Wikipedia

Geode

For other uses, seeGeode (disambiguation).
Not to be confused withGeoid.

Ageode(/ˈdʒiː.oʊd/;fromAncient Greekγεώδης(geṓdēs)'earthlike') is ageologicalsecondary formation withinsedimentaryandvolcanic rocks.Geodes are hollow, vaguely spherical rocks, in which masses ofmineralmatter (which may include crystals) are secluded. Thecrystalsare formed by the filling ofvesiclesin volcanic andsubvolcanic rocksby minerals deposited fromhydrothermal fluids;or by thedissolutionof syn-genetic concretions and partial filling by the same or other mineralsprecipitatedfrom water,groundwater,or hydrothermal fluids.

Quartz-filled geode, shown from inside (top) and outside (bottom)

Formation

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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 inigneous rocks,such as vesicles in basalticlava;or, as in the AmericanMidwest,in rounded cavities in sedimentary formations. After rock around the cavity hardens, dissolvedsilicatesand/orcarbonatesare 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

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Reddishchalcedonygeodes

Geode banding and coloration is the result of variable impurities.Iron oxideswill impart rust hues to siliceous solutions, such as the commonly observed iron-stained quartz. Most geodes contain clear quartz crystals, while others have purpleamethystcrystals. Still others can have agate,chalcedony,orjasperbanding or crystals such ascalcite,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 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

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Geodes are found where the geology is suitable with many of the commercially available ones coming fromBrazil,Uruguay,Namibia,andMexico.[1]Large, amethyst-lined geodes are a feature of the basalts of theParaná and Etendeka trapsfound in Brazil, Uruguay and Namibia. Geodes are common in some formations in the United States (mainly inIndiana,Iowa,Missouri,westernIllinois,Kentucky,andUtah). Geodes are also abundant in theMendip HillsinSomerset,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 anoduleorthunderegg.[4]

Crystal caves

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Very largeamethystand regular quartz geodes

'Crystal cave' is both an informal term for any large crystal-lined geode and also used for specificgeoheritagelocations such as theCrystal Cave (Ohio),discovered in 1887 at the Heineman Winery on Put-In-Bay, Ohio, theCave of the Crystals(Mexico), and thePulpi Geode,discovered in 1999 in Spain. In 1999, a mineralogist group discovered a cave filled with giantselenite(gypsum) crystals in an abandoned silver mine, Mina Rica, near Pulpi,Province of Almeria,Spain.The cavity, which measured 8.0 by 1.8 by 1.7 metres (26.2 ft × 5.9 ft × 5.6 ft), was, at the time, the largest crystal cave ever found. Following its discovery, the entrance to the cave was blocked by five tons of rock, with an additional police presence to prevent looters. In the summer of 2019 the cave, a significantgeotourismresource and now named the 'Geoda de Pulpi',Pulpi Geode,was opened as a tourist attraction, allowing small groups (max. 12 people) to visit the caves with a tour guide.[5]

See also

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References

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  1. ^ab"Geodes: The rocks with a crystal surprise inside!".geology.Retrieved2017-12-30.
  2. ^"Geodes".geology.
  3. ^"The Quarries".Dulcote, Somerset.Retrieved2017-12-30.
  4. ^"Definition of Geode".Mindat.Retrieved2023-05-22.
  5. ^"Geoda de Pulpi".Retrieved1 September2022.

Further reading

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