High-temperature cation ordering in olivine: an in situ Mössbauer study of synthetic (Mg0.55Fe0.45)2 SiO4

High-temperature in situ Mossbauer spectroscopy measurements (300–950°C) were done on synthetic olivine of composition (Mg0.55Fe0.45)2 SiO4 (=Fa45) in order to study the distribution of Fe2 + over the M1 and M2 octahedral sites. The spectra are fit with two doublets, which are assigned to Fe2 + at the M1 (smaller splitting) and M2 sites. The Fe2 + site-occupancies at M1 and M2, obtained from the Mossbauer relative areas, suggest that Fe2 + has a slight preference for the M1 site at temperatures below ~500°C, with a tendency of disordering around this temperature. At higher temperatures, Fe2 + again prefers to occupy the M1 site, where it shows a considerable order at this site up to 750°C. At still higher temperatures, the spectra indicated partial reduction of the Fa-component into metallic iron and the resolution of the doublets was severely deteriorated.