Key processes of long-term bentonite-water interaction at 90 °C: Mineralogical and chemical transformations

The results indicate that montmorillonite alteration, particularly illitization, could be excluded in this system due to the low concentration of K+ in the solution. In addition, cation exchange capacity measurements did not indicate any transformation process of montmorillonite. Slight modifications of montmorillonite basal reflection profiles in PXRD patterns were attributed to cation exchange reactions. The most reactive minerals included siderite, Fe-dolomite/ankerite, and calcite/Mg-calcite. Dissolution/precipitation of these minerals controlled the composition of the aqueous phase that affected the distribution of the exchangeable cations in the montmorillonite interlayer. The most significant change was the dissolution of siderite that induced partial dissolution of calcite, which resulted in the replacement of Na+ and Mg2 + by Ca2 +. The overall evolution of the water chemical composition could be predominantly explained by mineral phase equilibria. The initial water composition did not affect the geochemical processes, which took place in the bentonite-water system due to the strong buffering bentonite capacity.