High-temperature order-disorder in (Fe0.5Mn0.5)2SiO4 and (Mg0.5Mn0.5)2SiO4 olivines: an in situ neutron diffraction study
Redfern, Simon A. T.; Henderson, C. Michael B.; Knight, Kevin S.; Wood, Bernard J.
European Journal of Mineralogy Volume 9 Number 2 (1997), p. 287 - 300
published: Jun 26, 1997
manuscript accepted: Nov 13, 1996
manuscript received: May 10, 1996
ArtNo. ESP147050902006, Price: 29.00 €
Abstract 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 Ml 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 bond lengths, octahedral bond-angle variance, and cell parameters is described