Keokuk, Iowa is situated at the confluence of the Mississippi and Des Moines Rivers in the extreme southeastern portion of the state of Iowa. Illinois borders Iowa on the east and Missouri is to the south (area known locally as the "Tri-State area"). See map below for a visual view:
Map Credit/Reference: City Data (www.city-data.com)
Keokuk geodes originate in the lower Warsaw Formation, a Mississippian age rock unit approximately 250-300 million years old. The geodes are located in either a dolomitic mudstone (generally the upper portion of the lower Warsaw, a softer rock with smaller geodes, on average) or argillaceous dolomite (generally the lower portion of the lower Warsaw, a harder rock with a variety of geode sizes but, on average, the geodes are larger). Keokuk geodes range in size from peas (less than a half inch diameter) up to 30 inches (2.5 feet) in diameter. The most common size is between 3-5 inches in diameter across all locations but the average size varies considerably from location to location.
Below are three pictures that show Keokuk geodes in their host argillaceous dolomite. The geodes are the rounder shaped, brownish/tannish colored objects in the flatter, layered gray dolomite:
Keokuk geodes in their host argillaceous dolomite. The geode size varies from 4-12 inches (glove provided for scale). Copyright 2012 - The Geode Gallery.
Keokuk geodes (both opened and whole) in their host argillaceous dolomite. The opened geode near the middle of the photograph is approximately 10-11 inches in diameter and is lined with clear quartz crystals. Also note the somewhat irregular shapes of some of the geodes (especially the larger geode on the left side of the image) - not all geodes are round! Copyright 2012 - The Geode Gallery
Keokuk geodes in their host argillaceous dolomite. The geode size ranges from under 2 inches up to 8 inches across. A standard size rock pick is provided for scale. Note how the geodes occur in layers and the variety of sizes that can occur next to each other. Copyright 2012 - The Geode Gallery.
Two opened Keokuk Geodes in their host argillaceous dolomite. The two opened geodes are lined with quartz and also contain secondary crystals of calcite upon the quartz. Each matrix piece is approximately 7.50 inches across and the two sections fit back together. Copyright 2012 - The Geode Gallery.
Mineral Inclusions in Keokuk Geodes
Up to 17 minerals have been positively identified as being in Keokuk, Iowa area geodes strictly from the Warsaw Formation (not including geodes from the underlying Keokuk Formation), including (in no particular order) quartz (and chalcedony), kaolinite, marcasite, pyrite, chalcopyrite, barite, pyrolusite, goethite, sphalerite, calcite, dolomite, gypsum, hematite, aragonite, smithsonite, malachite, and jarosite. To assist the geode collector in identifying the minerals in his/her geodes, we are providing photographs of each geode mineral found in Keokuk, Iowa geodes below. We have also provided a short section highlighting additional minerals found in geodes from the Keokuk Formation and commonly misidentified minerals as well. Please note each geode is different so the mineral or assemblage of minerals in your geode may not look exactly like what is pictured below, but it is our hope that by looking at the variations in crystal structure and color in addition to the descriptions provided for each mineral that you will be able to confidently label your Keokuk geodes correctly.
Quartz is the most frequently encountered mineral in Keokuk geodes. Other than calcite geodes, all Keokuk geodes are lined with quartz crystals. Quartz crystals can appear in many colors, including clear, light blue, yellow, orange, smoky, red, sea green, and very infrequently a pale lavender or amethyst. Much of the coloring to quartz in Keokuk geodes is the result of pyrite, marcasite, and/or goethite weathering down and imparting a color in, under, or on the quartz crystal itself. Quartz "snowballs" may also reside in some geodes. "Snowballs" are chalcedony/silica growths (usually found in a round or semi-round shape) that are found on top of the initial quartz lining and may have a small or numerous contact points to the primary quartz lining. If evidence of the chalcedony/silica ball is present, the geode can be classified as a "snowball" regardless of how many contact points there are or the shape/nature of the growth. By definition, it fits Sinotte's (1969) description of a "snowball" geode if it has the chalcedony/silica structure present. Other geodes may be "dewdrop diamond" geodes which are amongst the most highly prized Keokuk geodes by collectors. Small quartzoid crystals (either clear or smoky) are found resting like sparkling dew drops on a white/grey chalcedony lining, hence the name.
Photographs of Quartz in Keokuk Geodes
Clear Quartz lined Keokuk, Iowa Geode. The half measures approximately 4 inches across.
Yellow-Orange Iron Stained Quartz lined Keokuk, Iowa Geode. The half measures approximately 3.50 inches across. The yellow-orange color is caused by the weathering of an iron mineral (usually pyrite but could also be marcasite or goethite). Geodes found loose in creeks, on sand or gravel bars, or in the softer mudstone unit of the Warsaw Formation frequently contain a yellow-orange coloring such as the geode pictured to the left. Natural citrine geodes do not occur in Keokuk geodes - even in those specimens where the yellow/orange coloring will not clean or wash out of the geode. It is still a weathering product of an iron mineral underneath the quartz lining and is not natural citrine. A Keokuk geode can be described as being "citrine colored", but it is not natural citrine.
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Please note this page is currently undergoing significant updates. Please visit http://www.geodegallery.com/keokuk.html for the old page. Please continue to check back to this page for the latest updates including new mineral inclusion images and much more over the coming months. Thank you for your patience while we update this page with higher quality images and more detailed text.