Electron spin resonance in natural kaolinites containing Fe³⁺ and other transition metal ions

Abstract

A number of natural kaolinites from a variety of world sources have been investigated using electron spin resonance at both X-band and Q-band. The results show systematic differences some of which are related to the crystallinity of the material, some to the presence of transition metal ions other than Fe3+ and hole-trapping defects.The results provide evidence for at least three sites for substitution of Fe3+ having large crystal fields : (i) a site with near maximum rhombic character having crystal field parameters D > 1·2, λ = ⅓ the occurrence of which is correlated with lack of crystalline perfection due to stacking defects or to the proximity of surfaces, (ii) a site with less rhombic character with λ = 0·234, D = 0·585 as likely parameters, and (iii) a site with parameters near the values λ = 0·207, D = 0·322.Mössbauer and ESR evidence suggests that the principal sites of Fe3+ substitution are octahedral. The presence of Fe3+ in adjacent cation sites leads to a very broad resonance centred near geff = 2. Other resonances in the spectra are attributed to the effect of trapped holes, some at least of which are situated at oxygen sites adjacent to Al3+ ions substituting in sites normally occupied by Si4+.Hyperfine effects due to the presence of the transition ions Mn2+ and (VO)2+ are also observed.

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

“…Low temperature 93 K recording was provided using a nitrogen-flow device, cooled by liquid nitrogen. This resulted in strong enhancement of the RID spectral intensity with respect to the background spectrum due to residual superparamagnetic iron-oxides (Bonnin et al 1982) and of the hyperfine structure of B-center (Meads and Malden 1975;Muller and Calas 1989;Clozel et al 1994).…”

“…They are trace elements (substituted Fe 3 § sorbed Mn 2+ and VO a § ions) together with defect centers (e.g., reviews by Hall 1980;Pinnavaia 1981;Muller and Calas 1993). Defect centers are detected in all natural kaolinites (Meads and Malden 1975;Muller and Calas 1993). They have been interpreted as positive holes trapped on oxygen atoms, stabilized by diamagnetic precursors Meads and Malden 1975;Cuttler 1980).…”

“…Defect centers are detected in all natural kaolinites (Meads and Malden 1975;Muller and Calas 1993). They have been interpreted as positive holes trapped on oxygen atoms, stabilized by diamagnetic precursors Meads and Malden 1975;Cuttler 1980). These hole centers have been demonstrated to be radiation-induced defects (RID), through irradiation experiments and studies of various natural alteration systems Clozel 1991;Muller et al 1990Muller et al , 1992.…”

Mineralogical characterization and beneficiation study of kaolin from Equador (RN) and Junco do Seridó (PB) to increase the brightness index

“…6) contained, in the region between 50 and 200 mT, resonance signals at 74 mT and in the triplet 135, 156 and 182 mT [10], indicating that the iron (III) ions were distributed in the internal phase substituting the aluminum ions in the octahedral sites of the kaolinite structure. Nevertheless, this phase did not show any alteration during the beneficiation and its signals were less intense, corroborating the results of the Hinckler index, indicating that the presence of iron (III) ions was not significant.…”

“…The electron paramagnetic resonance spectra were obtained with a Bruker ESP300E spectrometer with X-band of 9.75 GHz. The quantitative analysis was done with the absolute area percentage of the sigmoid between 200 and 400 mT regarding the area between 100 and 500 mT of the respective spectrum [10].…”