Smectite in sandstones: A review of the controls on occurrence and behaviour during diagenesis

Smectite comprises a chemically complex group of clay minerals and is common in sandstones, although there have been relatively few studies of their origin and distribution in comparison to both other clays in sandstones and smectites in mudstones. The occurrence of smectite in sandstones is a function of both depositional and diagenetic factors. It can occur as a component of primary sediment when the source terrain lithology, climate, duration of weathering and groundwater conditions are favourable. Smectite is characteristic of inhibited chemical weathering, for example, in arid, high-relief terrain. It thus is most common in aeolian, fluvial, lacustrine and turbidite sandstones, where the opportunity for advanced degrees of weathering and smectite breakdown (to other clays) is minimized. Smectite may be more abundant in deep-water settings because it tends to be finer grained than other clays and thus can be carried further into deep waters before settling. Smectites in sandstones are not always co-deposited with sand grade sediment but can be incorporated into the sand by bioturbation, soft-sediment deformation and mechanical infiltration. Smectite cements also can develop as authigenic minerals in sandstones owing to the presence and breakdown of unstable lithic grains and the formation of pseudomatrix resulting from compacted clay-rich clasts. Dioctahedral smectites in sandstones transform to illite and trioctahedral smectites transform to chlorite via mixed-clay intermediates during progressive diagenesis because they become unstable as the temperature and compositional environment changes during burial. The processes of smectite illitization and chloritization are not isochemical because typically they require movement of a wide range of elements (K, Fe, Mg, and also Al, and Si, etc.). Typically, the processes involve a net relative increase of the Al/Si ratio in the clay, frequently resulting in concomitant quartz cementation. The occurrence and rates of smectite illitization and chloritization in sandstones can be both similar and dissimilar to equivalent processes in neighbouring mudstones. If smectite in a sandstone is authigenic it may be relatively more stable during burial than detrital smectite in mudstones. Conversely, if smectite breakdown requires exotic components, the typically higher permeability of sandstones may facilitate faster transformation of the sandstone smectite than the mudstone smectite. The key controls on smectite transformation are the rock mineralogy, the pore-water geochemistry and the higher permeability of sandstones than mudstones.