A JOURNEY TO THE CENTER OF A GEODE: MICROMINERAL DISCOVERIES WITH MACRO-IMPLICATIONS

A geode is defined by a discrete outer shell with inwardly radiating crystals around a hollow cavity with a roughly spherical shape. Geodes are formed during the secondary mineralization phase of diagenesis as the influx of meteoric waters provide the solutions from which the geode minerals precipitate. For over a century, geodes have fascinated professional and amateur mineralogists with their exceptionally well-formed crystals and variety of mineral species. Many studies have described the overall minerology of geodes within a region as well as speculated on their paragenesis. A mineralogic study on the commercially mined geodes from Chihuahua, Mexico identified 18 different mineral species, seven of which were manganese minerals. Since Chihuahua is a known manganese mining district it can be concluded that the mineralogy of a geode can directly reflect the geochemistry of the region in which it was formed. In order to better understand the paragenetic process of a sedimentary geode, we have performed a petrographic and chemical analysis of several Keokuk geodes collected from Hamilton, Illinois. Through an extensive SEM/EDX analysis, we have tentatively identified 27 different minerals in just a single geode. The most peculiar mineral phases observed include sulfur grains with inclusions of Ni, Pb, Cu, Zn, Ti and Ce, a bismuth chloride, and bitumen globules containing halite, sylvite and REE crystals. We have also identified rutile, hollandite, a Ni-Cr-Mo bearing Fe-carbonate, a Ni-Cr bearing Fe-sulfide, and other mineral phases which have not been reported in previous studies of Keokuk geodes. Thin section analysis has provided additional information on intergrowths and inclusions linking multiple generations of growth into a one continuous yet extremely complex paragenetic sequence. Further research into the paragenesis of geodes could provide new and significant insights into the regional geochemical process of secondary mineralization which occurs during diagenesis. Additionally, the study of geode microminerals could generate valuable information about the movement of heavy metals and REEs in both igneous and sedimentary environments.