Micro-CT Scanning of Petrocalcic Horizon Materials from Mormon Mesa, Nevada

Recent investigations of macro- and micromorphic features and mineralogical and chemical characteristics of petrocalcic horizons at Mormon Mesa, Nevada reveal that complex, open-system processes occur within these highly indurated materials. These processes include the development of voids from the dissolution of primary minerals, and the reprecipitation and neoformation of minerals such as calcite, fibrous silicate clays, and barite. The internal macro-features observed from traditional cutting techniques include large cracks and void spaces that range in size from <1 mm to >2 cm. For this study, we utilized micro computed tomography (CT) to view the size, shape, and arrangement of internal features of four indurated calcium carbonate samples. Samples were scanned at 130kV for an exposure time of 190mA with a MS-8 Volume Micro CT Scanner. CT images show circular to elongated voids commonly filled with smaller grains, crystals or fragments of various densities that may represent remnants of the dissolved material, material that has accumulated from the partial collapse of the void space, and/or new mineral precipitates. Results also reveal density variations throughout the sample that could correspond to regions of palygorskite, calcite, and/or barite concentrations. Few penetrating cracks are partially filled with a material with high density that could indicate the presence of barite. Micro CT scanning is a useful tool for visualizing the internal features of highly indurated calcium carbonate materials. Results from this study show that the large voids may contain unconsolidated material that could be removed or altered when traditional cutting techniques are used. The material that fills these voids may provide clues to the complex processes taking place within these unique soils.