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Iron oxidation state in phyllosilicate single crystals using Fe-K pre-edge and XANES spectroscopy; effects of the linear polarization of the synchrotron X-ray beam

Manuel Munoz, Olivier Vidal, Clement Marcaillou, Sakura Pascarelli, Olivier Mathon and Francois Farges
Iron oxidation state in phyllosilicate single crystals using Fe-K pre-edge and XANES spectroscopy; effects of the linear polarization of the synchrotron X-ray beam
American Mineralogist (July 2013) 98 (7): 1187-1197

Abstract

We investigated the influence of linear polarization of the synchrotron X-ray beam on the determination of iron oxidation state in phyllosilicates. Fe K-edge XANES spectra and pre-edge peaks have been recorded for various orientations of single crystals of biotite (Bt), clinochlore (chlorite group; Cli), talc (Tc), and antigorite (serpentine group; Ant). Ab initio XANES calculations, performed for 6 orientations of the biotite structure, support the experimental results. Depending on crystal orientation, the experimental results show, (1) important changes both for XANES and pre-edge peaks, (2) characteristic changes of spectral signatures regardless of the mineral species, (3) uncorrelated changes between XANES and pre-edge peaks, and (4) important changes of the energy position of pre-edge peaks, but with no significant change for their integrated areas. Regarding the crystal orientation, the pre-edge peak centroid varies in energy by approximately 0.4 eV in the case of Bt and Tc, and by approximately 0.2 eV for Cli and Ant. Such variations correspond to XFe (super 3+) (i.e., Fe (super 3+) /Fe (sub total) ) of 0.22 and 0.15, respectively. Comparison with the analysis of powdered samples show that Fe-redox can be generally framed as follows: -(2/3)XFe (super 3+) < XFe (super 3+) (sub (powder)) < +(1/3)XFe (super 3+) . In good agreement with the so-called "magic angle" theorem for powdered samples, we propose an ideal orientation of the single crystals that provide similar pre-edge peaks as for powdered samples. With the wavevector (i.e., beam direction) perpendicular to (100) or (010), measurements should be done with an angle of 35 degrees between the electric-field vector and the [001] crystal direction. Moreover, measurements performed with the wavevector perpendicular to (001) systematically result in an overestimation of XFe (super 3+) up to 0.07. Finally, we show that the appropriate positioning of single crystals reduces the XFe (super 3+) uncertainty to the intrinsic error of pre-peak measurements. This approach opens possibilities for in situ analyses of Fe-bearing phyllosilicates in thin sections that are potentially relevant for scientific fields such as hydrothermal/metallogenic, metamorphic, meteoritic or environmental mineralogy.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 98
Serial Issue: 7
Title: Iron oxidation state in phyllosilicate single crystals using Fe-K pre-edge and XANES spectroscopy; effects of the linear polarization of the synchrotron X-ray beam
Affiliation: Universite de Grenoble 1, Institut des Sciences de la Terre, Grenoble, France
Pages: 1187-1197
Published: 201307
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 49
Accession Number: 2013-061390
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables
Secondary Affiliation: European Synchrotron Radiation Facility, FRA, FranceMuseum National d'Histoire Naturelle, FRA, France
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201337
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