Limestone—A Crucial and Versatile Industrial Mineral .
mostly of calcium carbonate (CaCO. 3), but to geologists, limestone is only one of several types of "carbonate rocks." These rocks are composed of more than 50% carbonate minerals, generally the minerals calcite (pure CaCO. 3) or dolomite (calciummagnesium carbonate, CaMg[CO. 3] 2) or both. Most carbonate rocks were deposited from seawater.