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 calcite is next to the outer chalcedonic layer, a second layer 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. The geode will expand in response to the pressure, either at the expense of surrounding limestone, which dissolves at the limestone-silica intertace, 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. Inc. We will keep looking for new information and update as we find it One place to find out more ahout rocks, and fossils is at the Falls of the Ohio State Park, P0 Box 1327, Jeffersonville, IN, 47131-1327. This site designed and hosted by Blueriver Networking Services, Inc.