Paramagnetic Fe³⁺: A Sensitive Probe for Disorder in Kaolinite

Abstract

Abstract--The Fe 3+ substituted for AP + at the 2 octahedral positions is one of the most common impurities in the kaolinite structure detected by electron paramagnetic resonance (EPR). Evidence has been provided for a relationship between the shape of EPR spectra for structural Fe and the structural disorder in kaolinite. It is proposed that the structural Fe be used as a sensitive probe for the degree of disorder of natural kaolinites. With this aim in view, an EPR disorder index (E) is defined from the width of selected EPR lines. Using reference kaolinites, it is shown that this index can account as well for longrange disorder detected by means of X-ray diffraction (XRD) as for local perturbations such as radiationinduced defects (RID). It is shown that the disorder observed through EPR has some points in common with the XRD-measured one. The influence on E of the presence of RID is shown by the study of artificially and naturally irradiated kaolinites.

The Characterization and Quantitative Analysis of Clay Minerals in the Athabasca Basin, Saskatchewan: Application of Shortwave Infrared Reflectance Spectroscopy

“…3a). The Hinckley index has been used widely to represent the structural disorder of kaolinite (Brindley et al 1986, Artioli et al 1995 and has been linked variably to vacancy displacements within the sheet of octahedra (Plançon & Tchoubar 1977) or substitutions of Fe 3+ impurities in the octahedral sites (e.g., Brindley et al 1986, Delineau et al 1994, Mosser et al 1996, Gaite et al 1997. The weak negative correlation between the total iron content as Fe 2 O 3 and the Hinckley index in the Athabasca kaolinite (Fig.…”

“…Similarly, Brindley et al (1986) showed that there are systematic relationships between the Hinckley index of kaolinite and the FTIR absorption features (i.e., frequency and half-width of the hydroxyl-stretching band). It should be emphasized that infrared spectra are influenced mainly by short-range order and are relatively insensitive to long-range order in minerals, whereas XRD patterns are more sensitive to long-range order (e.g., Gaite et al 1997). Therefore, information about the kaolinite structure derived from infrared spectroscopy is not expected to be identical to that from XRD (Fig.…”

Physical and chemical study of lattice kaolinites and their interaction with orthophosphate

“…Therefore, it is not only the structural diff erences of kaolinites caused by defects and diff erent octahedral shapes (Heller-Kallai and Rozenson 1981). Actually, the paramagnetic behavior observed reveals that kaolinite contains structural defects in its crystal lattice due to isomorphic substitution by Fe 3+ , causing disorder and changing the spin-spin interaction making the paramagnetic (Gaite et al 1997). Furthermore, the Mössbauer spectra are distinguishable showing polytypic modifi cations of kaolinite with diff erent line-widths.…”

Hyperspectral analysis of clay minerals

“…An additional peak at the 1.9 μm region in some illite separates is related to adsorbed water. It should be emphasized that infrared spectra are influenced mainly by short-range order and are relatively insensitive to long-range order in minerals, whereas XRD patterns are more sensitive to long-range order (e.g., Gaite et al 1997). Therefore, information about the clay mineral structure derived from infrared spectroscopy is not expected to be identical to that from XRD.…”