Kinetics of long-term illitization of montmorillonite—a natural analogue of thermal alteration of bentonite in the radioactive waste disposal system Original Research Article

A tertiary argillaceous rock formation and a quarternary intrusive rock which intruded into the former, distributed in the Nishikubiki district of Niigata prefecture in central Japan, has been investigated as a natural analogue of bentonite alteration in a repository for high-level radioactive waste. Lateral variation of clay mineral species in the aureole of intrusive rock has been examined and thermal analyses for the argillaceous rock has been carried out based on the cooling history of intrusive rock. Predominant clay mineral varies from montmorillonite to illite through illite/montmorillonite interlayers as the distance to the intrusive rock decreased. The thermal analyses indicate that temperature descended during the past 7.5×105 years from 543 to 288 K at a locality of argillaceous rock containing 75% illite in the interlayers. The apparent activation energy is evaluated to be about 107 kJ/mol for the reaction on the assumption that montmorillonite–illite conversion is governed by a first-order reaction. This value suggests little possibility of illitization of bentonite in the geological disposal system in the term of 105 years as long as the temperature of the repository environment is approximately 373 K or less.