Calcite is a carbonate mineral and the most stable of polymorph of calcium carbonate (CaCO3). The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470 degrees Celcius and vaterite is even less stable.
Calcite crystals are trigonal-rhombohedral, though actual calcite rhombohedra are rare as natural cyrstals. However, they show a remarkable variety of habits including acute to obtuse rhombohedra, tabular forms, prisms or v arious scalenohedra. Calcite exhibits several twinning types adding to the variety of observed forms. It may occur as fibrous, granular, lamellar or compact. Cleavage is usually in three directions parallel to the rhombohedron form. Its fracture is conchoidal, but difficult to obtain.
Calcite has a defining Mohs hardness of 3, specific gravity of 2.71 and its luster is vitreous in cyrstallized varieties. Color is white or none, though shades of gray, red, orange, yellow, green, blue, violet, brown or even black can occur when the mineral is charged with impurities.
Calcite is transparent to opaque and may occasionally show phosphorescence of fluorescence. A transparent variety called Iceland spar is used for optical purposes. Acute scalenohedral crystals are sometimes referred to as “dogtooth spar” while the rhombohedral form is sometimes referred to as “nailhead spar”.
Calcite is presented to different industrial sectors like a filling material. At a rate of 15-30% of micronized calcite is added to cellulose in the production of writing tab paper, wall paper and cardboard. Being in high whiteness, cheapness and because of other technical features, last ten years, starting from Europe in all over the word, Calcite has taken place of kaolin in paper sector. Since Calcite is harmless mineral to environment, In paper industry, using Calcium Carbonate reduces consumption of cellulose and optical bleaching agents as well as ensures reduction of environmental pollution. Also in paint industry, interior and exterior paints, water-based paint system, 25-30% of calcite is used and using Calcite cuts down consumption of TiO2 and lowers the cost of production.
Both natural ground and coated Micronized calcites are used for many reasons such as its color, chemical purity and cheapness,extensively in the plastics industry, particularly PVC pipes, profiles, panels, cables. Micronized calcite also used in adhesives, gum, food, toothpaste, ceramics, carpets and linoleum floor, foam and rubber industries.