Crystal chemistry of iron in natural grandidierites: an X-ray absorption fine-structure spectroscopy study |
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| Authors: | F Farges |
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| Institution: | Laboratoire des géomatériaux, Université de Marne-la-Vallée, 77454 Marne la Vallée Cedex, France?E-mail: farges@univ-mlv.fr, FR
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| Abstract: | Fe–K edge XAFS spectra (pre-edge, XANES and EXAFS) were collected for eight grandidierites from Madagascar and Zimbabwe,
as well as for Fe(II) and Fe(III) model compounds (staurolite, siderite, enstatite, berlinite, yoderite, acmite, and andradite).
The pre-edge spectra for these samples are consistent with dominantly 5-coordinated ferrous iron. The analysis of the XANES
and EXAFS spectra confirms that Fe(II) substitutes for Mg(II) in grandidierite, with a slight expansion of the local structure
around Mg by ∼2%. In addition, ferric iron was also detected in some samples 5–10 mol% of the total Fe or 500–1100 ppm Fe(III)].
Based on theoretical calculations of the EXAFS region, Fe(III) appears to be located in the 5-coordinated sites of Mg(II)
or in the most distorted 6-coordinated sites of Al (depending on the sample studied). Special attention is therefore required
when using grandidierite as a model for ferrous iron in C3v geometry, because of the possible presence of an extra contribution related to Fe(III). This additional contribution enhances
significantly the Fe–K pre-edge integrated area +40% for 1000 ppm Fe(III)]. Therefore, only a few grandidierite samples can
be used as a robust structural model for the study of the Fe(II) coordination in glasses and melts.
Received: 26 June 2000 / Accepted: 19 February 2001 |
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| Keywords: | Iron Grandidierite Redox XANES EXAFS |
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