Calcium carbonate is a chemical compound with the chemical formula CaCO3. It is a common substance found in rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skeletons and pearls. Materials containing much calcium carbonate or resembling it are described as calcareous. Calcium carbonate is the active ingredient in agricultural lime and is produced when calcium ions in hard water react with carbonate ions to form limescale. It has medical use as a calcium supplement or as an antacid, but excessive consumption can be hazardous and cause hypercalcemia and digestive issues.[8] Chemistry[edit] Calcium carbonate shares the typical properties of other carbonates. Notably it reacts with acids, releasing carbonic acid which quickly disintegrates into carbon dioxide and water: CaCO3(s) + 2 H+(aq) → Ca2+(aq) + CO2(g) + H2O(l) releases carbon dioxide upon heating, called a thermal decomposition reaction, or calcination (to above 840 °C in the case of CaCO3), to form calcium oxide, CaO, commonly called quicklime, with reaction enthalpy 178 kJ/mol: CaCO3(s) → CaO(s) + CO2(g) reacts with gaseous hydrogen to form methane and water vapor plus solid calcium oxide or calcium hydroxide depending on temperature and product gas composition. Various metals including palladium and nickel are catalysts for the reaction. Calcium carbonate reacts with water that is saturated with carbon dioxide to form the soluble calcium bicarbonate. CaCO3(s) + CO2(g) + H2O(l) → Ca(HCO3)2(aq) This reaction is important in the erosion of carbonate rock, forming caverns, and leads to hard water in many regions. An unusual form of calcium carbonate is the hexahydrate ikaite, CaCO3·6H2O. Ikaite is stable only below 8 °C.