Carbonate, KAHR buh nayt, is any compound that contains the carbonate ion. An ion is an atom or a group of atoms with an electric charge. The carbonate ion has two negative charges and consists of one carbon atom and three oxygen atoms. Chalk, marble, and other limestones are examples of a naturally abundant carbonate, calcium carbonate. Animal bones and teeth as well as egg and oyster shells have large amounts of calcium carbonate.