Carbonate

Carbonate refers to a salt or ester of carbonic acid (H2CO3) and, in inorganic chemistry, to the carbonate ion CO3^2-. In salts, carbonate is the dianion that combines with metal or other cations. Carbonates are widespread in geology, chemistry, and biology and form a range of minerals and rocks.

Common carbonate minerals include calcite (CaCO3) and its polymorph aragonite, as well as dolomite (CaMg(CO3)2). Other naturally occurring carbonates include magnesite (MgCO3), siderite (FeCO3), rhodochrosite (MnCO3), and smithsonite (ZnCO3). These minerals make up prominent sedimentary rocks such as limestone, chalk, and dolostone, and they occur in shells and skeletons of many marine organisms.

Chemistry and buffering: In aqueous systems, carbon dioxide dissolves in water to form carbonic acid, which equilibrates to bicarbonate (HCO3−) and carbonate (CO3^2−). The relative abundances depend on pH: at high pH carbonate dominates; at lower pH bicarbonate or dissolved CO2 prevails. The carbonate system plays a major role in buffering seawater and freshwater chemistry.

Geological and environmental role: Weathering of carbonate rocks consumes CO2 and releases calcium and bicarbonate into rivers and oceans, contributing to long-term carbon storage in carbonate sediments. Biological carbonate structures, such as shells and coral reefs, contribute significantly to marine carbonate deposits and ecosystems.

Industrial uses: Limestone and marble are quarried for cement, lime, and building materials; calcination of CaCO3 yields CaO, an important cement component. Carbonate minerals are also used as fillers, pigments, and buffering agents; related reagents such as sodium bicarbonate are common in baking, cleaning, and pH control.