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geode

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.

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geode

ge·ode / ˈjēōd/ • n. a small cavity in rock lined with crystals or other mineral matter. ∎  a rock containing such a cavity. DERIVATIVES: ge·od·ic / jēˈädik/ adj.

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geode

geode A hollow, rounded body, which has a lining of mineral crystals pointing inward, e.g. quartz or calcite. The crystals grow into the cavity unimpeded and form perfect crystals which are frequently collected and valued for their beauty.

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geode

geode. Piece of rock having a druse, or cavity lined with crystals, or a rounded hollow iron-stone nodule used e.g. in rock-rash facings.

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geode

geode XVII. — L. geōdēs — Gr. geṓdēs earthy. f. earth.

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geode

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Geode

Geode

Geodes are crystal-lined cavities in rock. Geodes are usually roughly spherical in shape and large geodes can be 12 in (30 cm) or more in diameter. Most commonly, the crystals growing within a geode are quartz, calcite, or fluorite, although other minerals can be found in geodes.

Most geodes are characterized by an outer shell of chalcedony, a dense microcrystalline form of quartz. The hard outer shell of the geode can usually be separated from the enclosing rock material. Geodes are most often formed in limestone or volcanic rocks, and are rarely found in mudstones and shales. The mineralogy of the geode and the rock in which it formed are commonly different. Because chalcedony is often harder and more weather resistant than the host rock, the hollow spheres resist the effects of erosion and are left behind as the host rock is eroded. Collectors identify the geodes in the field based upon their shape, the characteristic chalcedony shell, and the lower density arising from the hollow center.

The initial requirement for the formation of a geode is a cavity in the host rock. In a volcanic rock, voids commonly form as gas is released from lava. As the lava cools and hardens into rock, the gas bubbles are preserved as holes in the rock. In sedimentary rocks such as limestone, a hole may develop as groundwater dissolves the rock itself or as a result of the decay of biologic material buried at the time of deposition of the sediments. Groundwater within the sediments then carries dissolved minerals, including silica, through the host rock and into the cavity. The chalcedony shell is formed first, fully hardening only after an extended period of time. Subsequent mineralized groundwater flow may then deposit additional layers of minerals within the void. These crystal growths form first on the walls of the shell and then grow toward the center, producing the distinctive crystalline interior of the geode. The formation of a geode with large crystals may require tens or hundreds of millions of years.

Some of the most spectacular geodes, which can be as large as 1 yd (1 m) in diameter and contain very

large amethyst (purple quartz) crystals, come from Brazil. Once found, geodes are often cut open with a diamond-tipped saw blade and the cut surfaces of the sphere polished for display.

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Geode

Geode

Geodes are hollow rock masses that are lined with crystals that have grown toward the center of the cavity. Geodes are usually roughly spherical in shape, up to 12
in (30 cm) or more in diameter. Most frequently, the crystals growing within a geode are quartz, calcite, or fluorite, though occurrences of other minerals are found. These objects are prized by collectors for their well-formed crystals and outstanding beauty.

Geodes are typically characterized by an outer shell of chalcedony, a dense microcrystalline form of quartz. The hard outer shell of the geode can usually be separated from the enclosing rock material. Geodes are most often formed in limestone or volcanic rocks , though they are rarely found in mudstones. The mineralogy of the geode and the rock in which it formed are commonly different. Because chalcedony is often harder and more weather resistant than the host rock, the hollow spheres resist the effects of erosion and are left behind as the host rock is eroded. Collectors identify the geodes in the field based upon their shape, the characteristic chalcedony shell, and the lower density arising from the hollow center.

The initial requirement for the formation of a geode is the presence of a cavity in the host rock. In a volcanic rock, such voids are frequently a result of the release of gases from the molten lava. As the lava hardens, the gas bubbles are preserved as holes in the rock. In the cases of sedimentary rocks, such as limestone, a hole may develop as groundwater dissolves the rock itself or as a result of the decay of biologic material buried at the time of deposition of the sediments. Groundwater within the sediments then carries dissolved minerals, including silica, through the host rock and into the cavity. The chalcedony shell is formed first, only fully hardening after an extended period of time. Subsequent mineralized groundwater flow may then deposit additional layers of minerals within the void. These crystal growths form first on the walls of the shell and then grow toward the center, producing the distinctive crystalline interior of the geode. The formation of a geode with large crystals may require tens or hundreds of millions of years.

Some of the most spectacular geodes come from Brazil. Some of these may be as large as one meter in diameter and contain very large amethyst (purple quartz) crystals. Once found, the geodes are often cut open with a diamond-tipped saw blade and the cut surfaces of the sphere polished for display.

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