Intracrystalline Swelling of Mixed-Layer Illite-Smectite in K-Bentonites

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Abstract
To investigate the long-term stability of bentonite under final disposal conditions of highly radioactive waste, K-bentonites from Kinnekulle (Sweden) and from the Marias River Formation in the Montana disturbed belt (USA) were studied. After separating the mixed-layer illite-smectite (I-S) from the K-bentonite samples, the interlayer charge was calculated from the cation exchange capacity (CEC) and the amount of fixed interlayer K+ ions (Kfix). The interlayer charge was also determined by the alkylammonium method. According to both methods the interlayer charge was in the range for smectite. The results show that the amount of exchangeable cations increased linearly with decreasing Kfix. A small increase in the interlayer charge with increasing Kfix was observed as was a linear correlation between the intracrystalline swelling up to the second water layer, the CEC and the content of Kfix. Divalent exchangeable cations were then found to be surrounded by approximately 24 water molecules per cation. Fixed interlayer K+ ions were unhydrated. Forming the third and fourth water layer, swelling was presumably limited by free silica formed by the vitrification of the volcanic ash.