High-Temperature Study of Octahedral Cation Exchange in Olivine by Neutron Powder Diffraction
- 22 March 1996
- journal article
- other
- Published by American Association for the Advancement of Science (AAAS) in Science
- Vol. 271 (5256) , 1713-1715
- https://doi.org/10.1126/science.271.5256.1713
Abstract
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.Keywords
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