The impact of alkaline solutions (pH = 13.2) on the clay mineralogy of the Callovo-Oxfordian formation hosting the French underground laboratory for nuclear waste disposal investigation (Meuse-Haute Marne site) has been studied experimentally. Initially, each of the four samples selected as representative of the mineralogical transition in this Callovo-Oxfordian formation consists of a mixture of three main clay phases: discrete illite, discrete smectite and a randomly interstratified mixed-layered mineral (MLM) containing ~65% of non-expandable layers. Clay separates were altered in batch reactors at 60°C using high solution:solid ratios. The mineralogy of this clay fraction and solution chemistry were monitored as a function of reaction time. In addition, the interactions between organic matter and clay particles were investigated using scanning transmission X-ray microscopy (STXM).

The clay mineralogy is little affected even though the pH is still high after 1 y reaction time. The only significant mineralogical evolution is the partial dissolution of the discrete smectite component leading to the formation of a new randomly interstratified illite-expandable MLM. Additional mineralogical transformations lead, for one sample, to the dissolution of micro-crystalline quartz and, for another sample, to the crystallization of a tobermorite-like phase. The low reactivity of clay minerals may be attributed to the presence of organic matter in the samples. In their initial state, all outer surfaces of clay particles are indeed covered with organic matter. After 1 y reaction time, STXM studies showed the basal surfaces of clay particles to be devoid of organic matter, but their edges, which are the most reactive sites, were still protected.

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