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Oxidation state and coordination of Fe in minerals; an Fe-XANES spectroscopic study

Max Wilke, Francois Farges, Pierre-Emmanuel Petit, Gordon E. Brown and Francois Martin
Oxidation state and coordination of Fe in minerals; an Fe-XANES spectroscopic study
American Mineralogist (June 2001) 86 (5-6): 714-730

Abstract

High-resolution Fe K-edge XANES spectra of a series of crystalline Fe (super 2+) - and Fe (super 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 (super 2+) , including 5-coordinated trigonal bipyramidal Fe (super 2+) , and from 4 to 6 O atoms for Fe (super 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 (super 2+) and Fe (super 3+) is 1.4+ or -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 pre-edge position for mixed Fe (super 2+) -Fe (super 3+) compounds occurring between positions for Fe (super 2+) and Fe (super 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 (super 2+) and Fe (super 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. pre-edge intensity can be observed, depending on the coordination environment of Fe (super 2+) and Fe (super 3+) , with an accuracy in redox determination of + or -10 mol% provided that the site geometry for each redox state is known. These methods have been used to estimate the Fe (super 3+) /Fe (super 2+) ratio in 12 minerals (magnetite, vesuvianite, franklinite, rhodonite, etc.) containing variable/unknown amounts of Fe (super 2+) /Fe (super 3+) .


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 86
Serial Issue: 5-6
Title: Oxidation state and coordination of Fe in minerals; an Fe-XANES spectroscopic study
Affiliation: Universitede Marne la Vallee, Laboratoire des Geomateriaux, Marne la Vallee, France
Pages: 714-730
Published: 200106
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 101
Accession Number: 2001-049503
Categories: General mineralogy
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables
Secondary Affiliation: Stanford University, USA, United StatesEuropean Synchrotron Radiation Facility, FRA, FranceUniversite Paul Sabatier, FRA, France
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 200115
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