Mineralogical evolution of the Tournemire argillite after in situ interaction with steels

Titre du congrès :Bridging Clays 4ème Colloque du Groupe Français des Argiles et 43ème Réunion de la Clay Minerals Society américaine Ville du congrès :Ile d'Oléron Date du congrès :05/06/2006

Steels are possible materials for building nuclear waste containers used in long term geological disposal in argillaceous environments. The French Institute of Radioprotection and Nuclear Safety develops many experimental research programs on Toarcian argillites at its Tournemire site, Aveyron, France. One such program looks at the steel/argillite interaction in natural condition. Therefore, 3 types of steels (carbon and 2 stainless steels) were introduced in boreholes in two different areas: (1) the Excavated Disturbed Zone (EDZ) in wet condition, and (2) a zone located far from any mechanical disturbances in dry condition. These samples have been recovered after two years of contact (Foct et al., 2004 ) and after six years of contact (this study). In order to determine the petrological and mineralogical modifications of the argillite due to the interaction with steel samples, argillite samples were studied by XRD, SEM, FTIR and AAS for chemical analyses. Bulk-rock XRD semi-quantitative analysis from the initial state argillite indicate that quartz, calcite, micas and feldspars are the major constituents and that a few amount of chlorite, mixed-layered I/S, pyrite and dolomite occur. No significant changes are observed for the argillite in contact with the stainless steels but important modifications are noticed for the argillite in contact with the carbon steel. In this case, the development of a red alteration zone 0-2 millimetres thick is observed within the argillite. According to petrological SEM observations combined with EDS element maps, this altered zone shows a clear Fe-enrichment, a strong decrease in Ca-content and a slight decrease in Si, Al and K-contents. Some partly dissolved framboïdal pyrites are also observed in the Fe-rich zone. Moreover, semi-quantitative XRD analyses of this Fe-rich zone indicate dissolution of a part of the calcite whereas quartz and clay minerals remain. The formation of goethite, lepidocrocite and a few amount of gypsum suggests oxidizing conditions.