A white solid, CaCO3, which is only sparingly soluble in water. Calcium carbonate decomposes on heating to give calcium oxide (quicklime) and carbon dioxide. It occurs naturally as the minerals calcite (rhombohedral; r.d. 2.71) and aragonite (rhombic; r.d. 2.93). Rocks containing calcium carbonate dissolve slowly in acidified rainwater (containing dissolved CO2) to cause temporary hardness. In the laboratory, calcium carbonate is precipitated from limewater by carbon dioxide. Calcium carbonate is used in making lime (calcium oxide) and is the main raw material for the Solvay process.