Geode

A geode (

hydrothermal fluids; or by the dissolution of syn-genetic concretions and partial filling by the same or other minerals precipitated from water, groundwater

, or hydrothermal fluids.

Formation

Geodes can form in any cavity, but the term is usually reserved for more or less rounded formations in igneous and sedimentary rocks. They can form in gas bubbles in

Midwest, in 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.^[1]

When cut in half, visible bands corresponding to varied stages of precipitation may at times show patterns that reveal points of fluid entry into the cavity and/or varied colors corresponding to changes in chemistry.

Coloration

Geode banding and coloration is the result of variable impurities.

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 a particular area are usually similar in appearance.

Geodes and geode slices are sometimes dyed with artificial colors.^[2] Samples of geodes with unusual colors or highly unlikely formations have usually been synthetically altered.

Occurrence

Geodes are found where the geology is suitable with many of the commercially available ones coming from Brazil, Uruguay, Namibia, and Mexico.^[1] Large, amethyst-lined geodes are a feature of the basalts of the Paraná and Etendeka traps found in Brazil, Uruguay and Namibia. Geodes are common in some formations in the United States (mainly in Indiana, Iowa, Missouri, western Illinois, Kentucky, and Utah). Geodes are also abundant in the Mendip Hills in Somerset, England, where they are known locally as "potato stones".^[3] The term geode generally describes hollow formations. If the rock is completely solid inside, this would be classified as a nodule or thunderegg.^[4]

Crystal caves

'Crystal cave' is both an informal term for any large crystal-lined geode and also used for specific

Pulpi Geode, was opened as a tourist attraction, allowing small groups (max. 12 people) to visit the caves with a tour guide.^[5]

References

^ ^a ^b "Geodes: The rocks with a crystal surprise inside!". geology.com. Retrieved 2017-12-30.
^ "Geodes". geology.com.
^ "The Quarries". Dulcote, Somerset. Retrieved 2017-12-30.
^ "Definition of Geode". Mindat. Retrieved 2023-05-22.
^ "Geoda de Pulpi". Retrieved 1 September 2022.

External links

Indiana geode specimens, facts and stories
Video of a geode cracking using industrial soil pipe cutter
Australian Museum Fact sheet
Utah Geode Beds
Illinois State Geological Survey. "Geodes – Small Treasure Vaults in Illinois" (PDF). Archived from the original (PDF) on 2007-09-27.

[:0-1] "Geodes: The rocks with a crystal surprise inside!". geology.com. Retrieved 2017-12-30.

[2] "Geodes". geology.com.

[3] "The Quarries". Dulcote, Somerset. Retrieved 2017-12-30.

[4] "Definition of Geode". Mindat. Retrieved 2023-05-22.

[5] "Geoda de Pulpi". Retrieved 1 September 2022.

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