New Mexico Mineral Symposium — Abstracts

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A variety of spheroidal sedimentary structures occur in the American Midwest where they have been regarded as great curiosities. The rounded objects differ so much from the flat and layered rocks that host them that some have been given special names such as nodules, concretions, septaria, agates, and geodes. The focus of this talk is on geodes, perhaps the most interesting to the collector, because their often hollow centers may sometimes contain the surprise of beautiful and well-crystallized minerals.

Midwest geodes differ from and are easily separable from the carbonate rocks in which they occur. Geodes have an often thin, dense outer layer of microcrystalline, fibrous quartz (chalcedony) that is typically, but not always followed by an interior layer of white, interlocking, mosaic-textured crystalline quartz, and finally a center of inward-pointing quartz crystals. If growth is stopped early, that center remains open, providing a space in which other minerals may later crystallize.

Midwestern geodes occur only in rocks of Mississippian age, and while there are numerous outcrops where they may be found, this talk will cover the three most productive localities for minerals of collector interest: the Keokuk Geode Field, the Indiana Geode Field, and the Kentucky Geode Field.

For more information about Midwestern geodes, please refer to the article on this subject in the Jan/Feb 2017 issue of Rocks & Minerals magazine.