New Data on Thermal Decomposition of Kaolinite and Halloysite

Using single-crystal electron diffraction data it has been possible to demonstrate more directly the existence of considerable order in dehydroxylated kaolinite and halloysite. The two-dimensional (a-b) plane structure of dehydroxylated kaolinite is very similar to that of kaolinite. This phase, appropriately described by the term “metakaolinite,” decomposes first at about 825°C., γ-Al2O3 being the chief crystalline product. In the case of halloysite the γ-Al2O3 appears first at about 650°C. The products observed on the dehydroxylation of kaolinite depend to a large extent on the particle size of the sample used. Electron microscope studies of dehydroxylated material and the same material treated under very mild hydrothermal conditions show that the main phenomena accompanying dehydroxylation apparently are disalignment along the c-axis, the appearance of discontinuities in the sheets themselves (not visible directly in the crude image of the electron microscope), and the reduction of the effective particle size. The cause of the exothermic peak cannot be the formation of γ-Al2O3 but is assigned to the formation of mullite which is not necessarily detected in an X-ray powder pattern.