神戸層群豊岡凝灰岩部層中に生成しているスメクタイトの鉱物学的特徴 : 地すべりとの関係に着目して

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The Kobe Group distributed in the Sanda basin and eastern Kobe city, Hyogo Prefecture-southwest Japan, contains Palaeogene sedimentary rocks, such as tuff, mud and sand. Smectite and zeolite are formed characteristically in the tuff beds. Many landslides have occurred in areas where the Kobe group is widely distributed. It is generally considered that one of the mineralogical factors of landslide occurrence in these areas is the existence of smectite (Ozaki, 1986; Hirota, 1987). However, the mineralogical characteristics of smectite formed in these areas have not been discussed in detail with special reference to the landslide mechanism. In this study, the mineralogical characteristics of smectite from the Toyooka tuff formation of the Kobe group was analyzed to discuss the mechanism of landslide occurrence. Specimens were collected from drill cores obtained in the Kinkai, Toyooka and Toyooka-kita landslide areas. They were examined using X-ray powder diffraction (XRD), transmission electron microscope (TEM), and electron probe microanalysis (EPMA). A large amount of smectite accompanied with small amounts of kaolinite, illite and volcanic glass were identified within the tuff beds. Clinoptilolite in the crystallized form was also recognized in the tuff beds. All specimens from the cores were identified as a part of the montmorillonite clay mineral species. Based on the quantitative analysis of montmorillonite, it is mainly concentrated along the slip surface of these landslides, when compared with others landslides. An analysis of the montmorillonite interlayer cations revealed calcium as the main ion with a small amount of sodium. However, montmorillonite formed in the slip surface of these landslides composes of a large amount of sodium as the interlayer cation. The main interlayer cation of montmorillonite with clinoptilolite in a few formations was identified to be potassium. The analytical results suggest that the occurrences of montmorillonite is as follows; one was formed from volcanic glass in tuff, another was formed from the fine minerals in matrix, and the third was transformed from clinoptilolite; and were considered to be formed by weathering and/or diagenesis. The transmission electron microscope (TEM) results showed that the montmorillonite found in the slip surface was comparatively larger than the others in particle size. In a comparison of the montmorillonite formed along slip surfaces with the other areas, three characteristics were highlighted as follow, a large amount of montmorillonite was formed, montmorillonite consists mainly of sodium as the interlayer cation, montmorillonite crystals is comparatively larger than the others.
日本粘土学会の論文
2006-08-11

神戸層群豊岡凝灰岩部層中に生成しているスメクタイトの鉱物学的特徴 : 地すべりとの関係に着目して

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