Dehydration mechanism of interlayer water of halloysite heated at various temperatures was investigated by X-ray powder diffraction and weight loss analysis. Itwas found that halloysite 10Å changed to halloysite 7Å coherently and they were segregated each other during the dehydration. The idealized structure model in which dehydration progressed preferably perpendicular to the layers of each crystallite was considered. Reaction above 55°C was able to be approximated by the zero order which was expected from the above dehydration model, whereas it was influenced by the diffusion from the inner part of the particles formed by gathering many crystallites below 55°C. The activation energies obtained from the data of the temperature above and below 55°C were 6.8 and 11.9Kcal/mol, respectively.
