HIGH-TEMPERATURE ORDER-DISORDER IN (FE0.5MN0.5)(2)SIO4 AND (MG0.5MN0.5)(2)SIO4 OLIVINES - AN IN-SITU NEUTRON-DIFFRACTION STUDY

Time-of-flight neutron powder diffraction has been used to determine t he time-temperature dependence of M-site occupancies and crystal struc tures of MgMnSiO4 and FeMnSiO4 olivines between room temperature and 1 000 degrees C. Ln each sample Mn shows a preference for the M2 site at low temperature, but becomes increasingly disordered between M1 and M 2 on increasing temperature. The equilibrium non-convergent disorderin g behaviour is described by a Landau expression for the free-energy ch ange due to ordering. The kinetics of non-convergent ordering of the M -site cations in these samples have been analysed using a Ginzburg-Lan dau model, giving an activation energy for Fe-Mn exchange between M1 a nd 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,F e)(2)SiO4 olivine might be useful as a geospeedometer for relatively r apid cooling events, such as the cooling of small high-level basaltic intrusions. The relationship between M-site ordering in these Mn-beari ng samples and structural parameters such as <M-O> bond lengths, octah edral bond-angle variance, and cell parameters is described.