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Radiation-induced defects in dickites from the EI Berrocal granitic system (Spain)

Thierry ALLARD^1,*, Philippe ILDEFONSE, Luis Pérez Del VILLAR², Stéphanie SORIEUL¹, Marta PELAYO², Bruno BOIZOT³, Etienne BALAN¹ and Georges CALAS¹

¹ Laboratoire de Minéralogie-Cristallographie de Paris, UMR CNRS 7590, Universités Paris 6 et 7, IPGP, T 16–26, 2ème Etage, case 115, 4 Place Jussieu, F-75252 Paris cedex 05, France

² Centro de Investigaciones Energéticas, Medio Ambientales y Tecnológicas (CIEMAT), DIAE, CHE, Avenida Complutense, 22, E-28040 Madrid, Spain
³ CEA/DSM/DRECAM /Laboratoire des Solides Irradiés, Ecole Polytechnique, F-91128 Palaiseau Cedex, France

^* E-mail: allard{at}Imcp.jussieu.fr

This study deals with the radiation-induced defects in fracture filling clay-minerals from the EI Berrocal U-deposit (Spain), which is considered as a natural analogue of a high-level nuclear waste repository in granitic rock. Mineralogical anal yses shows the widespread occurrence of dickite as a secondary alteration phase, together with various amounts of associated smectite, illite, carbonates and primary minerals. Native radiation-induced defects accumulated in the dickite and smectite structure are identified by electron paramagnetic resonance spectroscopy (EPR). They are distinguished by their spectroscopic parameters and thermal stability, and can be reproduced by artificial irradiation. The dickite defects are similar to the well-characterized features in natural kaolinites, including defects stable at the scale of geological periods. Dickite can thus be used as a natural dosimeter to trace radioelement transfer in relevant geological systems. The concentration of stable defects in dickites is related to the present dose-rates, assuming dosimetry parameters from model kaolinites and several scenarii for dickite ages inferred from major tectonic events. This study demonstrates that the El Berrocal dickites correspond to different generations having recorded retention or migration events of radioelements within the whole system.

Key-words: dickite, clay minerals, radiation-induced defects, EPR..

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