Some Types of Geodes
"It's what's inside that counts."
Here's a few of the geodes found in our collection.
Approximately 6 to 8 million years ago (Miocene epoch), volcanic activity occurred in western Utah and deposited an extrusive igneous rock called rhyolite (See below). Trapped gasses formed cavities within the rhyolite, and millions of years of ground-water circulation allowed minerals to precipitate into the cavities.
Roughly 32,000 to 14,000 thousand years ago, a large body of water known as Lake Bonneville covered most of western Utah. The lake's wave activity eroded the geode-bearing rhyolite and redeposited the geodes several miles away in the Dugway geode bed area as lake sediments.
Figure 1. Photo by Jeff Patton.
Here's a picture of one:
Besides being a great sample of a geode, the Dugway also fluoresces a brilliant green under SW ultra-violet light.
Figure 3. (Photo taken with a 9W "Way Too Cool" triple wave UV light.)
The fluorescence comes from uranium salts trapped in the Chalcedony. The level of uranium is so low though, that it is undetectable, and totally harmless. Because of this, Dugway Geodes are highly sought after by collectors.
These started their formation approximately 150 million years ago when the gulf of Mexico reached what is now southern Utah.
Volcanic eruptions killed the sea life, and many became trapped in the sediment and formed mud balls. When the ocean receded, the balls were left to dry and crack. But when ocean returned, more shell life was deposited above the cracked mudballs.
As these later animals died and decomposed, the calcite from the shells seeped into the cracks of the mud balls, and calcite crystals formed.
A thin wall of calcite was transformed into aragonite dividing the bentonite clay (shale) exteriors from the calcite centers. Because of this dividing wall (septum in Latin), the geodes are called Septarians.
The septarian geode's golden star-like centers are the result of the crystallization of the mineral calcite, framed by aragonite (the brown lines). Some are filled in completely with calcite, while others left hollows showing the tooth-like calcite crystals and in rare cases, some perfect crystals of barite as well (white or clear crystals). The gray areas are limestone.
Keokuk geodes are found in the Warsaw formation, a layer of exposed rock in approximately a 50 mile radius around Keokuk, Iowa.
And while this layer also extends into Illinois and Missouri, they are still referred to as "Keokuk geodes."
This Warsaw formation is characterized by two distinct geode bearing layers, a gray shale layer and a brown carbonate rich-clay layer.
The gray shale (lower level) is very hard, and the geodes do not come out easily. However, when opened, they are very clean on the inside and don't require much clean up.
The brown layer is a carbonate rich clay, much softer than the gray shale.
Many rivers and creeks in the Keokuk area have cut though these layers and weathered out the geodes. So, you can find them along these waterways, and can easily picked them up.
Here a picture of a calcite geode from Keokuk.
As with other earth events and features, the unique composition of geodes can teach us about minerals, how they interact to produce structure, and ultimately the world we live on.
Definition of Calcite
- Noun. A common mineral consisting of crystallized calcium carbonate; a major constituent of limestone.
Definition of Chalcedony (kāl-sěd'n-ē)
- n. A cryptocrystalline, translucent variety of quartz, having usually a whitish color, and a luster nearly like wax.
www.hpwt.de cal·ced·o·ny (kāl-sěd'n-ē)
Definition of Rhyolite
- Noun. Very acid volcanic rock.
Here's a link to an April Holladay, science journalist for USATODAY.com, article “Whence geodes come”:
Here's a good geode site:
Slide Show Of A Few Of The Geodes Found
Here's a dealer at the Ark-La-Tex Gem & Mineral Society Rock Show, 2006. He shows how to crack a geode open and explains what's inside.
Here's a good reference dictionary:
Figures & Acknowledgments
Figure 1. geology.utah.gov
Figure 2. wasatchgemsociety.com
Figure 3. naturaluniquities.com
Figure 4. sticks-in-stones.com
Figure 5. riversedgeiowa.com
Figure 6. firstcrackgeodes.com
Figure 7. firstcrackgeodes.com