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1 Environmental Geochemistry Group, LGIT - Maison des Géosciences, University J. Fourier – CNRS, BP 53, 38041 Grenoble cedex 9, France
2 ANDRA, Parc de Croix Blanche, 1–7 rue Jean Monnet, 92298 Châtenay-Malabry cedex, France
3 Geological Institute, Russian Academy of Sciences, 7 Pyzhevsky street, Moscow 119017, Russia
4 Laboratoire de Géologie, Ecole Normale Supérieure - CNRS, 24 rue Lhomond, 75231 Paris cedex 5, France
5 Laboratoire HYDRASA, University of Poitiers - CNRS, 40 avenue du Recteur Pineau, 86022 Poitiers cedex, France
* E-mail address of corresponding author: claret{at}ine.fzk.de
A clay-rich Callovo-Oxfordian sedimentary formation was selected in the eastern Paris Basin (MHM site) to host an underground laboratory dedicated to the assessment of nuclear waste-disposal feasibility in deep geological formations. As described initially, this formation shows a mineralogical transition from an illite-smectite (I-S) mixed-layered mineral (MLM), which is essentially smectitic and randomly interstratified (R = 0) in the top part of the series to a more illitic, ordered (R
1) I-S in its deeper part.
This description has been challenged by using the multi-specimen method developed by Drits et al. (1997a) and Sakharov et al. (1999). It is shown that all samples contain a physical mixture of an unusually (?) illitic (~65% I) randomly interstratified I-Exp (illite-expandable MLM) and of a discrete smectite, in addition to discrete illite, kaolinite and chlorite. Structural parameters of the different clay phases vary little throughout the series. According to the proposed model, the mineralogical transition corresponds to the disappearance of smectite with increasing burial depth.
Comparison with clay minerals from formations of similar age (Oxfordian–Toarcian) throughout the Paris Basin shows that the clay mineralogy in the deeper part of the series originates from a smectite-to-illite transition resulting from a low-temperature burial diagenesis. The anomalous lack of evolution of clay minerals in the upper part of the series is thought to be related to specific interactions between organic matter and clay minerals.
Key Words: Callovo-Oxfordian • Clay Diagenesis • Clay Minerals • French Underground Laboratory • Illite-smectite • Interstratification • MHM Site • Mixed Layering • Organic Matter • XRD
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