Oxidation state and coordination of Fe in minerals: An Fe K- XANES spectroscopic study

2001/05/01 by Max Wilke, François Farges, Pierre-Emmanuel Petit +3 · 1,099 citations
Chemistry · Computer Science · Earth and Planetary Sciences · Energy · #Chemistry #Coordination number #Crystal structure #Crystallography #Geochemistry and Geologic Mapping #Geological and Geochemical Analysis #Ion #Iron oxide chemistry and applications #K-edge #Materials science #Metal #Oxidation state #Physics #Spectral line #Trigonal bipyramidal molecular geometry #XANES

paper · doi:10.2138/am-2001-5-612

published in American Mineralogist 86(5-6), 714-730 (Mineralogical Society of America)

openalex publication_date 2001/05/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/02

Abstract

High-resolution Fe K-edge XANES spectra of a series of crystalline Fe 2+ - and Fe 3+ -bearing model compounds were measured in an effort to correlate characteristics of the pre-edge feature with oxidation state and local coordination environment of Fe atoms. The model compounds comprise 30 natural minerals and synthetic compounds, with Fe coordination environments ranging from 4 to 12 O atoms for Fe 2+ , including 5-coordinated trigonal bipyramidal Fe 2+ , and from 4 to 6 O atoms for Fe 3+ . Most pre-edge spectra show two components (due to crystal-field splitting) that are located just above the Fermi level. The most useful characteristics of the Fe-K pre-edge for determining Fe oxidation state and coordination number are the position of its centroid and its integrated intensity. The separation between the average pre-edge centroid positions for Fe 2+ and Fe 3+ is 1.4 ± 0.1 eV. Thus, the position of the pre-edge feature can be used as a measure of the average Fe-redox state, with the average preedge position for mixed Fe 2+ -Fe 3+ compounds occurring between positions for Fe 2+ and Fe 3+ . The lowest pre-edge normalized heights and integrated intensities are observed for the most centrosymmetric sites of Fe, in agreement with previous studies (see Waychunas et al. 1983). Examination of the pre-edge features of mechanical mixtures of phases containing different proportions of Fe 2+ and Fe 3+ suggests that the pre-edge position and intensity for these mixtures can vary quite non-linearly with the average redox state of Fe. However, distinctly different trends of pre-edge position vs. preedge intensity can be observed, depending on the coordination environment of Fe 2+ and Fe 3+ , with an accuracy in redox determination of ±10 mol% provided that the site geometry for each redox state is known. These methods have been used to estimate the Fe 3+ /Fe 2+ ratio in 12 minerals (magnetite, vesuvianite, franklinite, rhodonite, etc.) containing variable/unknown amounts of Fe 2+ /Fe 3+ .

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