geode

geode
geodic /jee od"ik/, geodal /jee ohd"l/, adj.
/jee"ohd/, n.
1. a hollow concretionary or nodular stone often lined with crystals.
2. the hollow or cavity of this.
3. any similar formation.
[1670-80; < F géode < L geodes < Gk geódes earthlike. See GEO-, -ODE1]

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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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      hollow mineral body found in limestones and some shales. The common form is a slightly flattened globe ranging in diameter from 2.5 to more than 30 cm (1 to 12 inches) 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 of quartz, less often of calcite or dolomite, and sometimes of aragonite, ankerite, hematite, magnetite, pyrite, chalcopyrite, and sphalerite. Most often the metal sulfides are innermost, and a layer of calcite is next to the outer chalcedonic layer; a second layer of chalcedony sometimes is present.

      Geodes form by expansion from an initial fluid-filled cavity, such as the opening in a bivalve. The initial deposit, chalcedony, forms from a silica gel that surrounds and isolates the salt solution. If the water surrounding the forming geode becomes less saline, osmosis begins to bring the salinity inside into equilibrium with that outside (water seeps in, diluting the trapped solution), and the internal pressure increases. The geode will expand in response to the pressure, either at the expense of the surrounding limestone, which dissolves at the limestone-silica interface, or, if the limestone has not hardened, by pushing aside the lime mud. Expansion continues until the pressure difference becomes negligible. If the initial cavity is inside a fossil, the fossil is burst by the expanding geode. Ultimately, dehydration and crystallization of the silica gel occur, followed by shrinkage, cracking, and the entrance of water carrying dissolved minerals, which are deposited on the chalcedony wall.

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Universalium. 2010.

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  • géode — [ ʒeɔd ] n. f. • 1752; géodès 1556; du gr. geôdês « terreux » 1 ♦ Minér. Masse pierreuse sphérique ou ovoïde, creuse, dont l intérieur est tapissé de cristaux. 2 ♦ Pathol. Petite cavité bien circonscrite, constituée dans un tissu (surtout osseux) …   Encyclopédie Universelle

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  • geode — rounded stone with a hollow center lined with crystals, 1670s, from Fr. géode, from L. geodes, from Gk. geodes earthy, earth like, from ge earth (Homeric gaia), a word of pre I.E. origin + oides, adjective suffix, characterized by. Perhaps so… …   Etymology dictionary

  • geode — [jē′ōd΄] n. [Fr géode < L geodes, a precious stone < Gr geoidēs, earthlike < gē, earth + eidos, OID] 1. a globular stone having a cavity lined with inward growing crystals or layers of silica 2. a) such a cavity b) any formation like… …   English World dictionary

  • Geode — Ge ode (j[=e] [=o]d), n. [F. g[ e]ode, L. geodes, fr. Gr. ? earthlike; ge a, gh^, the earth + e i^dos form.] (Min.) (a) A nodule of stone, containing a cavity, lined with crystals or mineral matter. (b) The cavity in such a nodule. [1913 Webster] …   The Collaborative International Dictionary of English

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  • geode — ► NOUN 1) a small cavity in rock lined with crystals or other mineral matter. 2) a rock containing such a cavity. ORIGIN from Greek ge d s earthy …   English terms dictionary

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