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

Abstract

Abstract--Structural Fe 3+ 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 Fe 3+ ions in disordered local environments, including multimodal site-tosite 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, Fe 3+ ions are equally substituted in sites of axial symmetry (Fe(ii)sites, namely Fe(m a and Fe(mb) which were determined to be the two non-equivalent All and A12 sites of the kaolinite structure. In dickite, Fe 3+ ions were also found to be equally substituted for AP + in the two non-equivalent A1 sites of the dickite structure. In poorly ordered kaolinites, the distribution of the fine-structure parameters indicates that Fe 3+ ions are distributed between Fe(,> 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 Fe 3+ 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.

Combined dating of goethites and kaolinites from ferruginous duricrusts. Deciphering the Late Neogene erosion history of Central Amazonia

“…substituting for Al 3+ in octahedral sites of the structure, appears at low magnetic field (Balan et al, 1999). Two signals, socalled Fe(I) and Fe(II), compose the structural Fe 3+ spectrum (Balan et al, 1999), the relative contribution of which reveals that JIC kaolinite is clearly well crystallized when compared to ARIX2, consistently with the SEM observation of large booklets in JIC. This also indicates that the conditions of formation of these two kaolinites were different, resulting in two distinct generations with a different crystalline order.…”

“…The structural Fe 3+ of kaolinite, i.e. substituting for Al 3+ in octahedral sites of the structure, appears at low magnetic field (Balan et al, 1999). Two signals, socalled Fe(I) and Fe(II), compose the structural Fe 3+ spectrum (Balan et al, 1999), the relative contribution of which reveals that JIC kaolinite is clearly well crystallized when compared to ARIX2, consistently with the SEM observation of large booklets in JIC.…”

The Relationships between Kaolinite Crystal Properties and the Origin of Materials for a Brazilian Kaolin Deposit

“…The Fen) signal in high-defect kaolinites has recently been associated with dickite C layers (Balan et al, 1999). The F%I) spectrum (g = 3.5, 3.7, 4.9, and 9.2) is a consequence of Fe 3 § for A13 § substitution in low-defect kaolinites.…”

Quantitative Measurement of Paramagnetic Fe3+ in Kaolinite

“…7 R2 : XRD disorder index from Li6tard (Cases et al, 1982;Gaite et al, 1993 The kaolinite concentration of the samples varied from --85% (sample GB3) to near 100% (sample DCV). Most of these samples were recently studied by examining the shape of the Fe 3 § EPR signal and the defect nature of the kaolinite (Gaite et al, 1993(Gaite et al, , 1997Balan et al, 1999).…”

“…The present study determines a quantitative approach to measure dilute structural Fe 3+ by combining calculation of X-band EPR spectra (Balan et aL, 1999;Morin and Bonnin, 1999) with data from an Fe3+-doped c~-A1203 standard.…”