Calcite/
Introduction
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate. The other polymorphs are the minerals aragonite and vaterite. Calcite crystals are trigonal-rhombohedral, though actual calcite rhombohedra are rare as natural crystals. However, they show a remarkable variety of habits including acute to obtuse rhombohedra, tabular forms, and prisms.
Physical Properties
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. It has a defining Mohs hardness of 3, a specific gravity of 2.71, and its luster is vitreous in crystallized 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.
Chemical Properties
On a chemical level, calcite is stable in most geologic settings and can occur in all three major rock types: sedimentary, igneous, and metamorphic. It is reactive to acids, even weak ones such as vinegar, and will effervesce (bubble) vigorously. It is, therefore, often used in acid neutralization. Pure calcite is transparent and colorless but can take on many colors due to impurities. Chemically, it is known as calcium carbonate and has the formula CaCO3. It is most commonly found in sedimentary rock and is the primary component of the shells of marine organisms, snails, and eggshells.
Formation and Occurrence
Calcite is a common constituent of sedimentary rocks, limestone in particular, much of which is formed from the shells of dead marine organisms. Approximately 10% of sedimentary rock is limestone. It is the primary mineral in metamorphic marble. It also occurs in caverns as stalactites and stalagmites. Large individual crystals in transparent form are found in many locations; most notably from the Sweetwater Mine, Reynolds County, Missouri, USA, where it is popularly known as 'Missouri Marble'.
Uses and Applications
Calcite is used in many commercial and industrial applications. In construction, it is used as a building material and aggregate to produce cement and lime. In the chemical industry, it is used to produce soda ash and sodium phosphate. It is also used in the production of glass and ceramics. Calcite is used in the pharmaceutical industry as an inexpensive dietary calcium supplement and antacid. It is also used for alkalizing the body, as it is a natural form of calcium.