INTRACRYSTALLINE SWELLING OF MIXED-LAYER ILLITE-SMECTITE IN K-BENTONITES

To investigate the long-term stability of bentonite under final dispos al conditions of highly radioactive waste, K-bentonites from Kinnekull e (Sweden) and from the Marias River Formation in the Montana disturbe d belt (USA) were studied. After separating the mixed-layer illite-sme ctite (I-S) from the K-bentonite samples, the interlayer charge was ca lculated from the cation exchange capacity (CEC) and the amount of fix ed interlayer K+ ions (K(fix)). The interlayer charge was also determi ned by the alkylammonium method. According to both methods the interla yer charge was in the range for smectite. The results show that the am ount of exchangeable cations increased linearly with decreasing K(fix) . A small increase in the interlayer charge with increasing K(fix) was observed as was a linear correlation between the intracrystalline swe lling up to the second water layer, the CEC and the content of K(fix). Divalent exchangeable cations were then found to be surrounded by app roximately 24 water molecules per cation. Fixed interlayer K+ ions wer e unhydrated. Forming the third and fourth water layer, swelling was p resumably limited by free silica formed by the vitrification of the vo lcanic ash.