Compositional-structural characterization of the Opalinus Clay and Passwang Formation: New insights from Rietveld refinement (Mont Terri URL, Switzerland)

Middle Jurassic Opalinus Clay (OPA) in Switzerland and Southern Germany is considered as host rock for the disposal of heat-generating radioactive waste. For the present study, core samples from the Mont Terri rock laboratory (Switzerland) were investigated using a facies-based approach including mineralogical and geochemical analyses. Special focus was on the analysis of the degree of ordering of irregular illite-smectite interstratified clay minerals (I-S) responsible for sorption of radionuclides and swelling properties of the rocks.

The investigations support the classification of OPA into five main facies (instead of the established threefold division) and into further subfacies. The facies concept was extended to the Passwang Formation (PWF). The amount of clay fraction varies in samples from the different facies, which is consistent with variations in cation exchange capacity (CEC). X-ray diffraction (XRD) analyses of the <2 μm fraction revealed a homogenous composition of the main constituents, namely R1-ordered I-S, kaolinite, and illite. Crystal structure based Rietveld refinement indicates strong similarities in the nature of disorder of the interstratified illite-smectite minerals for all investigated facies. The I-S-phase can be described with the R1 model, which is together with the amount of illitic layers in the I-S an indicator for a maximum burial temperature between 90 and 105°C. The amount of illitic layer in the I-S varies from 73%–85% for all refinements and ordering types, and in all OPA and PW facies and subfacies. This, together with the homogenous composition of the clay size fraction points to (1) a homogenous overprint of the clay assemblage during burial and/or (2) a uniform siliciclastic sedimentary supply throughout the sedimentation period of the OPA. The results of this study highlight the need to combine lithofacial studies with mineralogical investigations in order to adequately capture the variability of clay-rich host rocks under consideration for radioactive waste disposal.