Geode

For other uses, see Geode (disambiguation).

Geode, halved and polished

Geode, halved and polished

Geodes in unusual rock formation

Large hollow geode at the Carefree Resort in Carefree, Arizona. Booklet shown for scale is 8.5 by 11 inches.

Geodes (Greek γεώδης - ge-ōdēs, "earthlike") are geological secondary sedimentary structures which occur in sedimentary and certain volcanic rocks. Geodes are essentially spherical masses of mineral matter that were deposited sygenetically within the rock formations they are found in. Geodes have a Chalcedony shell containing various minerals, usually quartz. Geodes differ from vugs in that they were formed as a separate entity from the surrounding rock, whereas vugs are voids or cavities within a rock formation. Geodes also differ from "nodules" in that a nodule is a mass of mineral matter that has accreted around the nodule nucleous. Both structures had the minerals contained within deposited from groundwater or hydrothermal processes. The exterior of most geodes generally consists of Chalcedony, a cryptocrystalline form of quartz. Inside the Chalcedony shell many minerals have been found such as Calcite, Pyrite, Kaolinite, Sphalerite, Millerite, Barite, Dolomite, Limonite, Smithsonite, and Quartz, which is by far the most common and abundant mineral found in geodes. Geodes are found mostly in basaltic lavas and Limestones of the Warsaw formation in the Keokuk region near the area where Missouri, Iowa, and Illinois all join together.

Formation

Geodes can form in any cavity, but the term is usually reserved for more or less rounded formations in igneous and sedimentary rocks, while the more general term "vug" is applied to cavities in fissures and veins. They can form in gas bubbles in igneous rocks, such as vesicles in basaltic lavas, or as in the American Midwest, rounded cavities in sedimentary formations. After rock around the cavity hardens, dissolved silicates and/or carbonates are deposited on the inside surface. Over time, this slow feed of mineral constituents from groundwater or hydrothermal solutions allows crystals to form inside the hollow chamber. Bedrock containing geodes eventually weathers and decomposes, leaving them present at the surface if they are composed of resistant material such as quartz.

See also

• Septarian nodule
• Lithophysa
• Thunderegg
• Bristol Diamonds

Further reading

• Rocks and Minerals, written by Fredrick H. Pough, ISBN 0-395-91096-X
• Gerard V. Middleton: Encyclopedia of Sediments and Sedimentary Rocks. Springer 2003, ISBN 9781402008726, p. 221 (restricted online copy, p. 221, at Google Books)
• Walter David Keller: The common rocks and minerals of Missouri. University of Missouri Press 1961, ISBN 9780826205858, S. 67 (restricted online copy, p. 67, at Google Books)
• Brian J. Witzke: Geodes: A Look at Iowa's State Rock Iowa Geological Survey
• Geodes Kentucky Geological Survey (University of Kentucky)

External links

• Geode images, locations and discussion
• Australian Museum Fact sheet
• Utah Geode Beds
• Illinois State Geological Survey. "Geodes—Small Treasure Vaults in Illinois" (PDF). {{cite news}}: Cite has empty unknown parameter: |coauthors= (help)