Abstract
Clay mineralogical studies were made of a yellow orange colored soil in the Ishigaki shima Is., Okinawa pref., where the humid sub-tropical climate prevails. The soil is located on the slope (altitude 200 m) of the Mt. Omotodake (525.8 m), and has been derived from biotite granite. The soil is “moist” enough, this being particularly true of the B2 horizon which is derectly underlaid by an impervious bedrock. However the soil has not the characteristics of hydromorphic soils (Pseudogley, Fragiaquult, etc.). The clay minerals were identified by the X-ray, thermal analysis and electron microscope observation after the treatments by Norrish and Taylor's method, Mehra Jackson's method and after heating treatments (180 and 350°).
It was found from the studies that the clays contain an appreciable amount of lepidocrocite (α-FeOOH) together with goethite (γ-FeOOH). Crystalline iron hydroxides (lepidocrocite plus goethite) amount to about 8 percent in the clay fractionsof the A1, B1 and B2 horizons. It is interesting to note that soil lepidocrocites occur in terrestrial soil without any features as associated with wetness because it has so far been reported to form in a hydromorphic soil environments. The mechanism of lepidocrocite formation in soil is get to be investigated hereafter.
