LOW-TEMPERATURE FORMATION OF CLAY MINERALS : CORRELATING LABORATORY EXPERIMENTS WITH NATURAL FIELD-SCALE PROCESSES

Clay minerals such as smectites are one of the most abundant silicate minerals, and is a major phase controlling the Earth's surface and near-surface materials' physical and chemical conditions. Their chemical composition highly varies due to extensive isomorphic substitution that depends much on environmental conditions. Numerous researches have focused on their natural formation and to duplicate these natural processes in the laboratory for highly tailored functionalities. Despite our ability to duplicate certain natural conditions in the laboratory, not all types of clays can be synthesized at low temperatures - a puzzle that is still undergoing heavy scrutiny. In a natural low-temperature setting, the physicochemical characteristics of waters circulating through Earth's materials (i.e. aqueous speciation) and the time involved to accumulate a detectable amount of precipitated clay materials are the main issues. Precipitation from low temperature solutions in conditions of low supersaturation minimizes kinetic factors and the composition of the precipitated smectites could approach maximum stability. On a microscopic scale, such may be the ideal scenario in natural systems and how it precipitates smectites at natural environmental conditions involves a larger volume of materials to accumulate detectable amounts of smectite. Laboratory experiments have shown that even at highly dilute solutions, precipitation takes place rapidly but requires a larger volume of solutions to precipitate a few milligrams of bulk nanomaterials. The inherent variability in composition and stability of smectites as shown by the natural system has also been exploited by duplicating specific types that can be used for novel applications. One of them is transition-metal-bearing smectites; they are rare and only forms in specific geochemical environments. At their nanometer-scale size, they have been shown to exhibit photocatalytic activity which has not been seen in bulk natural materials. Besides the well-known increase in chemical reactivity of smaller particles, this smectite nanomaterial could also exhibit properties akin to quantum-size effects.