Mineral composition and their origin of forest soils developed on Sedimentary rocks in the southern Yamizo mountainous.

摘要

The mineral composition of forest soils collected form southern Yamizo mountains were determined using X-ray diffraction on 6 grain size classes. The bedrock of the sampling sites consists of Jurassic sandstone, chert and Miocene dacitic lithic tuff. Pleistocene tephra and eolian deposits. Fine particles deposited on joints of weathered bedrock were also taken to identify the mineral composition. The result are as follows. For soils developed in sandstone dominated areas, observed minerals are quartz, plagioclase, K-feldspar and illite in a wide range of grain size classes from clay to granule fraction. Chlorite only occurs in the silt and fine sand size classes, and hornblende in the fine sand size class. For soils in chert dominated areas, quartz occurs in each grain size class, chlorite only in the silt size, and hornblende and magnetite in the coarse and fine sand size classes. For soils in dacitic tuff dominated areas, quartz, plagioclase and K-feldspar in all grain sizes, and hornblende, pyroxene and magnetite in the coarse and fine sand size classes. Based on the relationship between mineral occurrence and grain size, we conclude that several sources contributed in the forest soils. There are: (1) Weathering products of bedrock, such as quartz, plagioclase, K-feldspar and illite in sandstone areas, quartz in chert areas, and quartz and feldspars in dacitic tuff areas, (2) particles that originate from regional tephras, like hornblende, pyroxene and magnetite observed in the coarse and fine sand size in all bedrock types, (3) long-range transported eolian dust such as chlorite in the fine sand size fraction in most forest soils.