Experimental study of the transformation of smectite at 80 and 300°C in the presence of Fe oxides

The alteration and transformation behaviour of montmorillonite (Wyoming bentonite) was studied experimentally to simulate the mineralogical and chemical reaction of clays in contact with steel in a nuclear waste repository. Batch experiments were conducted at 80 and 300°C, in low-salinity solutions (NaCl, CaCl₂) and in the presence or otherwise of magnetite and hematite, over a period of 9 months. The mineralogical and chemical evolution of the clays was studied by XRD, SEM, transmission Mössbauer spectroscopy and EDS-TEM. Experimental solutions were characterized by ICP-AES and ICP-MS. The main results are that no significant change in the crystal chemistry of the montmorillonite occurred at 80°C, while at 300°C, the presence of Fe oxides leads to a partial replacement of montmorillonite by high-charge trioctahedral Fe²⁺-rich smectite (saponite-like) together with the formation of feldspars, quartz and zeolites.