XANES calibrations for the oxidation state of iron in a silicate glass

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⁺/ΣFe 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⁺/ΣFe 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⁺/ΣFe ratios determined from linear combinations of end-member spectra (Fe³⁺/ΣFe ∼O 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⁺/ΣFe 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⁺/ΣFe 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.