Any member of a family of minerals that contains the carbonate ion, CO32−, as the basic structural unit. The carbonates are among the most widely distributed minerals in the earth's crust; the most common are calcite, dolomite, and aragonite. Dolomite replaces calcite in limestones; when this replacement is extensive, the rock is called dolomite. Other relatively common carbonate minerals are siderite, rhodochrosite, strontianite (strontium-rich); smithsonite (zinc-rich); witherite (barium-rich); and cerussite (lead-rich).
Learn more about carbonate mineral with a free trial on Britannica.com.
Any member of two classes of chemical compounds—one inorganic and the other organic—that are derived from carbon dioxide (CO2) or its water solution, carbonic acid (H2CO3). Inorganic carbonates (MCO3 or M2CO3, where M is a metal atom of, e.g., calcium or sodium) are salts of carbonic acid. The shells and other hard parts of shellfish are calcium carbonate, as is the limestone they turn into. Many other minerals, including calcite, dolomite, and aragonite, consist of or contain carbonates. Sodium carbonate is one of the world's most important basic chemical commodities. Organic carbonates are esters of carbonic acid and various alcohol groups (methyl, ethyl, or phenyl). These are liquids used as solvents and to synthesize plastics and other compounds.
Learn more about carbonate with a free trial on Britannica.com.
Carbonate-containing salts are industrially and mineralogically ubiquitous. The term "carbonate" is also commonly used to refer to one of these salts or carbonate minerals. Most common is calcite, or calcium carbonate, CaCO3, the chief constituent of limestone. The process of removing carbon dioxide from these salts by heating is called calcination.
The term is also used as a verb, to describe the process of raising carbonate and bicarbonate concentrations in soda, see also carbonated water, either by the introduction under pressure of carbon dioxide gas into the bottle, or by dissolving carbonate or bicarbonate salts into the water.
The structure and bonding of the carbonate ion cannot be properly represented by its Lewis structure, which depicts CO32− with two long single bonds and one short double bond:
Resonance structures can be used to depict the carbonate ion:
In reality, CO32− has three equally long C-O bonds:
In aqueous solution, carbonate, bicarbonate, carbon dioxide, and carbonic acid exist together in a dynamic equilibrium. In strongly basic conditions, the carbonate ion predominates, while in weakly basic conditions, the bicarbonate ion is prevalent. In more acid conditions, aqueous carbon dioxide, CO2(aq), is the main form, which, with water, H2O, is in equilibrium with carbonic acid - the equilibrium lies strongly towards carbon dioxide. Thus sodium carbonate is basic, sodium bicarbonate is weakly basic, while carbon dioxide itself is a weak acid.
Carbonated water is formed by dissolving CO2 in water under pressure. When the partial pressure of CO2 is reduced, for example when a can of soda is opened, the equilibrium for each of the forms of carbonate (carbonate, bicarbonate, carbon dioxide, and carbonic acid) shifts until the concentration of CO2 in the solution is equal to the solubility of CO2 at that temperature and pressure. In living systems an enzyme, carbonic anhydrase, speeds the interconversion of CO2 and carbonic acid.
When pH is too high, the concentration of hydrogen ions in the blood is too low, so the kidneys excrete bicarbonate (HCO3−). This causes the equation to shift right, essentially increasing the concentration of hydrogen ions, causing a more acidic pH.
Significant carbonate deposits have not been found on Mars via remote sensing or in situ missions, even though Martian meteorites contain small amounts and groundwater may have existed at both Gusev and Meridiani Planum.