Using natural systems evidence to test models of transformation of montmorillonite

Abstract

Safety functions for the clay buffer in a repository for high-level radioactive waste (HLW) are fulfilled if the presence of montmorillonite with high swelling capacity and low permeability is maintained in the long-term. The transformation of montmorillonite to the non-swelling mineral illite is addressed in most safety assessments by using simple semi-empirical kinetic models, but this approach contrasts with all other near-field geochemical modelling activities that employ a full description of thermodynamic and kinetic mineral-fluid processes. The consistency of these two modelling approaches has been studied by simulating the montmorillonite to illite transformation in the marine sediment profile penetrated by the Ocean Drilling Program (ODP) Site 1174, offshore Japan. Illite in mixed-layer smectite-illite increases from 20% at <700 m below seafloor (mbsf) to 89% at 1100 mbsf. Illitization of smectite at Site 1174 using the semi-empirical approach has been shown by previous authors to provide a satisfactory match to the gradual change of illite content with depth, albeit with significant differences between model variants.In comparison, the approach used in the current study was to simulate the mineralogical and fluid chemical evolution of a ‘packet’ of a typical fluid-saturated sediment using a model involving full kinetic and thermodynamic treatment of mineral dissolution-precipitation reactions, along a temperature-time burial curve defined by published thermal conductivity data for Site 1174. The results of these simulations showed that the onset of illitization at a depth of 700 mbsf could be matched, but that the overall rate of conversion was significantly more rapid than observed, or as modelled by the simple semi-empirical kinetic approach. The onset and rate of illite growth was strongly linked to the rate of transformation of amorphous silica to quartz. Geochemical model simulations therefore err on the side of conservatism, but may produce unrealistic estimates of illitization. This comparison demonstrates that models involving full kinetic and thermodynamic treatment of mineral dissolution-precipitation reactions must be carefully applied to simulate other transformation reactions of montmorillonite relevant to the geological disposal of HLW, such as those arising from the interaction of montmorillonite with iron/steel, cement and/or groundwater.