The carbonate minerals are much simpler structurally than the silicates. This mineral group is composed of the carbonate ion(cos and one or more kinds of positive ions. The most common carbonate mineral is calcite, Caco(calcium carbonate). This mineral is the major constituent in two well- known rocks: limestone and marble. Limestone has many uses, including as road aggregate, as building stone, and as the main ingredient in Portland cement. Marble is used decoratively.