TEM Studies of Weathering Reactions: The Importance of Mineral Structure, Microstructure and Composition

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Chemical weathering is of fundamental importance because it is the process by which silicate minerals formed at high-temperatures and pressures are converted to the constituents of sediments and soils. Despite this, the structural and compositional controls on the mechanisms and rates of transformations of many reactions are still poorly understood. This proposal describes two research projects designed to use high-resolution transmission electron microscopy to investigate the microscopic characteristics of weathering reactions where significant structural exists between the parent and product minerals. The first of these involves hydration and oxidation of amphibole-group minerals. The second project focuses on perovskite weathering, where hydrous minerals are not formed and oxidation does not occur. The weathering reactions of a selection of microstructurally complex amphiboles with close to end-member compositions, intergrown and exsolved amphiboles, and amphiboles containing chain-width defects will be examined. Intergrown amphiboles offer the opportunity to compare weathering rates and mechanisms under the same physiochemical conditions and to evaluate the degree to which reactions are focused by coherency strain, defects, and grain boundaries. The Weathering of perovskite will also be examined. Hydrothermal dissolution of perovskite, a possible component of nuclear waste storage ceramics, has been studied extensively, but the natural weathering reactions have been largely ignored. The chemically simple transformation to TiO2 minerals may involve considerable structural inheritance. The aims these projects are to clarify the crystal chemical details of the natural weathering processes, thus enhancing our understanding of the mechanisms by which these reactions occur.