Pure limestone is made up of two component minerals: calcium carbonate and calcium-magnesium carbonate. The chemical formula for calcium carbonate is CaCO3. The chemical formula for calcium-magnesium carbonate is CaMg(CO3)2.
Calcium carbonate is commonly known as calcite,and calcium-magnesium carbonate is commonly known as dolomite. In nature, limestone deposits are very rarely composed of pure calcite or pure dolomite, although such deposits have been found.
Under the academic definition of limestone, any sedimentary rock with at least 50 percent calcite and dolomite content is considered limestone. Under the common commercial definition, sedimentary rock must contain at least 80 percent calcite and dolomite to be defined as limestone.