The mineral species containing the CO₃²⁻ unit as an essential structural and chemical component. Members are distinguished by chemical composition (type of metal cation and in some cases other anion present) and crystal structure. More than 200 varieties of carbonate minerals are known, but only a small number occur in abundance. Calcite (CaCO₃) and dolomite (CaMg(CO₃)₂) are the most common species, occurring as the principal mineral components in limestone and Mg-rich limestone sedimentary rocks. Carbonate minerals exhibit limited stability at high temperature unless high carbon dioxide (CO₂) pressure exists, which explains their occurrence dominantly in environments near the Earth's surface and in sedimentary rocks. However, important occurrences in metamorphic rocks include marble and hydrothermal deposits, with only rare occurrences in carbonatite and kimberlite igneous rocks.