Electron spin resonance studies of doped synthetic kaolinite. II

Abstract

Abstract It is shown from studies of synthetic kaolinites doped with Fe 3+ that the ESR spectrum of kaolinite at g = 4 consists of two components attributed to Fe 3+ 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 Fe 3+ replacing Al 3+ in the octahedral layer in a region of high crystallinity. Centre I is assigned to Fe 3+ occupying Al 3+ 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 Mg 2+ octahedral impurity or to an O 2 − ion trapped within the lattice.