Pyrite Morphology, Texture, and Trace Metals Across a Weathering Profile (from Parent Rock to Soil) of Ohio Coal Shales

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Acid mine drainage (AMD) refers to the acidic outflow of water from a mining site caused by the weathering of pyrite (FeS2) present in coal. Oxidation of pyrite, within underground mine work and surface waste, releases sulfuric acid and metals, including nickel, cobalt, arsenic, and lead into surface and subsurface waters. AMD negatively impacts water quality, wildlife, and human health. The aim of this study was to determine changes in pyrite particle size, morphology, texture, and composition during the weathering of the parent coal-shale rock. This was accomplished by collecting scanning electron microscopy (SEM) images and energy dispersive spectroscopy (EDS) element maps of pyrite in the following materials: (1) the parent coal-shale rock; (2) rock powder before and after simulated weathering; and (3) soils developing on historic mine waste. Shale samples were crushed to 63, 250, and 2000 µm and subjected to an artificial weathering process over several months to observe if particle size impacted the degree of weathering and its effect on mineral morphology. Soils were collected at various depths and prepared as thin sections. The commonest morphologies seen were framboids and octahedra. Oxidation rim and replacement textures were observed repeatedly, especially in the soils. Iron and sulfur oxide concentrations were universally observed over the range of crushed particle sizes and various surface topographies. Oxidation rim textures are most abundant on small particles consistent with prolonged weathering, resulting in greater degrees of oxidation. Preliminary results suggest AMD production increases with greater variability of surface topography and decreasing particle size. Understanding how the release of AMD will progress in an impacted system, controlled by the degree of weathering of parent rock, is important to assessing its environmental impact. Characterization of mine spoil is instrumental to proper planning and implementation of treatment systems.