Hematite neocrystallisation derived from the experimental alteration of montmorillonite in NaCl solution

Smectite-rich bentonites are widely used as engineered barriers, and consequently, their stability and alteration behaviour have been extensively studied. Small hematite crystals frequently form during the alteration of bentonite under oxidising conditions; however, the mechanism of Fe 2 O 3 formation remains poorly understood, especially its relationship to smectite alteration. This study presents mineralogical and geochemical analyses of hematite neocrystallisation from the experimental alteration of a purified montmorillonite fraction isolated from a Bavarian bentonite. Experiments were conducted in eight batch reactors containing monomineralic Na-saturated montmorillonite and a 0.1 M NaCl solution, at temperatures ranging from 160 to 220 °C and durations of 1–5 months, while other parameters were kept constant. Hematite crystallisation and smectite alteration mechanisms were investigated using transmission electron microscopy, X-ray diffraction and solution chemistry analyses. Results indicate that Fe ions released from partial dissolution (up to 7 wt%) of montmorillonite were mobilised and incorporated into neocrystallised hematite, producing log-normal crystal size distributions consistent with supply-controlled ripening mechanisms of crystal growth in a closed system. Time-dependent data indicate hematite growth is kinetically supply-controlled and self-limiting in low-Fe smectites, suggesting minimal long-term impact on engineered barrier performance. As iron oxides on clay minerals can mediate electron transfer in fluctuating redox environments, hematite formation may influence subsequent alteration processes, including microbial activity. Mass balance calculations suggest the formation of up to 6 % amorphous SiO 2 , potentially limiting further reaction progress under these conditions. • Experimental implications for the long-term stability of bentonite backfills. • Montmorillonite alteration mechanism in 0.1 M NaCl solution. • Precipitation mechanism of hematite derived from the partial dissolution of montmorillonites.