High-Temperature Study of Octahedral Cation Exchange in Olivine by Neutron Powder Diffraction

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Science  22 Mar 1996:
Vol. 271, Issue 5256, pp. 1713-1715
DOI: 10.1126/science.271.5256.1713


Time-of-flight, neutron powder diffraction to 1000°C provides precise octahedral site occupancies and intersite distribution coefficients for MnMgSiO4 and MnFeSiO4 olivines. Intersite exchange occurs in minutes down to 500°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 kilojoules per mole for magnesium-manganese and iron-manganese, respectively. Distribution coefficients deduced for FeMgSiO4 olivine suggest an exchange energy of 4.8 kilojoules per mole. Intersite exchange energies are consistent with diffusion coefficients in the order iron > magnesium > manganese. Geothermometry based on magnesium-manganese and iron-manganese exchange may be possible only for samples equilibrated below 500°C.