Mineralogical Characteristics of Smectite from the Landslide Area in the Neogene Kobe Group, Southwest Japan

This paper deals with smectite collected from the landslide area in the Neogene Kobe Group, southwest Japan. Samples were collected from throughout the region where the Kobe group was distributed. At the Kinkai area, clear outcrops are developed in which landslides horizon is recognizable. Therefore, numerous samples were collected in the Kinkai area and examined in detail.
Collected samples were first investigated by X-ray powder diffraction method. Among clay minerals, smectite was further inspected by Greene-Kelly test and by chemical analysis. In order to clarify mineralogical characteristics of smectite, differential thermal analysis, pH measurement and observation by scanning and transmission electron microscopy were carried out.
Obtained mineralogical characteristics of smectite from the landslide area were characterized as follows:
1) Only smectite is confirmed in the sedimentary rocks of Miocene sediments distributed around the landslide area of the Kinkai area as clay constituent minerals and amount of smectite is almost constant regardless of the horizon of the sediments.
2) The smectite is identified to be montmorillonite through mineralogical examinations, such as X-ray powder diffraction method, Greene-Kelly test and chemical analysis.
3) Montmorillonite with high Na+ ration is prevailed in the horizon corresponding to the sliding plane.
4) The montmorillonite is characterized by extraordinary expanding nature and the fact is regarded to be the significant cause of the landslide which occurs commonly in the district.
Based on the mineralogical and chemical characteristics of smectite collected from the landslide area, relations between smectite and landslides were discussed.