Fe-Speciation in Kaolins: A Diffuse Reflectance Study

Abstract

Abstract--Diffuse reflectance spectra of kaolins have been recorded in samples from different environments. They show the systematic presence of Fe-oxides, even in bleached kaolins, with no contribution from the Fe 3+ ions substituted in kaolinite. Second derivative spectra of various Fe-phases (hematite, goethite, lepidocrocite, maghemite, akaganeite, ferrihydrite and Fe-polymer) may be differentiated by the position of a diagnostic band corresponding to the 2QA0 ~ 2(4T~(4G)) transition. The systematic comparison of diffuse reflectance spectra of unbleached and bleached kaolins has demonstrated the differences between the Fe-oxides occurring as coatings and as occluded phases. The features observed in second derivative spectral curves are consistent with assignments of crystal field transitions to goethite, hematite, akaganeite, and aged hydrous ferric oxides. The optical determination of the Fe-phases associated to kaolins assists in the interpretation of the formation conditions of these minerals.

Use and Limitations of Second-Derivative Diffuse Reflectance Spectroscopy in the Visible to Near-Infrared Range to Identify and Quantify Fe Oxide Minerals in Soils

“…The difference in EPT and color between hematite and the other Fe oxides has been explained by the presence of face-sharing octahedra in the hematite structure leading to an increase of the magnetic coupling between neighboring Fe(III) centers (Sherman and Waite 1985). In contrast to the results of Malengreau et al (1994), however, the ranges of the EPT of all other Fe oxides generally overlapped, that is, the discrimination of these Fe oxides could not be performed with the EPT only.…”

“…To determine the positions of these bands, the resolution may be mathematically enhanced by calculating the derivatives of the measured spectra (Huguenin and Jones 1986). Deaton and Balsan (1991) used the first derivative, and Kosmas et al (1984Kosmas et al ( , 1986 and Malengreau et al (1994Malengreau et al ( , 1996 used the second derivative to detect and quantify different Fe oxides in soils or mineral mixtures. In a similar approach, Coyne et al (1990) found that second-derivative spectra were superior to first derivative or untransformed spectra in predicting the amount of structural or interlayer Fe(HI) in montmorillonite.…”

“…Recent studies have suggested very low detection limits for Fe oxides and the possibility of unequivocally differentiating between the various Fe oxides based on crystal field band positions (Malengreau et al 1994(Malengreau et al , 1996. Since these studies were carried out for relatively simple mineral mixtures and with a limited number of Fe oxide samples, a closer examination of the potential of second-derivative reflectance spectroscopy seemed necessary.…”

Nature and Stability of Radiation-Induced Defects in Natural Kaolinites: New Results and a Reappraisal of Published Works

“…Strong baseline perturbations arose from the presence of iron oxides which are always associated with natural kaolinites (Malengreau et al 1994). These oxides are responsible for a broad resonance (AB > 1000 G) which superimposes on the investigated EPR signal, whenever the oxides occur as distinct phases or as coatings (Angel and Vincent 1978;Bonnin et al 1982).…”

Quantitative Measurement of Paramagnetic Fe3+ in Kaolinite

“…Nanometric domains of concentrated Fe 3+ ions exhibit superparamagnetic or antiferromagnetic character. This is characterized by a broad line at high magnetic field, the intensity of which decreases with decreasing temperature, Diffuse reflectance spectroscopy (DRS) suggests that iron oxide or oxy-hydroxide nanophases may be responsible for this latter EPR response (Malengreau et al, 1994). Magic-angle spinning nuclear magnetic resonance studies by Schroeder and Pruett (1996) and Schroeder et aL (1998) also suggested that the ordering pattern of Fe 3+ between "dilute-Fe" and "clustered-Fe" may vary between different kaolinite samples.…”