Genesis of soils derived from various types of parent rock in Southwestern Japan

Abstract

Abstract Both the primary and the secondary mineralogical compositions of the soils derived from various parent rocks in southwestern Japan were investigated. With regard to the primary mineralogy, the peculiarity of the parent rock was conspicuous. Soils derived from felsic rocks were characterized by the dominance of feldspars, quartz, and mica, while those from mafic rocks by the larger amounts of heavy minerals such as amphiboles, pyroxenes, and magnetite. As regards the clay mineralogy, kaolin minerals were dominant in most of the soils examined. But the peculiarity of the parent rock was also reflected in the clay mineralogy. In the Kagawa soil derived from granite, the presence of interstratified illite-vermiculite regarded as a weathering product of mica was noted. The Aobayama soil derived from andesite and the Matsue soil from andesitic tuff were characterized by the dominance of interstratified halloysite-smectite that are considered to be weathering intermediates in the kaolinization of smectite. The presence of smectite was clearly found in the Ikoma soil derived from gabbro. In the Sekinomiya-2 soil derived from serpentine, chlorite was dominant in the upper part of the profile, while serpentine minerals in the C horizon. However, some soils were considered to have received additions of foreign materials such as volcanic ash or eolian dust, thus casting a doubt on the sedentary nature of these soil.