新潟県羽津にみられるモンモリロナイトのハロイサイトへの変化

Abstract

At Nagamine-hara near Shibata hydrothermally altered pyroclastic rocks of the Miocene are overlain by terrace deposits composed of red soil and strongly weathered gravel. White clay deposits are found underneath the unconformity plane. They are weathering products of the altered pyroclastic rocks. Various stages of transition from unweathered pyroclastic rocks to white clays are observed at outcrops A and B (Fig. 2-a and b). X-ray diffraction data (Fig. 3) of unweathered rocks (specimen B-4) show that a clay mineral contained in them is montmorillonite, though it is “abnormal” type which gives double endothermic peaks in the range of 500-700°C (Fig. 9). Whereas white clays are composed of halloysite (E-3) or mixed-layer halloysitemontmorillonite (A-1). The products in the intermediate stages of weathering are mixed-layer halloysitemontmorillonite in whhch halloysite layer increases in process of weathering. At the intial stage of weathering clays are composed of heterogeneous phases (B-3). Chemical compositions by electron microscopic analysis (Table 1, Fig. 10, 11) are compatible with X-ray and thermal data. The observation by transmission electron microscope (Fig. 12) indicates that thinning of montmorillonite particles and curling of their edges occur inthe earlier stage of weathering and that formation of tube-like form and mosaicking such as cell division appear in the later one.