Cave formations

Cave formations, also known as pretties, are the result of the interactions between water, rock, and the atmosphere within caves.

Most cave chemistry revolves around calcium carbonate (CaCO3, or limestone), a slightly soluble mineral whose solubility increases with the introduction of carbon dioxide. It is paradoxical in that its solubility decreases as the temperature increases, unlike the vast majority of dissolved solids. This decrease is due to interactions with the carbon dioxide, whose solubility is diminished by elevated temperatures; as the carbon dioxide is released, the calcium carbonate is precipitated.

Most other solution caves that are not in limestone are in gypsum (calcium sulfate), the solubility of which is positively correlated with temperature.

As water percolates through rock, it dissolves certain compounds; with caves, these compounds are usually the ones mentioned above (calcium carbonate, or calcium sulfate). Eventually, this water may reach a void within the rock, causing its solutes to precipitate. This precipitation may be a function of concentration through water removal (calcium sulfate, calcium carbonate), or through the loss of carbon dioxide (calcium carbonate). Formations may be produced on the ceiling, creating pendulous structures called stalactites or structures that "grow" from the floor of the cave upwards, called stalagmites These may grow together, given enough time.

Additional formations include cave pearls that are the result of water dropping from far above, and causing small "seed" crystals to turn over so often that they form into near-perfect spheres of calcium carbonate.

Other formations include bacon, soda straws, chandeliers, helictites, and many more. Occasionally they are colored because of the inclusion of specific elements, such as copper, but they are typically brown or mud-colored because of particulate inclusions from mud or silt.