Smectite clay minerals are ubiquitous phyllosilicate minerals commonly observed as weathering products in environments affected by past volcanic activity. Due to their unique physical properties they have found to be useful for many anthropogenic activities. For example, these minerals are highly prized for their water-impermeable characteristics and are used, or envisaged to be used, as part of a multi-barrier approach to hydraulically isolate mining, industrial, municipal and radioactive waste materials from surrounding sub-surface environments. There has also been widespread interest in the chemical properties of smectite clay minerals including sorption reactions, redox processes and, relatively more recently, the secondary mineralization products that crystallize as a result of interfacial electron transfer between sorbed Fe(II) and clay structural Fe(III) under anoxic conditions. Our most recent contribution to this research area has demonstrated that green rust has likely been an overlooked, but significant, secondary mineral to form when Fe(II) has ‘sorbed’ to the surface of smectite clay minerals (under certain conditions). In this presentation I will describe further studies that we have since undertaken to better define the range of ‘certain’ conditions that result in the secondary crystallization of green rust minerals. For example, whilst pH is an important variable, the speciation of aqueous Fe(II), and hence its propensity for sorption, can have a significant impact on green rust formation. We have also observed that smectite clay minerals template for the formation of the three major natural green rusts – where chloride, sulfate or carbonate are the interlayer anions – suggesting that the natural occurrence of these minerals, such as fougerite, could be related to local phyllosilicate mineralogy. Furthermore, these observations have relevance to the sub-surface storage of radioactive waste forms where the eventual secondary crystallization of carbonate green rust minerals have the potential to reductively-decrease radionuclide mobility over millennia time periods.

Smectite clay minerals: Templates for the crystallisation of green rust

2019 Spring National Meeting

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