High-temperature order-disorder in (Fe0.5Mn0.5) (sub 2) SiO (sub 4) and (Mg0.5Mn0.5) (sub 2) SiO (sub 4) olivines; an in situ neutron diffraction study

Time-of-flight neutron powder diffraction has been used to determine the T-time dependence of M-site occupancies and crystal structures of these two synthetic olivines at up to 1000 degrees C. In both, Mn shows a preference for the M2 site at low T, but becomes increasingly disordered between M1 and M2 with increasing T. The equilibrium non-convergent disordering behaviour is described by a Landau expression for the free-energy change due to ordering. The activation energy for Fe-Mn exchange between M1 and M2 iscalculated to be 193 + or - 3 kJ/mol and for Mn-Mg exchange 172 + or - 3 kJ/mol. The M-site occupancy at room-T is shown to be a function of the cooling rate of the crystal, indicating the possibility that (Mg,Fe) (sub 2) SiO (sub 4) olivine might be useful as a geospeedometer for realtively 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 <<<$I>M-O>><$I> bond lengths, octahedral bond-angle variance and cell parameters is described.