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Wilson, J.*; Cressey, G.*; Cressey, B.*; Cuadros, J.*; Ragnarsdottir, K. V.*; Savage, D.*; Shibata, Masahiro
Geochimica et Cosmochimica Acta, 70(2), p.323 - 336, 2006/01
Times Cited Count：104 Percentile：86.35(Geochemistry & Geophysics)This work investigates montmorillonite stability in the presence of metal iron, magnetite under hydrothermal conditions. Two series of experiments were conducted. In the first, mixtures of Na-montmorillonite, native Fe, (magnetite, calcite,) and NaCl solutions were reacted at 250
C for about 100 days. In the second, mixtures of Na-montmorillonite, native Fe and FeCl
solutions were reacted at 80
250C for about 90 days. In the first series, the starting montmorillonite was transformed to Fe(II)-rich smectite only when the Fe metal was predominantly added. The reaction product was oxidised to Fe(III)-rich form on exposure to air. The expansion of this material on ethylene glycol solvation was much reduced compared to the starting montmorillonite. TEM imaging shows that partial loss of tetrahedral sheets, resulting in adjacent layers becoming H-bonded with a 7 angstrom repeat. Solute activities corresponded to the approximate stability field for Fe(II)-saponite. In the second series, significant smectite alteration was only observed at 250C and the product contained a small proportion of a 7
repeat structure, observable by XRD. In these experiments, solute activities coincide with berthierine.
Wilson, J.*; Savage, D.*; Cuadros, J.*; Shibata, Masahiro; Ragnarsdottir, K. V.*
Geochimica et Cosmochimica Acta, 70(2), p.306 - 322, 2006/01
Times Cited Count：102 Percentile：58.97(Geochemistry & Geophysics)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.
