The Effect of iron on montmorillonite stability, 1; Background and thermodynamic considerations

Wilson, J.*; Savage, D.*; Cuadros, J.*; Shibata, Masahiro ; Ragnarsdottir, K. V.*

In some geological repository designs, steel canister and bentonite backfill are placed in contact. Some previous studies indicate that the montmorillonite component of the backfill could react with steel corrosion products to produce non-swelling Fe-rich phyllosilicates (e.g. chamosite, berthierine) or Fe-rich smectite. If montmorillonite were altered to non-swelling minerals, the expected swelling capacity of the bentonite backfill could be reduced. This paper discuss Fe-rich phyllosilicate mineral stability at the canister-backfill interface using thermodynamic modelling. Estimates of thermodynamic properties were made for Fe-rich clay minerals in order to construct approximate phase-relations for end-member/simplified mineral compositions in activity space. The diagrams (for the system AlO-FeO-FeO-MgO-NaO-SiO-HO) suggest that if pore waters are supersaturated with magnetite, Fe(II)-rich saponite is the most likely alteration product (if fo values are significantly lower than magnetite-hematite equilibrium). Therefore, the alteration of montmorillonite may not be detrimental to HLW repositories that include Fe, as long as the swelling behaviour of the Fe-rich smectite produced is maintained. If fo exceeds magnetite-hematite equilibrium, and solutions are saturated with magnetite, berthierine is likely to be more stable than smectite minerals. The alteration of montmorillonite to berthierine could be detrimental to the performance of the repositories.