Compositional-structural properties of the Opalinus Clay in southern Germany: Combining facies-based studies and mineralogical analyses

The Middle Jurassic Opalinus Clay (OPA) in Switzerland and southern Germany is regarded as a potential host rock for the disposal of high-level radioactive waste. This study investigates sediment samples from drill cores taken from the Swabian Alb region (southern Germany), and employs a facies-based approach combined with mineralogical analyses, measurements of cation exchange capacity (CEC), and Rock-Eval pyrolysis. Special focus was given to the analysis of the degree of ordering of irregular illite-smectite interstratified clay minerals (I-S) that are responsible for the sorption of radionuclides and the swelling properties of the rocks. The observations support the classification of OPA at the study site into three facies associations (FA-1: clay-rich lower part, FA-2: silty middle part, FA-3: silty upper part with interbedded calcareous-sandy beds), and further into subfacies (SF), in addition to the established twofold lithostratigraphic division into the Zillhausen and Teufelsloch Member. The percentages of clay fraction (<2 μm) varies across different lithofacies, correlating with variations in CEC. Crystal structure-based Rietveld refinement indicates that the proportion of illitic layers in the I-S phase averages between 77 % and 83 % across all refinements and ordering types. No significant differences in the illite:smectite ratio of the I-S phases were observed between the two studied sites, however, a difference in the degree of ordering was identified. This variation may be attributed to differences in the burial history of the two localities and/or a mixture of different domains. A preliminary comparison with results from the OPA in Switzerland reveals that the average proportion of illite layers in the I-S (approximately 77 % for Mont Terri) closely aligns with the results observed in southern Germany. The findings of this study underscore the importance of integrating lithofacies studies with mineralogical investigations to effectively assess the variability and comparability of clay-rich host rocks suitable for radioactive waste disposal. • First-time characterization of I-S structural parameters for Opalinus Clay from southern Germany • The proportion of illite layers in the I-S phase averages between 77 % and 83 % • I-S ratio measurements from southern Germany align with those from Switzerland • This consistency facilitates the calibration/extrapolation of mineralogical models or geophysical logs • The applicability of these calibrations to other clay formations should be critically examined