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Calcite

Calcite
Calcite.jpg
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.


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