HIGH-TEMPERATURE M-SITE DISORDERING IN Mn-OLIVINES BY IN-SITU NEUTRON DIFFRACTION. Simon A.T. Redfern², C.M.B. Henderson¹, K.S. Knight³, B.J. Wood⁴. ¹Department of Earth Sciences, University of Manchester, Manchester, M13 9PL, UK; ²Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK; ³ISIS, Rutherford Appleton Laboratory, Oxon, OX11 0QX, UK; ⁴Department of Geology, University of Bristol, Queens Road, Bristol, BS8 1RJ, UK

The nonconvergent order/disorder behavior over M1 and M2 sites has been determined as a function of temperature for 50:50 (Mg:Mn) and (Fe:Mn) olivines between 293 and 1273 K. Rietveld structure refinements of the high-temperature neutron diffraction data reveal that the amount of Mn in M2 increases (approaching its equilibrium value) on increasing temperature to around 770 K, and then decreases along its equilibrium path towards 50%. These data are interpreted within a Landau model for non-convergent order/disorder in olivine. We find that M-site order/disorder is non-quenchable and only observable by high-temperature in-situ study. The room temperature site occupancy can be used as a geospeedometer, since it depends only on the cooling rate of the crystal and is related to the temperature at which the M1:M2 occupancies depart from equilibrium on cooling. The activation energy for Fe:Mn exchange is calculated as some 193 kJ/mol. The results are used to infer cooling rates of natural and synthetic Mn-olivines using data culled from the literature, where previous structural studies list room-temperature M-site occupancies.