CATION ORDERING IN SYNTHETIC MG-FE-MN OLIVINE

Abstract

An olivine of the intermediate composition (Mg.33, Fe.33, Mn.33)2SiO4, along with the olivine end members forsterite, Mg2SiO4, fayalite, Fe2SiO4, and tephroite, Mn2SiO4, were synthesized to a fair degree of purity, and the cation ordering of the intermediate composition determined. Neutron diffraction was carried out on the end members for calibration, and a combination of x-ray and neutron diffraction was done on the intermediate olivine, using the Rietveld refinement method. The divalent Mn ion was found to be preferentially ordered in the larger M2 site, while the divalent Fe and Mg ions showed no preference for either site. This confirms other studies which indicate a preference of Mn for the M2 site and an absence of preference of Mg or Fe for either site in a variety of different olivine compositions between forsterite, fayalite, and tephroite. Since only the Mn has a preference for a certain site, and because the Mn is not totally ordered, one application of these results is the use of intermediate olivines as geothermometers.