Clay mineralogy of Hijiori and Numazawa Ando soils

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Abstract
The clay mineralogy of Hijiori and Numazawa Ando soils was investigated with special reference to the occurrence and weathering of hornblende and biotite in the parent materials. The profiles of both soils underlain by pumice beds were developed from tephras of different ages. The ages of the Hijiori and Numazawa pumice beds were approximately 10,000 and 5,000 years old, respectively. The rock types of all the parent materials belonged to dacite and the existence of relatively large amounts of quartz was noted in almost all of the bulk soil samples. The primary mineral compositions were characterized by large amounts of hornblende in the heavy fractions and small amounts of biotite and bipyramidal quartz in the light fractions. Though all the soil samples had similar primary mineral compositions, they were divided into two groups on the basis of their clay mineralogical compositions. The clay fractions were characterized by the presence of relatively large amounts of opallne silica and vermiculitechlorite intergrades in soil group 1 including younger present-day and shallowly buried soils, and by the notable presence of imogolite and aIlophane in soil group 2 of older buried soils. Detailed investigations on the occurrence and weathering of hornblende and biotite by X-ray and optical methods, indicated that these minerals were not the main origin of 2:1 layer silicate minerals in Hijiori and Numazawa Ando soils.