Euroclay 1999 : conference of the European clay groups association : program with abstracts

Trivalent iron is the most common impurity in kaolinites. X-band Electron Paramagnetic Resonance (EPR) spectra of natural kaolinites exhibit three superimposed signals due to paramagnetic Fe(3+) ions located in distorted sites and a broader signal related to superparamagnetic nanocrystalline Fe oxides. Paramagnetic Fe(3+) signals are referred to as Fe(I), Fe(II)A, and Fe(II)B. The Fe(II) signals correspond to Fe(3+) substituted at Al(3+) crystallographic sites Al1 and Al2 in the kaolinite structure whereas the Fe(I) signal occurs in the spectra of poorly ordered kaolinites. We present a detailed EPR investigation of kaolinites and dickites from different environments at 9 GHz (X-band) and 34 GHz (Q-band) which : (i) shows that the Fe(I) signal is due to the occurrence of dickite-like sequences in the structure of poorly ordered kaolinites, (ii) allows quantification of the distribution of Fe(3+) between isolated ions and concentrated phases. Using an inversion procedure based on the calculation of X and Q-band EPR spectra, we determined the distribution of fine-structure parameters related to the stacking disorder inkaolinites. This distribution indicates that, in poorly ordered kaolinites, Fe(3+) ions are distributed between Fe(II) sites and other sites with the symmetry of the dickite sites. Calculation of the X-band spectrum of kaolinites, and comparison with the EPR spectrum of an Al2O3 sample doped with Fe(3+) as an external standard, allowed a quantitative assessment of site occupancies regardless the stacking order of the kaolinite. (Résumé d'auteur