Experimental investigation of the effect of high-pH solutions on the Opalinus Shale and the Hammerschmiede Smectite H. TAUBALD 1 , A. BAUER 2, *, T. SCHA ¨ FER 2 , H. GECKEIS 2 , M. SATIR 1 AND J. I. KIM 2 1 Universita ¨tTu ¨bingen, Institut fu ¨r Mineralogie,Petrologieund Geochemie, Lehrstuhl fuer Geochemie, Wilhelmstr.56, D-72074 Tu ¨bingen, and 2 Forschungszentum Karlsruhe, Institut fu ¨r Nukleare Entsorgungstechnik, PO Box 3640, D-76021 Karlsruhe, Germany (Received 20 June 1999; revised 20 September 1999 ) ABSTRACT: The alteration and transformation behaviour of the Tertiary Hammerschmiede Smectite andthe Jurassic Opalinus Shale in an alkaline solution was studied in column experiments. The Hammerschmiede Smectite is proposed as potential backfill material and the Opalinus Shale as host rock for the Swiss low-level nuclear waste storage site. Over a period of 18 months, the evolution of permeability, pH and solution concentrations were measured. After the experiment, the columns were cut into pieces to study the mineralogical and the chemical evolution of the clays. X-ray diffraction (XRD) revealed no significant appearance or disappearance of diffraction peaks at the end of the experiments. The scanning electron micrographs of the clays revealed that both clays exhibited a precipitation zone, which extends from 0 to 2 cm below the infiltration surface. Both clays showed significant differences in the evolution of pH and hydraulic conductivity. The solution front crossed the Opalinus Shale column entirely after only 11weeks and the initial values for K + and Na + were conserved in the solution. For both clays, the salt concentrations in the percolating fluids mirror the evolution of pH. K EYWORDS: Opalinus Shale, Hammerschmiede Smectite, hydraulic conductivity. The effect of alkaline solutions on clay minerals has been the subject of study for a number of years (Velde, 1965; Eberl & Hower, 1977; Mohnot et al., 1987; Carroll-Webb & Walther, 1988; Carroll & Walther, 1990; Chermark, 1992; Chermark, 1993; Eberl et al., 1993; Huang, 1993; Adler et al., 1998; Bauer et al., 1998; Bauer & Velde, 1999). The stability of kaolinite in aqueous solutions of varying pH has been the object of more numerous investigations (May et al., 1986; Carroll-Webb & Walther, 1988; Carroll & Walther, 1990; Nagy et al., 1991; Wieland & Stumm, 1992; Ganor et al., 1995; Bauer et al., 1998; Bauer & Berger, 1998). One objective of such studies has been to determine the ultimate stability of clays as barriers to the migration of aggressive solutions emanating from man-made repositories. These clay barriers are designed to contain nuclear waste products, as well as municipal waste concentrates. The major concern in these cases is the rate of reaction of clays. Such information is important for risk assessments or evaluations and calculations concerning the safety of repositories. High-pH solution-mineral reactions occur in a variety of natural geological environments as well in environments modified by human activity in various engineering projects. Two common ‘engi- neered ’ high-pH environments include emplaced concrete (Atkinson, 1985; Andersson et al., 1989; Lunden & Andersson, 1989; Savage et al., 1992) Clay Minerals (2000) 35, 515–524 * E-mail: bauer@ine.fzk.de # 2000 The Mineralogical Society