Structural Fe3+ in Natural Kaolinites: New Insights from Electron Paramagnetic Resonance Spectra Fitting at X and Q-Band Frequencies

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Abstract
Structural Fe3+ in kaolinites and dickites covering a broad range of disorder was investigated using electron paramagnetic resonance (EPR) spectroscopy at both the X and Q-band frequencies. A procedure based on a numerical diagonalization of the spin Hamiltonian was used to accurately determine the second and fourth-order fine-structure parameters. A least-squares fitting method was also developed to model the EPR spectra of Fe3+ ions in disordered local environments, including multimodal site-to-site distributions. Satisfactory fits between calculated and observed X and Q-band spectra were obtained regardless of the stacking order of the samples.In well-ordered kaolinite, Fe3+ ions are equally substituted in sites of axial symmetry (Fe(II)sites, namely Fe(II)a and Fe(II)b) which were determined to be the two non-equivalent Al1 and Al2 sites of the kaolinite structure. In dickite, Fe3+ ions were also found to be equally substituted for Al3+ in the two non-equivalent Al sites of the dickite structure. In poorly ordered kaolinites, the distribution of the fine-structure parameters indicates that Fe3+ ions are distributed between Fe(II) sites and other sites with the symmetry of the dickite sites.Hence, when stacking disorder prevails over local perturbations of the structure, the near isotropic resonance owing to Fe3+ ions in rhombically distorted sites (Fe(I) sites) is a diagnostic feature for the occurrence of C-layers in the kaolinite structure, where C refers to a specific distribution of vacant octahedral sites in successive layers.