The scanning electron microscopic studies of the clay minerals have usually been made on air-dried samples. The air-drying method is, however, not always preferable for H(2)O rich clay minerals such as imogolite because of their strong contraction in the dehydration process. The present writers examined the critical point drying for H(2)O rich clay mineral (imogolite) and proved that this drying method is excellent in preservation of natural surface of the minerals than that of air-drying. Imogolite which examined was separated from Kurayoshi pumice beds. The samples were treated first with OsO(4) solution and then were dehydrated progressively with ethanol. After completing above treatments, the imogolites which sealed in vessel were dried successively at critical point of CO(2). The samples were mounted onto brass stubs and coated first with carbon and then with Au-Pd alloy. The Japan Electron Optics Laboratory Co. scanning electron microscope Model JSM-50A which was installed in the Division of Rehabilitation Medicine in the present Institute was used at the operating conditions: accelerating voltage 25kV, magnifications ranging from 3000 to 40000. The scanning electron micrographs of the air-dried imogolite are shown in plate I, and cry tical point dried ones are in plate II and III. The former shows twig-like shape which indicated the effect of contractility, whereas the latter exhibits softly curled and entangled threads. The diameter of these threads ranges from 280A to 560A
