The chemical formula of calcite is CaCO3, usually containing Fe, Mn and Zn, etc. The other polymorphs are the minerals aragonite and vaterite. Calcite belongs to trigonal crystallographic system. Calcite crystals are usually trigonal-rhombohedral; the aggregate usually shows as drusy, granular, stalactite shape, oolitic, denseclumps or sinter, etc. Mohs hardness: 3; density: 2.6-2.8g/cm3; refractive index: 1.486-1.685; birefringence: 0.172, optically negative; transparent and glassy; colorless or white, sometimes stained to other colors due to other impurities included. The pure, colorless and transparent calcite is known as Iceland spar. Calcite foams strongly with diluted hydrochloric acid. Formed by various geological processes, it has a wide distribution in the nature and is the main component of limestone and marble.