MICRO-MINERALOGY OF A SINGLE KEOKUK GEODE; HOW COMPLEX CAN IT GET?

From a suite of Keokuk geodes collected in Hamilton, Illinois, we randomly selected a single geode for a detailed micro-mineralogical study. These 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 around 340 million years ago. Optical microscopy using a scanning electron microscope with an electron dispersive x-ray spectrometer reveals a surprisingly complex mineralogy. A comprehensive micro-analysis exposed an elaborate network of intergrowths linking multiple generations of quartz, calcite, and siderite which exhibit a wide array of crystal habits. Platy hexagonal kaolinite crystals are littered throughout the geode as inclusions in quartz and calcite and are also associated with siderite. Micro-crystals of acicular siderite grew from the edges of the hexagonal crystals creating spike-rimmed kaolinite grains. 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 the points of micro-druzy quartz and exhibit a variable iron composition corresponding to the rhodocrosite-siderite series including ponite. Siderite appears as hallow spheres with growth rims indicating multiple phases of varying iron concentrations. Siderite also coats calcite euhedra with a multi-layered crust a few microns thick and incorporates spike-rimmed kaolinite grains in some areas. Most unusually, siderite appears as a box-like rectangular-to-rhombic lattice of sub-botryoidal textured walls which incorporate the siderite spheres. This siderite structure creates a ‘framework’ where some of the resultant ‘boxes’ are filled with calcite. Several additional minerals have been tentatively identified including: chalcedony, goethite, pyrite, sylvite, hollandite, and an unidentified REE carbonate. Organics are also present in the form of micro-sized amorphous hydrocarbon particles. The mineralogy of this geode offers a microcosm illustrating the complex geologic process of secondary mineralization that has occurred in the Midwest region.