Thermodynamic formulations of (Ca,Fe,Mg)2SiO4 olivine.

The multi-site regular solution model is applied to the intracrystalline exchange reaction of Fe2+ and Mg2+ between M1 and M2 sites of olivine. Effects of Ca on the Fe–Mg intracrystalline distribution coefficient KD[=(XMgM2XFeM1)⁄(XFeM2XMgM1] are derived from this solution model, which indicates that Fe preferentially orders onto the lager M2 site accompanying with increasing Ca. This reaction would be assumed to be temperature-independent. The difference between Fe–Mg mixing parameters of M1 and M2 sites is very small and temperature-independent, so that the fayalite-forsterite olivine would be approximated as a binary regular solution.The miscibility gaps between high-Ca and low-Ca olivines are sufficiently described by the disordered multi-site regular solution model, which is very similar to a symmetric regular solution model. The present estimates for Margules parameters WMgCaM2 is consistent with Davidson and Mukhopadhyay’s (1984) analysis of miscibility gap in Ca–Fe–Mg olivines, and is slightly less non-ideal than the Adams and Bishop’s (1985) results estimated from the mon-ticellite-forsterite solvus. The Ca–Fe mixing is more non-ideal than the previous estimates.