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 (µ-CaCO₃) and has a JCPDS number of 13-192. Vaterite, like aragonite, is a metastable phase of calcium carbonate at ambient conditions at the surface of the earth. As it is less stable than either calcite or aragonite, vaterite has a higher solubility than either of these phases. Therefore, once vaterite is exposed to water, it converts to calcite (at low temperature) or aragonite (at high temperature: ~60 °C). 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. Vaterite is usually colorless, its shape is spherical, and its diameter is small, ranging from 0.05 to 5 µm.