Abstract
A mechanism of shape transformation from spheres to tubes in halloysite is proposed from the standpoint of curvature elasticity theory, using the size and morphological data of samples separated from chronological series of tephras in Hachijo Island. Transmission electron microscopic observations indicated that when the diameters of halloysite particles exceeded about 200nm, the shape showed prominent deviation from spherical to tubular and/or partially curled platy ones. This fact leads to that the spherical halloysite with 200nm in diameter is subjected to the curvature elastic energy of about 9kJ/m^2 due to the anti-bending of layers, which is an estimate using approximately the elastic constants of kaolinite and taking into account a preferable curvature for a single layer of halloysite. The curvature elastic energy acts as a driving force for the shape transformation. In reality irreversible dehydration process plays a trigger to transform such flustrated spherical halloysites to more stable shapes such as tubular ones.
