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geode

   Also found in: Dictionary/thesaurus, Medical, Acronyms, Wikipedia, Hutchinson 0.09 sec.
geode (jē`ōd), hollow, globular rock nodule ranging in diameter from 1 to 12 in. (2.54–30.5 cm) or more. Most geodes are partly filled with mineral matter; they have a thin layer of chalcedony ("wavy" quartz) covering an inner lining of inward-projecting crystals. These spectacular crystals, generally quartz or, less often, calcite, make geodes highly prized by collectors. Geodes are formed in a cavity such as might be found inside a fossil shell buried in sediment. At the beginning, this cavity is probably filled with a concentrated salt solution. The first step in the creation of a geode is the formation along the inner cavity wall of a layer of gelatinous silica, which will eventually be transformed into the chalcedony layer. As the water surrounding the layer becomes less salty, osmosis induces migration of fluids into the cavity. This results in a buildup of pressure, causing the cavity to expand until the water inside and outside is equally salty. When the silica gel dehydrates, crystallizes to form chalcedony, and cracks, mineral-bearing water enters to slowly deposit the inward-projecting crystals. A geode measuring 26 ft (8 m) long and 6 ft (1.8 m) across was found in an old silver mine near Almería, NE Spain, in 2000. See also concretion concretion, mass or nodule of mineral matter, usually oval or nearly spherical in shape, and occurring in sedimentary rock. It is formed by the accumulation of mineral matter in the pore spaces of the sediment, usually around a fossil or fossil fragment acting as a
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geode

Hollow mineral body found in limestones and some shales, commonly a slightly flattened globe 1–12 in. (2.5–30 cm) in diameter and containing a chalcedony layer surrounding an inner lining of crystals. The hollow interior often is nearly filled with inward-projecting crystals, new layers growing on top of old. The crystals are often of quartz but sometimes of other minerals.


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Co-ordinator Martha Cunningham-Closs says the formation of GEODE resulted as a solution to assist people who suffered financial hard-ship from an Inco strike about 10 years ago.
The cave, known as a geode (hollow crystal-lined rock), is the largest ever found in the world.
 
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