Geode
Geode. Near Well 23 Canning Stock Route a very remote part of Western Australia. Geodes form from expansion of an initial cavity filled with fluid, often a salt solution. Chalcedony, from silica gel, then forms on the edges of the cavity. This isolates the salt solution. If the surrounding water is less salty, then the internal salinity, though osmosis, creates outward directed pressure and the geode expands until equilibrium is reached. Crystallisation and dehydration of the silica gel leads to cracking, and sometimes, to entry of mineral-bearing fluids. These fluids may precipitate minerals leading to geodes which are completely crystallised (infilled with crystals). Sometimes, the term geode is applied to any spherical cavity within rocks, but strictly speaking, it should only be applied to the above rocks
