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Mixed-layer minerals such as illite-smectite (I-S) are very common in geologic environments. Their formation through illitization of smectite, in relation to increasing burial depth and temperature, has been extensively studied in sedimentary basins (Perry & Hower, 1970; Weaver & Beck, 1971; Hower et al., 1976; Boles & Franks, 1979; Hoffman & ttower, 1979; Nadeau & Reynolds, 1981). However, recent studies have demonstrated that the illitization process is more complex than previously described. Experimental works by Eberl & Hower (1976), Roberson & Lahann (1981), Inoue (1983), Inoue et at. (1988), Howard & Roy (1985), Whitney & Northrop (1988), Whitney (1990, 1992), Huang et al. (1993), and by Cuadros & Linares (1995) have shown that several chemical and physical factors such as temperature, time, reacting fluid compositions, nature of smectite interlayer cations and rocl~ / water ratio may affect the course and the rate of reaction. Smectite and smectite-rich rocks have been selected for nuclear waste disposal purposes as potential buffer materials (Pusch, 1978, 1980; Bird, 1979; Bird & Cameron, 1982) because of their ability to fill space between the waste canister and the host rock and to ensure an efficient permeability barrier against the incoming groundwaters. Smectite exhibits good adsorption and fixation capacities which are essential for the retardation of the eventual radionuclide migration. However, the long-term stability of the smectites under waste package conditions, e.g. under temperatures close to 100~ near the waste canister contact, under a pressure range of 50 to 100 bar corresponding to hydrostatic pressure within the 500-1000 m depth, and under solution chemistries of natural ground9 1999 The Mineralogical Society 276 R. Mosser-Ruck et al. waters, is poorly documented. Most experimental works have been carried out under much higher P conditions than those expected for waste disposals, because most of them deal with the smectite illite transition in sedimentary basins, and frequently with rather extreme solution chemistry (e.g. high K availability, to enhance illite formation). Preliminary experiments carried out on three homoionic smectites (K, Na, Ca) under 300~ and low pressure (100 bar) have shown that the Ksmectite was far more reactive than the two other end-members (Cathelineau et al., 1993). In order to evaluate the maximal changes which could affect smectites, a detailed experimental study of Ksmectite reactivity has been carried out under the same conditions. The main objective was to determine the nature of the mineralogical changes and to assess the effect of several variables such as fluid composition (using Na/K ratio close to those of natural fluids) and time on the rate and extent of smectite transformation. The originality of this study lies in combining the determination of the expandability of I-S by XRD, and detailed examination of the clay chemistry using TEM and microprobe analysis.

Hydrothermal Reactivity of K-smectite at 300°C and 100 bar: Dissolution-Crystallization Process and Non-Expandable Dehydrated Smectite Formation