Carbonatites: Rare intrusive or extrusive carbonate igneous rocks that are formed by magmatic or metasomatic processes. Carbonatites usually consist of 50 percent or more (by volume) primary carbonate minerals, such as calcite, dolomite, and (or) ankerite. They are genetically associated with, and therefore typically occur near, alkaline igneous rocks. The carbonate minerals in carbonatites do not form due to alteration processes; rather, carbonatites crystallize from a magma that is super-saturated in calcium and carbon dioxide. Several theories have been advanced to explain the genesis of carbonatites. The most popular theory is that carbonatites form due to partial melting of peridotites in the upper mantle of the Earth.