Calcium carbonate is a chemical compound with the chemical formula . It is a common substance found in rocks as the minerals calcite and aragonite (most notably as limestone, which is a type of sedimentary rock consisting mainly of calcite) and is the main component of eggshells, gastropod shells, shellfish skeletons and pearls. Things containing much calcium carbonate or resembling it are described as calcareous. Calcium carbonate is the active ingredient in agricultural lime and is created when calcium ions in hard water react with carbonate ions to create 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. Chemistry Calcium carbonate shares the typical properties of other carbonates. Notably it reacts with acids, releasing carbon dioxide (technically speaking, carbonic acid, but that disintegrates quickly to and ): CaCO_3(s) {+} 2H^{+}(aq) -> Ca^{2+}(aq) + CO2(g) + H_2O(l) releases carbon dioxide upon heating, called a thermal decomposition reaction, or calcination (to above 840 °C in the case of ), to form calcium oxide, CaO, commonly called quicklime, with reaction enthalpy 178 kJ/mol: CaCO3(s)->[\Delta]CaO(s){+}CO2\uparrow 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, . Ikaite is stable only below 8 °C. Preparation The vast majority of calcium carbonate used in industry is extracted by mining or quarrying. Pure calcium carbonate (such as for food or pharmaceutical use), can be produced from a pure quarried source (usually marble). Alternatively, calcium carbonate is prepared from calcium oxide. Water is added to give calcium hydroxide then carbon dioxide is passed through this solution to precipitate the desired calcium carbonate, referred to in the industry as precipitated calcium carbonate (PCC) This process is called carbonatation: CaO{+}H2O->Ca(OH)2 Ca(OH)2{+}CO2->CaCO3{+}H_2O In a laboratory, calcium carbonate can easily be crystallized from calcium chloride (CaCl2), by placing an aqueous solution of CaCl2 in a desiccator alongside ammonium carbonate (NH4)2CO3. In the desiccator, ammonium carbonate is exposed to air and decomposes into ammonia, carbon dioxide, and water. The carbon dioxide then diffuses into the aqueous solution of calcium chloride, reacts with the calcium ions and the water, and forms calcium carbonate. Structure The thermodynamically stable form of