This thesis is divided into six chapters. In the first chapter relevant previous studies of kaolinite are summarised and in particular a review of past electron spin resonance studies of kaolinite is included. The experimental results are presented in four chapters. Chapter 2 describes an investigation of the vanadium impurities associated with natural kaolinites from Georgia, U.S.A. By following the changes in the electron spin resonance spectra of these samples with magnetic separation, thermal and chemical treatments, preferential orientation and relative humidity, it is shown that vanadium is substituted in the kaolinite structure, probably in the octahedral layer, as V4+ ions. The synthesis and electron spin resonance study of vanadium-doped kaolinite is reported in chapter 3. I t is shown that vanadium is substituted in the tetrahedral layer of these kaolinites and also that vanadium may be adsorbed on the surface of kaolinite. It is demonstrated that the comparison of electron spin resonance spectra of kaolinite samples equilibrated in atmospheres with low and high values of relative humidity provides a convenient method to distinguish between vanadium substituted in the structure and vanadium adsorbed on the surface. Chapter 4 describes the synthesis of kaolinites doped with iron (II) ions. It is shown that ions of this type can stabilise paramagnetic defects produced by X-irradiation and reproduce the asymmetric two-line resonance at g = 2 which is common to most natural kaolinites. An investigation using electron spin resonance that identifies different types of iron oxide phases associated with the surfaces of two groups of kaolinites from England and the United States is described in chapter 5. It is shown that the English kaolinites are coated with a lepidocrocite-like phase which is readily removed by de Endredy's method of deferrification. In contrast the American kaolinites are coated with a hematite- or goethite- like phase which is not removed by similar treatment. Throughout the course of this work, the effects of the various physical and chemical treatments on the brightness values of the kaolinites were examined. The results are summarised and discussed in chapter 6 and ideas for further work are suggested.

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