Calcite

Calcite a carbonate mineral the most stable polymorph of calcium carbonate [CaCO3].The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470 °C, and vaterite is even less stable.


Calcite exhibits several twinning types adding to the variety of observed forms. It may occur as fibrous, granular, lamellar, or compact. Cleavage is usually in three directions parallel to the rhombohedron form. Its fracture is conchoidal, but difficult to obtain.Calcite crystals are trigonal-rhombohedral, though actual calcite rhombohedra are rare as natural crystals. However, they show a remarkable variety of habits including acute to obtuse rhombohedra, tabular forms, prisms, or various scalenohedra.