Much of the research over the past ten years on the chemical weathering of silicate minerals has focused on mineral-water interfacial chemistry. The application of advanced surface techniques has permitted an unprecedented view at the nanoscale of the near-surface structure and chemistry of altered minerals. Such observations are a key to determining the mechanism of alteration. Here we show examples of chemically weathered silicate minerals, based on samples altered in the laboratory and in the field. Two main types of chemical alteration predominate: the first is characterized by the formation of an amorphous, hydrated and silica-rich surface layer on the parent mineral; the second is characterized by replacement of the parent mineral by a secondary mineral. Here we present nanoscale data of weathered silicate minerals that show the formation of amorphous silica surface layers. These layers form on surfaces, irrespective of the degree of bulk solution saturation. Most importantly, the spatially coincident structural and chemical interfaces at the mineral-surface layer boundary are extremely sharp. Based on advanced transmission electron microscopy (TEM), the chemical gradients rarely exceed a few nm, while the structural gradients are even more abrupt. Taken together, these data are not in accord with the traditional 'leached layer' mechanism. Instead, it is more likely that the controlling mechanism is coupled interfacial dissolution-reprecipitation. This mechanism is based on stoichiometric dissolution within a very thin interfacial fluid in contact with the parent phase; this dissolution is tightly coupled to the precipitation of an amorphous phase within the thin film. The physical and chemical properties of thin fluid films are much different than those of the bulk fluid, and it is these differences that are the key to this mechanism. The same mechanism is also likely to control mineral-mineral replacement reactions.

Chemical weathering: Evidence for a dissolution-reprecipitation mechanism

247th National Meeting (2014)

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