Vaterite (CaCO3) is a mineral, a polymorph of calcium carbonate. It was named after the German mineralogist Heinrich Vater. It is also known as mu-calcium carbonate (?-CaCO3) and has a JCPDS number of 13-192. Unlike the other calcium carbonate polymorphs, calcite and aragonite, vaterite is thought to be a metastable phase and has higher solubility than the other forms of calcium carbonate. Therefore, once vaterite is exposed to water, it converts to calcite (at low temperature) or aragonite (at high temperature: ~60C). However, vaterite does occur naturally in mineral springs, organic tissue, gallstones, and urinary calculi. In those circumstances, some impurities (metal ions or organic matter) may stabilize the vaterite and prevent its transformation into calcite or aragonite. The color of vaterite is usually white, its shape is spherical, and its diameter is small, ranging from 0.05 to 5 ?m.

Vaterite can be produced as the first mineral deposits repairing natural or experimentally induced shell damage in some aragonite-shelled molluscs (e.g., gastropods). Subsequent shell deposition occurs as aragonite.