High-temperature order-disorder in (Fe_0.5Mn_0.5)₂SiO₄ and (Mg_0.5Mn_0.5)₂SiO₄ olivines: An in situ neutron diffraction study

Time-of-flight neutron powder diffraction has been used to determine the time-temperature dependence of M-site occupancies and crystal structures of MgMnSiO₄ and FeMnSiO₄ olivines between room temperature and 1000°C. In each sample Mn shows a preference for the M2 site at low temperature, but becomes increasingly disordered between M1 and M2 on increasing temperature. The equilibrium non-convergent disordering behaviour is described by a Landau expression for the free-energy change due to ordering. The kinetics of non-convergent ordering of the M-site cations in these samples have been analysed using a 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 M-site occupancy at room temperature is shown to be a function of the cooling rate of the crystal, indicating the possibility that (Mg,Fe)₂SiO₄ olivine might be useful as a geospeedometer for relatively rapid cooling events, such as the cooling of small high-level basaltic intrusions. The relationship between M-site ordering in these Mn-bearing samples and structural parameters such as <M-O> bond lengths, octahedral bond-angle variance, and cell parameters is described.