Insights into the compositional-structural properties of the Opalinuston-Formation: combining facies-based studies and mineralogical analyses

The Middle Jurassic Opalinuston-Formation (OPA) in Switzerland and Southern Germany is considered as a potential host rock for the deep geological disposal of high-level radioactive waste. For the present study, core samples from the Mont Terri rock laboratory (Switzerland) and from the BGR research project "Sequence Stratigraphy of the Aalenian in Southern Germany" (SEPIA) are investigated using a facies-based approach including mineralogical and geochemical analyses. Special focus is put on the degree of ordering of the irregular illite-smectite (I-S) interstratified clay minerals, which are responsible for the sorption of radionuclides and swelling properties of the rocks.

The results support the classification of OPA into several facies types. The clay fraction present in the samples varies according to these facies, consistent with variations in cation exchange capacity. For the Mont Terri site, crystal structure-based Rietveld refinement indicates strong similarities in the nature of disorder of the interstratified I-S. In all facies and subfacies of the investigated samples from the Mont Terri rock laboratory, the amount of illitic layers in the I-S varies between 73% and 85 % for all refinements and ordering types.

Our ongoing effort is to compare these results with crystal-structure based Rietveld measurements from the OPA in Southern Germany. This comparison will improve our understanding of how results from the Mont Terri rock laboratory can be transferred to claystone formations in Germany.