EXAFS and XANES Spectroscopy Study of the Oxidation and Deprotonation of Biotite

Bernd Güttler*, Wilhelm Niemann and Simon A. T. Redfern*
* Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, U.K.
Haldor Topsøe Research Laboratories, DK-2800 Lyngby, Denmark

Abstract: The coupled thermal oxidation and deprotonation in air of iron-rich biotite (FeO + Fe2O3 = 34%) has been investigated by EXAFS and XANES spectroscopy at the Fe-K edge and by XANES spectroscopy at the Ti-K edge. Samples annealed for 5 h at temperatures between 250° to 600°C have been studied. Distortions mainly of the Fe-Fe correlation within the octahedral layers are reflected in increasing Debye-Waller factors of the Fe-Fe correlation peak proportional to the annealing temperature. Unchanged Fe-O nearest-neighbour and Fe-Fe next-nearest-neighbour coordination numbers show that these distortions, nonetheless, do not change the structural topology of the octahedral layers. A model is introduced to demonstrate that increasing distortions are compatible with the expected heterogenous deprotonation mechanism in biotite. Titanium occurs in octahedral coordination. It was found to be unaffected by the coupled oxidation/deprotonation process. Both the coordination number and the valence state stay constant during the annealing process, in spite of dramatic changes of the Fe2+/Fe3+ ratio. Thermally activated hopping conduction involving Ti according to Fe2+Ti4+ → Fe3+Ti3+ is, therefore, not a significant process during thermal deprotonation and oxidation in biotite.

Keywords: EXAFS • XANES • deprotonation • layer silicates • biotite

Mineralogical Magazine; December 1989 v. 53; no. 373; p. 591-602; DOI: 10.1180/minmag.1989.053.373.10
© 1989, The Mineralogical Society
Mineralogical Society (www.minersoc.org)