Stalactite of the abundance of limestone caves. The corresponding formation on the floor of the cave is known as a stalagmite. The most common stalactites are speleothems, which occur in limestone caves. They form through deposition of calcium carbonate and other minerals, which is precipitated from mineralized water solutions. Limestone is the chief form of calcium carbonate rock which is dissolved by water that contains carbon dioxide, forming a calcium bicarbonate solution in underground caverns. The chemical formula for this reaction is: This solution travels through the rock until it reaches an edge and if this is on the roof of
Amorphous calcium carbonate following Ostwald's step rule, which states that the least stable polymorph will precipitate first. But while ACC is the first product to precipitate, it rapidly transforms into one of the more stable polymorphs within seconds. When in pure CaCO, ACC transforms within seconds into one of the crystalline calcium carbonate polymorphs. This transformation from amorphous to crystalline is proposed to be a dissolution-reprecipitation mechanism. Despite ACC’s highly unstable nature, some organisms are able to produce stable ACC. For example, the American Lobster "Homarus americanus", maintains stable ACC throughout its yearly molt cycle. Studies of biogenic ACC have also shown that