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

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.

对高达1000°C的温度进行飞行时间中子粉末衍射，可得出MnMgSiO₄和MnFeSiO₄橄榄石精确的八面体占位情况和位间分配系数。在低至500°C的温度下，位间交换在数分钟内即可发生。平衡分配系数表明，锰在较大八面体位置的有序性随温度升高而降低。镁 - 锰和铁 - 锰的交换能分别为每摩尔15.7千焦和10.1千焦。对FeMgSiO₄橄榄石推导出的分配系数表明其交换能为每摩尔4.8千焦。位间交换能与扩散系数的顺序铁＞镁＞锰相符。基于镁 - 锰和铁 - 锰交换的地质温度计可能仅适用于在500°C以下达到平衡的样品。