at 4 GPa and 800 °C; the starting sample, which was annealed at 800 °C and room pressure, gave K_D = 8.3(1).