| Summary: | The long-term safe storage of CO<sub>2</sub> in geological reservoirs requires the understanding of the impact of CO<sub>2</sub> on clay-rich sealing cap rocks. The reactivity of the mixed layer of illite-smectite was investigated to determine the reaction pathways under conditions of supercritical CO<sub>2</sub> (scCO<sub>2</sub>) conditions in the context of geological CO<sub>2</sub> storage. A common clay (blue marl from the Guadalquivir Tertiary basin, southern Spain) was tested under brine scCO<sub>2</sub> conditions (100 bar and 35 °C) for 120 and 240 h. The clay sample (blue marl) contains calcite, quartz, illite, smectite, and the corresponding mixed-layer and kaolinite. X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), and inductively coupled plasma optical emission spectroscopy (ICP-OES) analyses were performed. The illitization of mixed-layer illite-smectite was observed by XRD and confirmed by a variation in the content of different elements (K, Mg, Na, Ca, and Fe) of the transformation, as well as an increase in the specific surface (SSA) of the clay (36.1 to 38.1 m<sup>2</sup>/g by N<sub>2</sub>, 14.5 to 15.4 m<sup>2</sup>/g by CO<sub>2</sub> adsorption). Furthermore, these reactions lead to mineral dissolution and secondary mineral formation along the CO<sub>2</sub>–water–clay intercalations of the source rock were responsible for a change in porosity (7.8 to 7.0 nm pore size). The implications of illitisation, mineral destruction, and precipitation processes on CO<sub>2</sub> storage and clay layer integrity should be explored before deciding on a geological storage location.
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