Compositional-structural characterization of the Opalinus Clay: New data from core samples from the Mont Terri URL, Switzerland

Middle Jurassic Opalinus Clay (OPA) is considered as host rock for the disposal of heat-generating high-level radioactive waste. For the present study, samples from the Mont Terri rock laboratory (Switzerland) were analyzed using sedimentological, mineralogical and geochemical methods. Thereby, the ordering of irregular illite-smectite interstratified clay minerals (I-S) was of particular interest as it is responsible for sorption of radionuclides and swelling properties.

Results support the classification of OPA into five main facies instead of the well-established threefold division, and further into six subfacies. The clay fraction present in the samples varies according to these facies, consistent with variations in cation exchange capacity. X-ray diffraction analyses of this < 2 µm fraction revealed a homogenous composition of the main constituents, namely R1-ordered I-S, illite and kaolinite. Crystal-structure based Rietveld refinement indicates strong similarities in the nature of disorder of the interstratified illite-smectite minerals. In all facies and subfacies the amount of illitic layers in the I-S varies from 73 - 85 % for all refinements and ordering types. Collectively, results point 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.

This uniformity will facilitate the calibration of mineralogical models and the extrapolation of geophysical logging data where no data is available. However, the transferability of these calibrations to both other OPA locations and claystone formations should be critically examined.