Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratch hardness comparison. Large calcite crystals are used in optical equipment, and limestone composed mostly of calcite has numerous uses.
Rhombohedral calcite
Scalenohedral calcite
Prismatic calcite
Prismatic calcite
Carbonate minerals are those minerals containing the carbonate ion, CO2−3.
Calcite crystals from the Sweetwater Mine, Viburnum Trend District, Reynolds County, Missouri; 6.2 × 6 × 3.3 cm
Rhodochrosite, Sweet Home Mine, Alma, Colorado; 5.2 × 4.2 × 2.3 cm
Smithsonite, Silver Bill Mine, Dragoon Mts, Cochise County, Arizona; 4.8 × 4.1 × 2.4 cm
Dolomite with calcite and chalcopyrite from the Picher Field, Tri-State district, Cherokee County, Kansas; 12.0 × 9.7 × 4.3 cm