EXAFS and XANES spectroscopy study of the oxidation and deprotonation of biotite

The coupled thermal oxidation and deprotonation i air of iron-rich biotite (FeO + Fe203 = 34%) has been investigated by EXAFS and XANES spectroscopy atthe Fe-K edge and by XANES spectro-scopy at the Ti-K edge. Samples annealed for 5 h at temperatures between 250 ~ to 600 ~ 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 temper-ature. Unchanged Fe-O nearest-neighbour and Fe-Fe next-nearest-neighbour c ordination umbers 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 coordina-tion. It was found to be unaffected by the coupled oxidatio~/deprotonation pr cess. Both the coordina-tion number and the valence state stay constant during the annealing process, in spite of dramatic changes of the Fe2+/Fe 3+ ratio. Thermally activated hopping conduction involving Ti according to Fe2+TP + ~ Fe3+Ti 3+ is, therefore, not a significant process during thermal deprotonation a d oxidation in biotite