長崎

Red-Yellow soils are widely distributed on the mountains, hills, and plateaus in Nagasaki Prefecture, located at the most western part of Japan, and used as an important agricultural land. They are derived from various kinds of parent rock including basalt, andesite, andesitic tuff breccia, welded tuff, Tertiary sedimentary rock, and crystalline schist. The purpose of the present study is to investigate the clay mineralogical composition of the Red-Yellow soils in relation to their parent rocks. The clay fraction (< 2 μm) was separated by repeated sonification-sedimentationsiphoning at an alkaline medium after H&-treatment. The clay mineralogical composition of the clay fraction was analyzed by the X-ray diffraction method. Free iron oxides of the clay fraction was determined by the DCB method, gibbsite by the differential thermal analysis, and amorphous materials by the acid oxalate method. Total content of the free iron oxides, gibbsite, and amorphous materials was estimated to be 10 to 20% at the most. The mineralogical composition of layer silicates in the clay fraction was closely related to the parent rocks. Most of the Red-Yellow soils derived from basalts were mainly composed of kaolin mineral (mostly halloysite) or kaolin mineral and 2:1 mineral (vermiculite or smectite)-chlorite intergrade. Small numbers of the basalt-derived soils contained kaolin mineral and smectite as principal clay minerals. The Red-Yellow soils derived from andesites and andesitic tuff breccias were dominated by kaolin mineral or kaolin mineral and 2:1 mineral-chlorite intergrade, similar to those of the soils derived from basalts. The principal clay minerals of the Red-Yellow soil derived from welded tuff was vermiculite-chlorite intergrade and mica accompanied by chlorite, vermiculite, and kaolin mineral. The Red-Yellow soils derived from Tertiary sedimentary rocks were mainly composed of kaolin mineral (kaolinite) and 2:1 mineral-chlorite intergrade. Smectite was also contained in the soils derived from the sedimentary rocks of marine-origin. The Red-Yellow soils derived from crystalline schists contained mica, chlorite, kaolin mineral, or vermiculite-chlorite intergrade. The composition of principal clay minerals of the soils varied from site to site. Some of the basalt-derived Red-Yellow soils distributed in the northern part of Nagasaki Prefecture contained mica and/or chlorite as principal clay minerals. Mica and chlorite are not originally contained in basalt and also not newly formed in the soil, leading to the possibility that the mica and chlorite is eolian-origin. Thus it is considered that the basalt-derived Red-Yellow soils containing mica and/or chlorite in a considerable amount were more or less influenced by the tropospheric dust. The clay content of the Red-Yellow soils was affected by the parent rock followed by the clay mineralogical composition, regional division, and landform. The parent rock is a determining factor of both type and amount of clay minerals of the Red-Yellow soils in Nagasaki Prefecture.