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Definition of geode


Geodes (Greek γεώδης - ge-ōdēs, "earthlike") are commonly independent spherical masses of resistant mineral matter that are usually hollow. The extreme filling of a geode so that it is "solid" is also common. Some people refer to completely in-filled geodes as nodules, but the two names are not always consistently used. Geodes differ from "nodules" in that a nodule is a solid mass of mineral matter, but some nodules may have a tiny void space in their interiors and the distinction between a geode and a nodule may be barely discernible if the cavity is miniscule. Geodes differ from vugs in that geodes can be separated free from any matrix, because the walls of geodes are strong enough to maintain the integrity of their initial shapes. 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 canyons, talus, rock detritus, etc. Many geode localities occur is desert areas and are elluvial, not alluvial, meaning the geodes have not been transported by water.

The most common geodes are dominantly quartz, but geodes may be composed of other minerals such as calcite, goethite, etc. Many quartz geodes consist of concentric layers of several varieties of quartz, such as chalcedony, agate, common opal, and visibly crystalline quartz. The order of the kinds of mineral layers varies with the particular history of the formation of geodes. The interior of geodes may also contain a wide variety of independently crystallized minerals: calcite, pyrite, kaolinite, sphalerite, millerite, baryte, dolomite, smithsonite and quartz. Geodes have been found in regions that have basaltic lavas, where they fill voids left by gas bubbles and in cavities in sedimentary rocks, especially limestones. Occasionally, geodes may be able to be mined, as in Brazil and Uruguay, when the enclosing rock is easily separable from the geode structure.

Geodes are often named according to a particular feature or mineral they exhibit or mention where the particular geodes were found. Common informal names for geodes include: quartz geodes, amethyst geodes, agate geodes, enhydro geodes, Oco geodes, Keokuk geodes, coconut geodes, etc. Thunder eggs may occur as geodes or nodules. See also Thunder Egg.

Note that the Spanish language cognate "geoda" simply means any crystal-lined vug, so the English meaning used here is narrower.

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