An unusual form of kaolin mineral, filling cracks in weathered prophyritic granite in the county of Piedade, S>o Paulo, Brazil, consists of visibly fibrous material which, in the electron microscope, appears as thin tubular fibers of considerable length; platy particles are almost entirely absent. X-ray rotation diagrams of dried fiber bundles and selected area electron diffraction patterns of single fibers show an appreciable degree of structural regularity in the kaolin layer structure, with b as the fiber axis. The results show general agreement with previous data by Honjo, Kitamura and Mihama. Prior to drying, the material as collected can be described as consisting of ⅔ endellite and ⅓ halloysite. The endellite component has the swelling and shrinking behavior characteristic of this mineral with respect to ethylene glycol, water, and gentle heat treatments. This may be the first observation of a mineral of endellite-halloysite type existing in a directly visible fibrous form.

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