Fe K-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of anorthite-diopside eutectic glasses containing 1 wt% ⁵⁷Fe₂O₃ quenched from melts equilibrated over a range of oxygen fugacities at 1409 °C. The Fe³⁺/∑Fe ratios were determined previously by ⁵⁷Fe Mössbauer spectroscopy and vary between 0 (fully reduced) and 1 (fully oxidized). Using the Mössbauer results as a reference, various methods for extracting Fe³⁺/∑Fe ratios from XANES spectra were investigated. The energy of the 1s → 3d pre-edge transition centroid was found to correlate linearly with the oxidation state. Correlations also exist with the energy of the K absorption edge and the area of peaks in the derivative spectrum associated with the 1s → 4s and crest (1s → 4p) transitions. The Fe³⁺/∑ Fe ratios determined from linear combinations of end-member spectra (Fe³⁺/∑ Fe ~0 and ~1) were found to deviate significantly from the Mössbauer values. This may indicate the susceptibility of this method either to errors arising from the treatment of the background or to changes in Fe²⁺ or Fe³⁺ coordination with the Fe³⁺/∑ Fe ratio. The general applicability of any XANES calibration for determining oxidation states is limited by variations in the Fe coordination environment, which affects both the intensity and energy of spectral features. Thus previous calibrations based on mineral spectra are not applicable to silicate glasses. Nevertheless, systematic trends in spectral features suggest that Fe³⁺/∑ Fe values may be obtained from XANES spectra, with an accuracy comparable to Mössbauer spectroscopy, by reference to empirical calibration curves derived from compositionally similar standards.

Fe³⁺/∑Fe 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 fO₂~ 10⁵ bars (Fe³⁺/∑Fe = 1) to 10⁻¹³ bars (Fe³⁺/∑Fe = 0) at 1682 K. Fe³⁺/Fe²⁺ was found to be proportional to fO₂ 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⁺/∑Fe 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⁺/∑Fe ratios were then determined for samples of the anorthite-diopside eutectic composition equilibrated at selected values of fO₂, to which up to 30 wt% Fe₂O₃ had been added. Fe³⁺/∑Fe was found to vary with ∑Fe (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⁺ ∞fO₂^1/4) 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⁺/∑Fe 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 Fe³⁺/∑Fe 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⁺/∑Fe on fO₂ 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.