Structural Fe3+ in natural kaolinites:: New insights from electron paramagnetic resonance spectra fitting at X and Q-band frequencies

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 All and A12 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.