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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 concretionconcretion,
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 nucleus.
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a form of natural mineral aggregate. It is represented by closed cavities in certain rocks, which are filled with cryptocrystalline or eucrystalline aggregates of minerals. Geodes may be isometric or round or have other shapes. The mineral substance in a geode is frequently deposited by layers, forming concentric zonal layers (for example, agates). Geodes may not be filled completely with minerals; in this case a hollow place remains in the middle that is usually strewn with vugs of crystals, stalactites, and so forth. They may be more than 1 m long in cross section; the minimum size of a geode is fractions of a centimeter (so-called amygdaloids).