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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.

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.
The above definition was borrowed from Wikipedia.

Another definition with particular reference to the geodes from Iowa commonly known as Keokuk geodes after the town of Keokuk, in the far south eastern part of Iowa near where geodes were/are found
The word "geode" is derived from Latin meaning "earthlike," a reference to their rounded shape. Most Iowa geodes are roughly spherical, often lumpy or cauliflower-like in external form, with diameters typically ranging between about two and six inches. However, specimens up to 30 inches are known. The most prized geodes have hollow interiors, although many geodes are solid objects in which crystal growth has filled most or all of the interior volume. Although the distinction may seem subtle, it is important to contrast geodes with other crystal-lined cavities or "vugs." Geodes differ from vugs in possessing an outer mineral layer which is more resistant to weathering than the host rock. As such, complete geodes commonly weather out of rock exposures and accumulate in stream bottoms. Crystal-lined vugs would not weather in such a manner.
This definition was borrowed from http://www.igsb.uiowa.edu/browse/geodes/geodes.htm [Link Broken? Jul 2014]

Note that the spanish language cognate "geoda" simply means any crystal-lined vug, so the english meaning is a bit narrower, but spanish speakers often misuse it in the broader sense when writing in english.

See also Thunder Egg.

Classification of Geode

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Other Names for Geode


Other Information

Health Warning:No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.

Mindat.org articles about Geode

Article entries:
GeodesDave Crosby
ThundereggsDave Crosby

Internet Links for Geode

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    Localities for Geode

    The map shows a selection of localities that have latitude and longitude coordinates recorded. Click on the symbol to view information about a locality. The symbol next to localities in the list can be used to jump to that position on the map.
    (TL) indicates type locality. ? indicates mineral may be doubtful at this locality. All other localities listed without reference should be considered as uncertain and unproven until references can be found.
    • Chubut
      • Mártires Department
        • Las Plumas
    Van King
    Raúl Jorge Tauber Larry
      • Wanda mines
    Raúl Jorge Tauber Larry
    Raúl Jorge Tauber Larry
      • Tafí del Valle Department
        • Estancia Anchilla
    R. J. Tauber Larry (2012). Visita a los cuerpos pegmatíticos de Estancia Anchilla, Departamento Tafí del Valle, Provincia de Tucum´´an.; R. J.Tauber Larry (2012). Visita a los cuerpos pegmatíticos de Estancia Anchilla, Departamento Tafí del Valle, Provincia de Tucumán.
    Raúl J. Tauber Larry (2012).
    Hugo A. Peña (1970): Minerales y rocas de aplicación de la Provincia de Tucumán. Secretaría de Estado de Comercio, Industria y Minería. Dirección Provincial de Minas. Tucumán. República Argentina.
      • Trancas Department
    Ávila, J.C. y Fogliata, A.S., 1992. Descripción de las geodas de Vipos, provincia deTucumán. Nota Breve. 1ra Reunión de Mineralogía y Metalogenia y 1ra Jornada de Mineralogía, Petrografía y Metalogénesis de Rocas Ultrabásicas. Instituto de Recursos Minerales, UNLP. Publicación 2: 29-34.
      • Yerba Buena Department
    Raúl Jorge Tauber Larry
    • Veszprém Co.
      • Balaton Uplands
    Kiss, G. & Molnár, F. (2006) Mineralogy And Origin Of Geodes In The Balatonfelvidék Sandstone Formation At Csopak Village, Balaton Highland, Hungary. Acta Mineralogica-Petrographica, Abstract Series 5, Szeged, p54
    • Maharashtra
    • Chihuahua
      • Mun. de Aldama
    Karl Volkman
    • Colorado
      • Fremont Co.
    Minerals of Colorado (1997) E.B. Eckels
    Minerals of Colorado (1997) E.B. Eckels
      • Larimer Co.
        • Rocky Mountain National Park
    Minerals of Colorado (1997) E.B. Eckels
      • Las Animas Co.
    Minerals of Colorado (1997) E.B. Eckels
      • Rio Grande Co.
    Minerals of Colorado (1997) E.B. Eckels
      • San Miguel Co.
        • San Juan Mts
    Minerals of Colorado (1997) E.B. Eckels
    • Illinois
    Van King
    • Indiana
      • Brown Co.
        • Trevlac
    Van King
    • Nevada
      • Humboldt Co.
        • Lone Pine District
    USGS Bull 1538D
    • New Mexico
      • Luna Co.
        • Carrizalillo District
    NMBGMR Open-file Report OF-459
        • Cooks Peak District (Cooke's Peak District)
    NMBGMR Open-file Report OF-459
    • Tennessee
      • Sevier Co.
    Economic Geology; 1 August 1971; v. 66; no. 5; p. 777-791
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