Time-temperature-dependent M-site ordering in olivines from high-temperature neutron time-of-flight diffraction

The application of neutron diffraction to the study of non-convergent cation order/disorder in minerals is illustrated with the behaviour of M cations in Mn-bearing olivines. The results may be described using a Landau expression for the free-energy change due to ordering. Non-convergent ordering kinetics of the M-site cations at the 50: 50 composition in tephroite-olivine solid solutions have been analysed using the Ginzburg-Landau model, giving an activation energy for Fe-Mn exchange between M1 and M2 of 193 ± 3 kJ/mol and for Mg-Mn exchange of 172 ± 3 kJ/mol. The order-disorder process is more rapid than the similar phenomenon observed in orthopyroxenes. The M-site occupancy observed at room temperature is a function of the cooling rate of the olivine, raising the possibility that olivine might be useful as a geospeedometer for relatively rapid cooling events. These results show the utility of probing mineral structure in situ by techniques such as neutron TOF powder diffraction, where strong scattering contrast between target nuclei, combined with the ability to control sample environment, provide a unique route to investigating the behaviour of minerals under the conditions of their formation in the Earth.