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AAPG Southwest Section Annual Convention

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Unique Microstrutures And Complex Micro-Mineral Associations in a Single Keokuk Geode


From a suite of Keokuk geodes collected in Hamilton, Illinois, we selected a single geode for a detailed micro- mineralogical study. Keokuk geodes are specific to the dolomitic beds of the lower Warsaw unit in the Keokuk Formation. This sedimentary marine deposit is a consequence of a regressing epicontinential sea during the Mississippian period. Optical microscopy and scanning electron microscopy revealed a surprisingly complex network of elaborate intergrowths linking multiple generations of minerals which exhibit a wide array of crystal habits. This geode has a chalcedony shell as well as quartz and calcite euhedra, typical of geodes from this locality. Platy hexagonal kaolinite crystals are littered throughout the geode on and in quartz, calcite, and siderite. Kaolinite included calcite exhibits several habits including: flow structures with ripple marks, a stair-stepped box texture, and euhedral crystals. Manganese carbonate micro-spheres grew from micro-druzy quartz and exhibit a variable Fe and Mn composition corresponding to the rhodocrosite- siderite series. Siderite appears as hallow spheres, about 5-10 microns in diameter, with growth rims indicating multiple phases with varying iron concentrations. Siderite also coats calcite euhedra with a multi-layered crust a few microns thick. Micro-crystals of acicular siderite grow from edges of kaolinite grains that settled atop the siderite-coated calcite. Most unusually, siderite appears as a box-like rectangular-to-rhombic lattice with a sub-botryoidal texture and incorporates the larger siderite spheres. This unique microstructure creates a ‘framework’ resulting from the systematic replacement of calcite along cleavage planes. The subsequent dissolution of calcite leaves the framework exposed to oxidation, resulting in the iron oxide goethite. Organics are also present in the form of 50-100 micron bitumen particles which host microcrystals of K and Na salts. Several additional minerals have been tentatively identified including: ponite, pyrite, sylvite, hollandite, feldspar, barite, and an unidentified REE carbonate. The mineralogy of this geode offers a microcosm illustrating the complex geologic process of secondary mineralization that occurs during the lithification and diagenesis of sedimentary beds.