Calcium Carbonate

a common chemical compound in nature with the formula CaCo3. In the form of calcite, it is an abundant MINERAL in rocks especially LIMESTONE, CHALK and MARBLE. It is used by man in the manufacture of cement and fertilizer.

Taken from Dictionary of Science

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Calcium carbonate is a chemical compound with the chemical formula CaCO₃. It is a common substance found in rock in all parts of the world, and is the main component of shells of marine organisms, snails, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime, and is usually the principal cause of hard water. It is commonly used medicinally as a calcium supplement or as an antacid, but excessive consumption can be hazardous.

Taken from Wikipedia