geode

This article relies largely or entirely upon a single source. Please help improve this article by introducing appropriate citations of additional sources. (October 2008)

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 - geōdēs, "earthlike") are geological rock formations which occur in sedimentary and certain volcanic rocks. Geodes are essentially rock cavities or vugs with internal crystal formations or concentric banding. The exterior of the most common geodes is generally limestone or a related rock, while the interior contains quartz crystals and/or chalcedony deposits. Other geodes are completely filled with crystal, being solid all the way through. These types of geodes are called nodules.

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 surrounding 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.

Coloration

Most geodes contain clear quartz crystals, while others have purple amethyst crystals. Still others can have agate, chalcedony, or jasper banding or crystals such as calcite, dolomite, celestite, etc. There is no easy way of telling what the inside of a geode holds until it is cut open or broken apart. However, geodes from any one locality usually have a more restricted variety of interior mineralization.

Geodes and geode slices are sometimes dyed with artificial colors. Samples of geodes with unusual colors or highly unlikely formations have usually been synthetically altered.

Prevalence

Geodes are common in some formations in the United States (mainly in Indiana, Iowa, Missouri, Kentucky, and Utah). They also are common in Brazil, Namibia, and Mexico. A large geode was discovered in Put-in-Bay, Ohio in the early 20th century. It is known as Crystal Cave, and tours are possible during the summer. In 1967, Iowa designated the geode as the official state rock, and it has a Geode State Park.

See also

• Septarian nodule
• Lithophysa
• Thunderegg

Sources

• Rocks and Minerals, written by Fredrick H. Pough, ISBN 0-395-91096-X

External links

Wikimedia Commons has media related to: Geode

• Geode images, locations and discussion
• Australian Museum Fact sheet
• Utah Geode Beds
• Illinois State Geological Survey. "Geodes—Small Treasure Vaults in Illinois". http://www.isgs.uiuc.edu/maps-data-pub/publications/geobits/pdf-files/geobit3.pdf.