Abstract
Weathering state of the diluvial sediments widely distributed in northwest Taiwan where the climate is subtropical hydrometeor was investigated mineralogically and chemically. Some observed varieties on the clay mineral composition and the chemical properties of weathered sediments were attributed to the differences of topographical location of sediments.
1) When the sediments are located in the well-drained slopes or the top of the diluvial plateau, the clay fraction of the weathered sediments which are colored in red shows the praperties of about 5 meq/100g-clay in CEC, high degree of base unsaturation, strong acidity (pH 4.5) and about 2.2 in SiO2/Al2O3 molar ratio. The clay is composed of illite, hydrous illite, kaolin, hydroxy-Al interlayered 14Å mineral and about 15% hydrous iron oxides in the upper part of the profile, while illite is dominant in the lower part.
2) Where the sediments are in the wet depressions, a large amount of black organic matter and montmorillonite are predominant at the top of the profile. Illite, kaolinite and montmorillonite are found in the lower part. Although the sediments are slightly acidic (pH 6), the hydrological conditions, under which silica, bases and colloidal substances are being supplied from the upstream or the upper part of the profile, seem to be favorable to develop these clay minerals. The molar ratio, SiO2/Al2O3, of the clay is abont 3.2.
