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There are many theories out there to choose from, but the most agreed upon by geologists is that, originally the geodes were nodules with crystal growth around a nucleus or core, and the solid concretions some how got desolved leaving a hollow void. The minerals we find inside were transported by groundwater solutions and left behind as replacements of the geode walls or as crystalline growths within there interior cavities. (Geode brochure, Iowa DNR, 1995) This is good, but I believe geodes are formed when a void caused by decaying organic material is filled with mineral rich groundwater fluids deep on the ocean floor. The fluids get trapped in the void when the elements in the geodes form minerals, most commonly quartz - Si02. These minerals are forced to the interior walls of the void due to the extreme pressures caused by the weight of the ocean water. Once the minerals settle, the microcrystalline quartz crystals form layers trapping the larger elements inside, thus starting the ... more
firstcrackgeodes.com
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We don't know but we have a theory for two types. Partly hollow balls filled mostly with quartz crystals or agate — commonly called geodes — have at least two entirely different origins, says geologist professor Gary A. Smith of the University of New Mexico. They form in: • lava (rhyolite) and • dolomite, a sedimentary rock resembling limestone. No matter what kind of rock, however, embeds the geodes originally, the rock must contain cavities for the geode's crystals to grow into, says geologist Jonathan H. Goodwin of the Illinois State Geological Survey. So, how do the cavities form, and how do the crystals grow inside? It depends on the original rock. Let's consider lava and other igneous rock first. Long ago, as molten rock cooled, dissolved gasses formed bubbles (like dissolved air forms little bubbles in ice cubes as the water freezes). When the lava hardened, the bubbles became cavities — the barest beginning of thundereggs. The minerals forming the crystals could have come ... more
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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". more
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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\". more
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(Golly, someplace, Michigan) ... My three geodes, each lined with sparkling quartz crystals. Photo by author. ... A: A trio of geodes gleams on my desk, white crystals glittering in the light. Geode — the name comes from the Greek (geoides), meaning earthlike, a reference to their rounded shape. more
dictionary.reference.com
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