Electron spin resonance studies of doped synthetic kaolinite. II

Abstract

It is shown from studies of synthetic kaolinites doped with Fe³⁺ that the ESR spectrum of kaolinite at g = 4 consists of two components attributed to Fe³⁺ ions occupying two distinct substitutional sites (Centres I and II). The relative intensity of the two components can be correlated with the X-ray crystallinity of the samples. The ESR spectrum is influenced by artificial changes in crystallinity produced by subjecting samples to high stresses or by intercalation. It is concluded that Centre II is due to Fe³⁺ replacing Al³⁺ in the octahedral layer in a region of high crystallinity. Centre I is assigned to Fe³⁺ occupying Al³⁺ sites distorted by changes in the hydroxyl orientations associated with layer stacking disorder or disruption of interlayer bonding.A stable defect centre in kaolinite which produces an ESR spectrum at g = 2·0 is attributed to either a hole centre located on an Si—O bond adjacent to an Mg²⁺ octahedral impurity or to an O₂⁻ ion trapped within the lattice.