Synthetic Kaolinites Doped with Fe²⁺ and Fe³⁺ Ions

Abstract

Abstract--Previous studies by Electron Spin Resonance (ESR) have established the substitution of Fe 3+ and Mg 2+ in the kaolinite structure. It is shown that Fe E+ can substitute in kaolinite and stabilize defects which are detectable by ESR in a manner identical to Mg 2+. The development of methods of preparing a synthetic kaolinite doped with Fe 2+ is described in detail. It is shown that the main ESR signals, which occur at g = 2.0 in natural kaolinites and which previously have been interpreted in terms of iron and magnesium, can be attributed to iron alone.

Application of Electron Paramagnetic Resonance and Mössbauer Spectroscopy in the Investigation of Kaolinite-Group Minerals

“…Nacrite contains iron in both the ferric and ferrous states. Although the EPR study did not determine whether Fe 3+ was present in the tetrahedral or octahedral layer, isomer shifts of the M6ssbauer spectra prove that Fe 3+ has octahedral symmetry in all of the samples studied, in agreement with results reported elsewhere (Malden and Meads, 1%7;Meads and Malden, 1975;Angel et al, 1977). Thus, in the minerals of the kaolinite group, Fe 3+ occurs in ionic substitution for A13+ in octahedral sheets.…”

“…Electron paramagnetic resonance (EPR) and Mrssbauer spectroscopy have been used frequently to study Fe 3+ in kaolinite (Boesman and Schoemaker, 1961;Jones et al, 1974;Meads and Malden, 1975;Jefferson et al, 1975;Angel et al, 1977). The present work attempts to relate the type of EPR spectrum observed to the form of iron in the kaolinite (iron in ionic substitution; adsorbed iron on the surface of kaolinite plates; separate iron minerals as impurities).…”

Hydrothermal (200°C) synthesis and crystal chemistry of iron-rich kaolinites

“…Although previous authors (Angel et al, 1977;Cuttler, 1980;Calvert, 1981) have recorded incorporation of Fe z+ in the kaolinite structure, M~ssbauer spectroscopy has shown that synthesis of ferric kaolinites is possible under strictly oxidizing conditions. No trace of Fe 2+ was found in any of the experiments, and no pure ferric phase could be detected.…”

“…Angel et al (1975) noticed that acid conditions favoured a better kaolinite morphology and that crystallinity decreased for small concentrations of iron (0.05%) and increased for higher concentrations (0.5-1%). Angel et al (1977) and Cuttler (1980) synthesized 57Fe-substituted kaolinites for examination by M6ssbauer spectroscopy, and found that the iron substituted in the kaolinite structure, especially as FEZ+; it substitutes trioctahedrally in the gibbsite-like sheet with 3 Fe 2+ replacing 2 A13+ to maintain charge neutrality, even after heating the starting material in oxygen to 1400~ Calvert (1981 also concluded that a very small quantity of the iron used for synthesis was incorporated into the kaolinite structure, and that it was mainly ferrous.…”

ESR and IR Evidence for Chromium in Kaolinites

“…The studies of Angel et al (1974), Angel et al (1977 and Cuttler (1980Cuttler ( , 1981 of synthetic kaolins doped with divalent ions have shown that it is possible to produce similar centres by X-irradiation. Muller & Calas (1989) and Muller et al (1990Muller et al ( , 1992 have shown clearly the existence, for natural kaolinites, of a relation between this signal and natural radioactivity.…”