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

TLDR: In this article, a series of anorthite-diopside eutectic glasses containing 1 wt% 57 3+ /ΣFe ratios from X-ray absorption near edge structure spectra were investigated.

Abstract: Fe K-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of anorthite-diopside eutectic glasses containing 1 wt% 57 3+ /ΣFe ratios from XANES spec- tra 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) transi- tions. The Fe 3+ /ΣFe ratios determined from linear combinations of end-member spectra (Fe 3+ /ΣFe ~0 and ~1) were found to deviate significantly from the Mossbauer values. This may indicate the sus- ceptibility of this method either to errors arising from the treatment of the background or to changes in Fe 2+ or Fe 3+ coordination with the Fe 3+ /ΣFe ratio. The general applicability of any XANES calibra- tion 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 Fe 3+ /ΣFe values may be obtained from XANES spectra, with an accuracy com- parable to Mossbauer spectroscopy, by reference to empirical calibration curves derived from com- positionally similar standards.