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 syngenetically 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. The Warsaw Formation in the Keokuk region near the area where Missouri, Iowa, and Illinois join contains abundant geodes.

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.

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.

In 2000, a team of geologists found a cave filled with giant gypsum crystals in an abandoned silver mine near Almería, Spain. The cavity, which measures 1.8 × 1.7 meters and is 8 meters in length, would be the largest geode ever found. The entrance of the cave has been blocked by five tons of rocks, and is under police protection (to prevent looters from entering). According to geological models, the cave was formed during the Messinian salinity crisis 6 million years ago, when the Mediterranean sea evaporated and left thick layers of salt sediments (evaporites). The cave is currently not accessible to tourists.

The Cave of the Crystals is many meters in size, holding gargantuan crystals up to 12 m in length. It may be composed of additional chambers that are obstructed by the giant crystals, so its total size is currently unknown to researchers.

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 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 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". http://www.isgs.uiuc.edu/maps-data-pub/publications/geobits/pdf-files/geobit3.pdf.