Natural and Experimental Weathering of Pyroxenes and Pyroxenoids

点击翻译按钮获取中文摘要

9317082 Banfield The research in this proposal aims to bridge the gap between natural and experimental mineral weathering studies by combining high-resolution electron microscopic characterization of both sets of reactions with dissolution kinetic data. Research involves chain silicate minerals. Chain silicates have been selected because of their geochemical importance and because their structures are sufficiently similar to those of clay products to allow us to predict that transformations might, under some conditions, involve some structural inheritance. Furthermore, through collaborative arrangements, a suitable selection of experimentally reacted materials and dissolution rate data are available. We have tested the proposed approach by comparing naturally and experimentally weathered pyroxenoid minerals. Results reveal sub-nanometer-scale details of both experimentally and naturally reacted surfaces and provide insights into reaction mechanisms. Initial results show Si-rich leached layers on experimentally reacted pyroxenoids. Thus, models to explain measured congruent dissolution rates must account for the fact that Si entering solution may be largely derived from the leached-layer-solution interface. We find preliminary evidence for recrystallization of the gel layer. Thus, rates may also be limited by transport properties, solubility, and crystallinity of the gel. Initial data for natural weathering reactions are consistent with at least partial inheritance of polymerized units by smectite reaction products. Thus, we hypothesize that rates for pyroxenes may reflect incomplete depolymerization of the silicate chains under both laboratory and natural conditions. The proposed work has two major goals. First, we will characterize the compositional, volumetric, mineralogical, and structural aspects of natural pyroxene reactions and examine the influence of microstructures (e.g., exsolution lamellae, grain boundaries, antiphase bounda ries, chain width defects, dilocations) on relative reaction rates. Second, we will characterize experimentally reacted pyroxenes and pyroxenoids and combine these data with rate measurements to clarify details of possible dissolution kinetic models. Through this approach we hope to enhance current understanding of the mechanisms and products of weathering reactions and the conditions under, and extent to which, experimentally measured rates apply to natural processes.