Preferential dissolution along misoriented boundaries in heterogenite
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Preferential dissolution along misoriented boundaries in heterogenite R. Lee Penn1,2, Alan T. Stone2, and David R. Veblen1 Department of Earth and Planetary Sciences 2 Department of Geography and Environmental Engineering Johns Hopkins University, Baltimore, MD 21218, U.S.A.
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ABSTRACT High-Resolution Transmission Electron Microscopy (HRTEM) results show a strong crystalchemical and defect dependence on the mode of dissolution of synthetic heterogenite (CoOOH) particles. As-synthesized heterogenite particles are micron-size plates (aspect ratio ~ 1/30) constructed of crystallographically oriented ~ 3-nm primary particles or are single ~ 21-nm unattached heterogenite platelets (aspect ratio ~1/7). Reductive dissolution, using hydroquinone, was examined in order to evaluate morphology evolution as a function of reductant concentration. Two end-member modes of dissolution were observed: 1) non-specific dissolution of macroparticles and 2) preferential dissolution along misoriented boundaries. In the case of non-specific dissolution, average macrocrystal size and morphology are not altered as building block crystals are consumed. The result is web-like particles with similar breadth and shape as undissolved particles. Preferential dissolution involves the formation of channels or holes along boundaries of angular misorientation. Such boundaries involve only a few degrees of tilt, but dissolution occurs almost exclusively at such sites. Energy-Filtered TEM thickness maps show that the thickness of surrounding material is not significantly different from that of undissolved particles. Finally, natural heterogenite from Goodsprings, Nevada, shows morphology and microstructure similar to those of this synthetic heterogenite. INTRODUCTION Central to understanding the geochemical cycling of the elements is elucidating the processes that liberate chemical species and allow them to move. Dissolution is one such process and can liberate chemical species by producing either aqueous molecular complexes or nanocrystalline particles that become important components of mobile fluids in our environment. Understanding growth and dissolution processes requires a holistic characterization of the phases involved. Coupling high-resolution transmission electron microscopy (HRTEM) with wet chemical methods allows direct correlation of atomic-scale changes in dissolving materials with changes in solution and surface chemistry. Dissolution experiments utilizing particles with different types of surface area have the potential to provide the link between understanding reactive surface area and the chemical behavior of a mineral surface. This work’s focus involves dissolution experiments utilizing heterogenite (CoOOH) particles formed by oriented assembly. This mineral belongs to a class of minerals, the oxyhydroxides, that are important because they comprise a large proportion of the reactive surface area in the environment (e.g., in soils and as mineral weathering products [2]). In ad60 dition, Co is a radionuclide and an appreciable compone
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