A Mössbauer study of the oxidation state of Fe in silicate melts

Fe³⁺/ΣFe ratios were determined from Mössbauer spectra recorded for a series of 17 anorthite-diopside eutectic glasses containing 1 wt% ⁵⁷Fe₂O₃ quenched from melts equilibrated over a range of oxygen fugacities from f_O2 ∼ 10⁵ bars (Fe³⁺/ΣFe = 1) to 10- ¹³ bars (Fe³⁺/ΣFe = 0) at 1682 K. Fe ³⁺/Fe²⁺ was found to be proportional to f _O2 to the power of 0.245 ± 0.004, in excellent agreement with the theoretical value of 0.25 expected from the stoichiometry of the reaction Fe²⁺O + 0.25 O₂ = Fe³⁺O_1.5. The uncertainty in the Fe³⁺ /ΣFe ratios determined by Mössbauer spectroscopy was estimated as ± 0.01 (1σ) from the fit of the data to the theoretical expression, which is significantly less than that quoted for previous measurements on silicate glasses; this results from fitting the spectra of a large number of systematically varying samples, which allows many of the ambiguities associated with the fitting procedure to be minimized. Fe³⁺/ΣFe ratios were then determined for samples of the anorthite-diopside eutectic composition equilibrated at selected values of f_O2, to which up to 30 wt% Fe ₂O₃ had been added. Fe³⁺/Σ Fe was found to vary with ΣFe (or FeO_T), but both the 1 wt% and high FeO_T data could be satisfactorily fit assuming the ideal stoichiometry (i.e., Fe³⁺/Fe²⁺ ∝ f _O2 ¹/⁴) by the inclusion of a Margules term describing Fe²⁺-Fe³⁺ interactions. The large negative value of this term indicates a tendency toward the formation of Fe²⁺-Fe³⁺ complexes in the melt. The resulting expression, using the ideal exponent of 0.25, gave a fit to 289 Fe³⁺/ΣFe values, compiled from various literature sources, of similar quality as previous empirical models which found an exponent of ∼0.20. Although the empirical models reproduce Fe ³⁺/ΣFe values of glasses with high FeO _T reasonably well, they describe the data for 1 wt% FeO_T poorly. The non-ideal values of the exponent describing the dependence of Fe³⁺/ΣFe on f_O2 at high FeO_T are an artifact of models that did not include a term explicitly to describe the Fe²⁺-Fe³⁺ interactions. An alternative model in which Fe in the silicate melt is described in terms of three species, Fe²⁺O, Fe³⁺O _1.5, and the non-integral valence species Fe ^2.6+O_1.3, was also tested with promising results. However, at present there is no model that fits the data within the assessed accuracy of the experimental measurements.