HIGH-TEMPERATURE STUDY OF OCTAHEDRAL CATION-EXCHANGE IN OLIVINE BY NEUTRON POWDER DIFFRACTION

Time-of-flight, neutron powder diffraction to 1000 degrees C provides precise octahedral site occupancies and intersite distribution coeffic ients for MnMgSiO4 and MnFeSiO4 olivines. Intersite exchange occurs in minutes down to 500 degrees C. Equilibrium distribution coefficients show that manganese ordering into the larger octahedral site decreases with increasing temperature. Exchange energies are 15.7 and 10.1 kilo joules per mole for magnesium-manganese and iron-manganese, respective ly. Distribution coefficients deduced for FeMgSiO4 olivine suggest an exchange energy of 4.8 kilojoules per mole. Intersite exchange energie s are consistent with diffusion coefficients in the order iron > magne sium > manganese. Geothermometry based on magnesium-manganese and iron -manganese exchange may be possible only for samples equilibrated belo w 500 degrees C.