Accurate Fe³⁺/Fe_tot ratio from XAS spectra at the Fe K-edge

Highlights: • Extraction of the Fe³⁺/Fe_tot ratio from the pre-edge structure at the Fe K-edge. • Fitting of the pre-edge with 4 Gaussians representative of ferric and ferrous ion. • Calibration line combining the pre-edge analysis and Mössbauer spectroscopy data. • Method suitable to investigate Fe dynamics in complex systems such as amphiboles. - Abstract: A new approach to extract accurate information on the Fe oxidation state has been tested on three different Fe-rich amphiboles annealed at 194–520 °C. The method is based on the analysis of the pre-edge structure of the X-ray absorption spectra at the Fe-K edge. The pre-edge band is deconvolved into four Gaussian components where the integrated intensities of the pairs of lower- and higher-energy bands are related to the ferrous to ferric ion ratio. These Fe³⁺/Fe_tot ratios have been compared with those obtained with independent techniques (X-ray diffraction and Mossbauer spectroscopy) on the same samples. The experimental data allowed establishing a calibration curve that is in good agreement with a recent statistical analysis based on spectroscopic data for the same type of minerals. The method could be extended to the analysis of other complex systems containing both Fe³⁺ and Fe²⁺ in octahedral coordination or to study the dynamics of iron under non-ambient conditions.