Additional observations on the ferric ion (Fe3+) content of melt-grown fayalite (Fe2SiO4)

Finch et al. (1980) reported the Czochralski (melt) growth of fayalite (Fe₂SiO₄) and recorded solution colorimetric analyses for Fe³⁺ in representative crystals. The best material, grown at an oxygen fugacity of 10⁻¹² bar, was quoted to contain 0.27 to 0.51 wt.% Fe³⁺. The dark-colored samples (not attracted to a small magnet) had been dissolved in HCl under argon, and the solution complexed with thiocyanate ion at 295 K. The results appeared consistent with literature data, and the dark color of the crystals was thought to be due to Fe²⁺ ⇄ Fe³⁺ transitions (Wickersheim and Lefever, 1962). In addition, the Fe³⁺ analyses were supported by the presence of magnetite (Fe₃O₄) inclusions in regions grown under non-optimal conditions.