Any member of two classes of chemical compoundsone inorganic and the other organicthat are derived from carbon dioxide (CO) or its water solution, carbonic acid (HCO). Inorganic carbonates (MCO or MCO, 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.