Octahedral cation ordering in olivine at high temperature. II: an in situ neutron powder diffraction study on synthetic MgFeSiO4 (Fa50)

The partitioning of Fe and Mg between the M1 and M2 octahedral sites of oli vine has been investigated by in situ time-of-flight neutron powder diffrac tion. The degree of M-cation order was determined from direct measurements of site occupancies in a synthetic sample of Fo50Fa50 heated to 1250 degree sC at the Fe-FeO oxygen buffer. Fe shows slight preference for M1 at temper atures below about 600 degreesC, progressively disordering on heating to th is temperature. Above 630 degreesC, the temperature at which site preferenc es cross over (T-cr), Fe preferentially occupies M2, becoming progressively more ordered into M2 on increasing temperature. The cation-ordering behavi our is discussed in relation to the temperature dependence of the M1 and M2 site geometries, and it is suggested that vibrational entropy, crystal fie ld effects and changes in bond characteristics play a part in the cross-ove r of partitioning behaviour. The temperature dependence of site ordering is modelled using a Landau expansion of the free energy of ordering of the ty pe DeltaG = -hQ + gTQ + a/2 (T - T-c)Q(2) + b/4 Q(4), with a/h = 0.00406 K- 1, b/h = 2.3, T-c = 572 K and g/h = 0.00106 K-1. These results suggest that the high-temperature ordering behaviour across the forsterite-fayalite joi n will have a bearing on the activity-composition relations of this importa nt rock-forming mineral, and indicate that Fe-Mg olivine solid solutions be come less ideal as temperature increases.