Calcium carbonate is a chemical compound that forms the primary ingredient in limestone and exists in the chalk cliffs of Dover, the marble statues of Rome, and the eggshells of birds. It serves as a universal building block that connects the inorganic world with the living one and defines the texture of the planet's crust and the chemistry of its oceans.
Calcium carbonate exists in three distinct crystalline forms: calcite, aragonite, and vaterite. Calcite is the most common and thermodynamically stable form with a trigonal crystal structure, aragonite is the denser orthorhombic form found in marine shells, and vaterite is the least stable polymorph that quickly transforms into calcite.
Calcium carbonate dissolves when water saturated with carbon dioxide transforms it into soluble calcium bicarbonate, a reaction that carves out vast underground caverns. This process creates hard water that clogs pipes and forms limescale in kettles while influencing the carbonate compensation depth ranging from 4,000 to 6,000 meters below sea level.
Calcium carbonate serves as a filler, stabilizer, and structural component in paper, paint, plastic, cement, and steel production. It is also used as a firming agent in canned vegetables, a calcium supplement in fortified foods, and agricultural lime to neutralize acidic soils.
Molluscs, arthropods, and corals secrete calcium carbonate through biomineralization using specific proteins and macromolecules to control crystallization. These organisms can select between calcite and aragonite forms to create shells that are either strong and durable or flexible and lightweight depending on their needs.
Calcium carbonate neutralizes the effects of acid rain in rivers and lakes through a process called liming which has been practiced on a large scale in Sweden since the 1970s. It is also used in flue-gas desulfurization to eliminate harmful emissions from coal and fossil fuel power stations and in carbon capture technologies.