MICROMINERALOGICAL ANALYSIS OF OLD KEOKUK GEODES REVEALS NEW MYSTERIES

For over a century, Keokuk geodes have fascinated professionals and amateur mineralogists with their variety of minerals and exceptionally well-formed crystals. During the last 50 years, many studies have described the mineralogy of the geodes as well as speculated on their paragenesis. This study offers the first micromineralogical analysis of two Keokuk geodes, and reveals a surprisingly large number of unusual phases that indicate a far more complex paragenesis than previously described. The geodes selected for this study were collected in Hamilton Illinois, where the dolomitic beds of the lower Warsaw Formation (~340 my) are exposed. A detailed SEM/EDX analysis has revealed many unusual microstructures and mineral associations. The most curious feature of these geodes is a remnant siderite “framework” which appears as a rhombic-to-saddled lattice of sub-botryoidal textured walls, left exposed after the dissolution of calcite and dolomite. This framework includes discrete siderite spherules, around 10 microns in diameter, which exhibit growth rims with varying Mn and Fe concentrations. Micron-sized spheres with similar variations in concentration grow from druzy quartz and represent individual members of the rhodochrosite-siderite solid solution series, including the middle member ponite. Perplexing erythrocyte-shaped rhodochrosite grains were observed in what looks like “crater impact” sites associated with the siderite framework. Rhodochrosite growths appear as flat beveled disks and “ring worm” like masses atop quartz euhedra. The most unusual mineral associations include sulfur grains with inclusions of Ni, Pb, Cu, Zn, Ti and Ce, bitumen globules containing halite, sylvite and REE crystals, and a bismuth chloride. We have also tentatively identified rutile, hollandite, and feldspar, none of which been previously reported from Keokuk geodes. This study has revealed a microcosm of mineral mysteries that illustrates the complexity of the paragenetic processes within these geodes. Further research could contribute valuable information about the secondary mineralization processes that occurred during the diagenesis of these sedimentary beds and could provide insight into the movement of heavy metals and REEs through the Midwest region.