Natural evidence for the cementation in bentonite buffer based on transmission electron microscopy observation of bentonite ores

Kikuchi, Ryosuke*; Horiuchi, Misato*; Saito, Masaki*; Ishiwata, Tobimaru*; Nishiki, Yuto*; Sato, Tsutomu*; Takayama, Yusuke  ; Mitsui, Seiichiro  

In geological disposal of high-level radioactive wastes, the use of swelling clay is intrinsic as a buffer around waste packages. In order to achieve long-term safety, longevity of montmorillonite, a major constituent of bentonite buffer, under disposal related conditions is important. One of the alteration processes of concern is cementation of clay layers due to precipitation of secondary minerals such as silica. In this study, we examined natural bentonite ores that could serve as natural analogues for the cementation process, especially focusing on microstructure of consolidated bentonite. Bentonite ores were collected from the Tsukinuno Mine in Yamagata Prefecture, Japan. This bentonite is thought to have been formed by the diagenesis of volcanic ash. A dry polishing technique was used to form a smooth surface that well shows the original microstructure of bentonite ores, allowing petrographic discussion as in the observation of non-clayey rocks. Besides coarse quartz (probably pyroclastic origin), fine silica with grain sizes ranging from a few microns to submicron is widely observed in bentonite ores. The distribution of fine silica is characterized by discrete grains or short chains or small clusters in the fine-grained montmorillonite matrix, suggesting that they formed in closed system diagenesis and a low mobility of silica in clay matrix. Thin foils including fine silica and montmorillonite were extracted from polished sections using a focused ion beam apparatus and observed by transmission electron microscopy (TEM). TEM observations demonstrated a microstructure where montmorillonite edge is directly bonded to the fine silica particles. The microstructure as seen in this study, is interpreted to be formed by the authigenic silica precipitation during diagenesis, not by a physical mixture. The silica coating of montmorillonite edge could prevent water access to its interlayer space and free swelling, which leads to a decrease in swelling pressure.