calcium carbonate

calcium carbonate (CaCO₃), chemical compound consisting of one atom of calcium, one of carbon, and three of oxygen that is the major constituent of limestone, marble, chalk, eggshells, bivalve shells, and corals. Calcium carbonate is either a white powder or a colorless crystal. When heated, it produces carbon dioxide and calcium oxide (also called quicklime). Calcium carbonate has a molecular weight of 100.1 grams per mole.

Calcium carbonate occurs naturally in three mineral forms: calcite, aragonite, and vaterite. Calcite, the most common form, is known for the beautiful development and great variety of its crystals. A large percentage of calcite occurs in limestones, and calcite is also the chief component of marls, travertines, calcite veins, most cave 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 crystalline axes at right angles to one another) form of calcium carbonate. Though frequently deposited in nature, it is metastable at room temperature and pressure and readily inverts 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 to hydrated lime [Ca(OH)₂], and mixed with sand to make mortar. Limestone is one of the ingredients used in the manufacture of portland cement and is often employed as a flux in metallurgical processes, such as the smelting of iron ores. Crushed limestone is used widely as riprap, as aggregate for both concrete and asphalt mixes, as agricultural lime, and as an inert ingredient of medicines.

As marble, calcium carbonate is used for statuary and carvings and is a popular facing stone as polished slabs. The term marble is used differently in the marketplace from the way it is used in geology: in the marketplace, it is applied to any coarse-grained carbonate rock that will take a good polish rather than to metamorphic carbonate-rich rocks exclusively. Some coarsely crystalline diagenetic 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 as paper, ceramics, glass, plastics, and paint. Synthetic calcium carbonate, called “precipitated” calcium carbonate, is employed when high purity is required, as in medicine (antacids and dietary calcium supplements), in food (baking powder), and for laboratory purposes.