Synthetic samples in the system Fe2SiO4 (fayalite structure, orthorhombic)-Zn2SiO4 (wil-lemite structure, trigonal), equilibrated at 1000°C, were investigated by means of X-ray diffraction, scanning-electron microscope, microprobe, and Mossbauer spectroscopy (77-700 K).
The two-phase region is estimated to occur in the interval ∼ 17-48 mol% of Zn2SiO4 at 1000°C and 0 bar. It is obvious from calculated densities that the fayalite structure is ∼ 17% more dense at comparable compositions. The volumes per formula unit increase with Fe content in both phases, allowing the determination of the composition from X-ray diffraction data. The fayalite structure contains two octahedral metal positions M(l) (1 symmetry) and M(2) (m symmetry), and it is shown that Zn is preferentially occupying the M(l) site with KD ⋍ 0.43 and ΔG ⋍ 8.9 kj/mol for samples equilibrated at 1000°C and quenched to room temperature. There are three tetrahedral-cation positions in Zn2SiO4 (willemite structure), Zn(l), Zn(2), and Si, and it is shown that Fe occupies the Zn positions in the willemite structure, with a preference for one of the positions. The obtained KD and ΔG values (0.49 and 7.6 kj/mol, respectively, at 1000°C and 0 bar), are nearly the same as the ones obtained for the fayalite structure.