Vermicular gibbsite aggregates found in the weathered pumice layer (the Kurayoshi pumice) at Yotsuzuka, western part of Tottori Prefecture, range in size from 5 to lOmm (Fig.1). They are more or less twisted and show some cleavages perpendicular to the long axis. Vermicular gibbsite are milky white colored and usually coated with dark brown color. X-ray diffraction patterns show strong renections for gibbsite, kaoline minerals, hydrobiotite and a relatively small amount of hydrated halloysite (Fig.3), Kaoline minerls are rather abundant at the white part. On the other hand, the dark brown part is rich in hydrobiotite. Differential thermal analysis shows the endothermic reaction for the dehydration of gibbsite at 310℃ and of kaoline minerals, hydrated halloysite and hydrobiotite at 550℃ (Fig.4). Electron microscopy shows that the gibbsite consist of peculiarly warped rods (Plate 1). The clay minerals which are looked like
"cracked rice grain" are seemingly kaoline minerals or hydrated halloyslte. By the electron microprobe scanning across the vermicular gibbsite, the triform structure is revealed, such as the gibbsite rich part, the hydrobiotite rich part and kaoline minerals rich part (Fig.5 and 6). Gibbsite and clay minerals (hydrobiotite or kaoline minerals) grow alternately zone by zone (Fig.7). The width of each zone ranges in size from 20 to 40 micron.
