A speleothem (/ˈspiːliːəθɛm/; Greek: "cave deposit"), commonly known as a cave formation, is a secondary mineral deposit formed in a cave. Speleothems are typically formed in limestone or dolostone solutional caves.

Origin and composition

While more than 250 cave mineral deposits exist, the vast majority of speleothems are calcareous, composed of calcium carbonate in the form of calcite or aragonite, or calcium sulphate in the form of gypsum. Calcareous speleothems form via carbonate dissolution reactions. Rainwater in the soil zone reacts with soil CO₂ to create weakly acidic water via the reaction:

As the lower pH water travels through the calcium carbonate bedrock from the surface to the cave ceiling, it dissolves the bedrock via the reaction:

When the solution reaches a cave, degassing due to lower cave pCO₂ drives precipitation of CaCO₃:

Over time the accumulation of these precipitates forms stalagmites, stalactites, and flowstones, which compose the major categories of speleothems.