Calcite

Calcite
A one-inch calcite rhomb that shows the double image refraction property
General
Category	Carbonate minerals
Formula (repeating unit)	CaCO3
Strunz classification	5.AB.05
Crystal system	Trigonal
Crystal class	Hexagonal scalenohedral (3m) H-M symbol: (3 2/m)
Space group	R3c
Unit cell	a = 4.9896(2) Å, c = 17.0610(11) Å; Z = 6
Identification
Color	Colorless or white, also gray, yellow, green,
Crystal habit	Crystalline, granular, stalactitic, concretionary, massive, rhombohedral.
Twinning	Common by four twin laws
Cleavage	Perfect on [1011] three directions with angle of 74° 55'
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	3 (defining mineral)
Luster	Vitreous to pearly on cleavage surfaces
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.71
Optical properties	Uniaxial (-)
Refractive index	nω = 1.640–1.660 nε = 1.486
Birefringence	δ = 0.154–0.174
Solubility	Soluble in dilute acids
Other characteristics	May fluoresce red, blue, yellow, and other colors under either SW and LW UV; phosphorescent
References

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO₃). The Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 3 as "calcite".

Other polymorphs of calcium carbonate are the minerals aragonite and vaterite. Aragonite will change to calcite at 380–470 °C, and vaterite is even less stable.

Calcite is derived from the German Calcit, a term coined in the 19th century from the Latin word for lime, calx (genitive calcis) with the suffix -ite used to name minerals. It is thus etymologically related to chalk.

When applied by archaeologists and stone trade professionals, the term alabaster is used not just as in geology and mineralogy, where it is reserved for a variety of gypsum; but also for a similar-looking, translucent variety of fine-grained banded deposit of calcite.

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, prisms, or various 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.