calcium carbonate (CaCO₃),chemical compound consisting of oneatom ofcalcium, one ofcarbon, and three ofoxygen that is the majorconstituent oflimestone,marble,chalk, eggshells,bivalve shells, andcorals. Calcium carbonate is either a white powder or a colorless crystal. When heated, it producescarbon dioxide andcalcium oxide (also called quicklime). Calcium carbonate has amolecular weight of 100.1 grams permole.

Calcium carbonate occurs naturally in threemineral forms:calcite,aragonite, and vaterite.Calcite, the most common form, is known for the beautiful development and great variety of itscrystals. A large percentage of calcite occurs in limestones, and calcite is also the chief component ofmarls,travertines, calcite veins, mostcave deposits, many marbles and carbonatites, and some ore-bearing veins. Calcite is the stable form of calcium carbonate at most temperatures and pressures.Aragonite is the orthorhombic (i.e., having three unequal crystallineaxes at right angles to one another) form of calcium carbonate. Though frequently deposited in nature, it ismetastable at room temperature and pressure and readilyinverts to calcite.Vaterite, the hexagonal form of calcium carbonate, is extremely rare and transforms into calcite or aragonite or both.

Calcium carbonate has many uses. Since ancient times,limestone has been burned to quicklime (CaO), slaked tohydrated lime [Ca(OH)₂], and mixed with sand to makemortar. Limestone is one of the ingredients used in the manufacture ofportland cement and is often employed as aflux in metallurgical processes, such as thesmelting ofiron ores. Crushed limestone is used widely as riprap, asaggregate for bothconcrete andasphalt mixes, as agriculturallime, and as an inert ingredient of medicines.

Asmarble, calcium carbonate is used for statuary and carvings and is a popular facing stone as polished slabs. The termmarble is used differently in the marketplace from the way it is used in geology: in the marketplace, it is applied to any coarse-grainedcarbonate rock that will take a good polish rather than tometamorphic carbonate-rich rocks exclusively. Some coarsely crystallinediagenetic limestones are among the most widely used commercial “marbles.” Travertine and onyx marble (banded calcite) are also popular facing stones, usually for interior use.

Calcium carbonate obtained from its natural sources is used as a filler in a variety of products, such aspaper,ceramics,glass,plastics, andpaint.Synthetic calcium carbonate, called “precipitated” calcium carbonate, is employed when high purity is required, as in medicine (antacids and dietarycalcium supplements), in food (baking powder), and for laboratory purposes.