Calcite
A one-inch calcite rhomb that shows the double image refraction property
General
Category	Carbonate mineral
Chemical formula	CaCO3
Strunz classification	05.AB.05
Crystal symmetry	Trigonal 32/m
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.
Crystal system	Trigonal hexagonal scalenohedral (32/m), Space Group (R3 2/c)
Twinning	Common by four twin laws
Cleavage	Perfect on [1011] three directions with angle of 74° 55' [1]
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	[2][3][4]

Crystal structure of calcite

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO₃). The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C,^[5] and vaterite is even less stable.^{[citation needed]}

Properties[link]

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.

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.

Calcite is transparent to opaque and may occasionally show phosphorescence or 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".

Single calcite crystals display an optical property called birefringence (double refraction). This strong birefringence causes objects viewed through a clear piece of calcite to appear doubled. The birefringent effect (using calcite) was first described by the Danish scientist Rasmus Bartholin in 1669. At a wavelength of ~590 nm calcite has ordinary and extraordinary refractive indices of 1.658 and 1.486, respectively.^[6] Between 190 and 1700 nm, the ordinary refractive index varies roughly between 1.6 and 1.4, while the extraordinary refractive index varies between 1.9 and 1.5.^[7]

Calcite, like most carbonates, will dissolve with most forms of acid. Calcite can be either dissolved by groundwater or precipitated by groundwater, depending on several factors including the water temperature, pH, and dissolved ion concentrations. Although calcite is fairly insoluble in cold water, acidity can cause dissolution of calcite and release of carbon dioxide gas. Calcite exhibits an unusual characteristic called retrograde solubility in which it becomes less soluble in water as the temperature increases. When conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together or it can fill fractures. When conditions are right for dissolution, the removal of calcite can dramatically increase the porosity and permeability of the rock, and if it continues for a long period of time may result in the formation of caves. On a landscape scale, continued dissolution of calcium carbonate-rich rocks can lead to the expansion and eventual collapse of cave systems, resulting in various forms of karst topography.

Use and applications[link]

High-grade optical calcite was used in World War II for gun sights, specifically in bomb sights and anti-aircraft weaponry.^[8]

Natural occurrence[link]

The largest documented single crystals of calcite originated from Iceland, measured 7×7×2 m and 6×6×3 m and weighed about 250 tons.^[9][10]

Doubly terminated calcite crystal

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.

Calcite is the primary mineral in metamorphic marble. It also occurs as a vein mineral in deposits from hot springs, and it occurs in caverns as stalactites and stalagmites.

Lublinite is a fibrous, efflorescent form of calcite.^[11]

Calcite may also be found in volcanic or mantle-derived rocks such as carbonatites, kimberlites, or rarely in peridotites.

Calcite is often the primary constituent of the shells of marine organisms, e.g., plankton (such as coccoliths and planktic foraminifera), the hard parts of red algae, some sponges, brachiopods, echinoderms, most bryozoa, and parts of the shells of some bivalves (such as oysters and rudists). Calcite is found in spectacular form in the Snowy River Cave of New Mexico as mentioned above, where microorganisms are credited with natural formations. Trilobites, which are now extinct, had unique compound eyes. They used clear calcite crystals to form the lenses of their eyes.

Calcite in Earth history[link]

Calcite seas existed in Earth history when the primary inorganic precipitate of calcium carbonate in marine waters was low-magnesium calcite (lmc), as opposed to the aragonite and high-magnesium calcite (hmc) precipitated today. Calcite seas alternated with aragonite seas over the Phanerozoic, being most prominent in the Ordovician and Jurassic. Lineages evolved to use whichever morph of calcium carbonate was favourable in the ocean at the time they became mineralised, and retained this mineralogy for the remainder of their evolutionary history.^[12] Petrographic evidence for these calcite sea conditions consists of calcitic ooids, lmc cements, hardgrounds, and rapid early seafloor aragonite dissolution.^[13] The evolution of marine organisms with calcium carbonate shells may have been affected by the calcite and aragonite sea cycle.^[14]

Gallery[link]

Wikimedia Commons has media related to: Calcite

• 

  Calcite crystals inside a test of the cystoid Echinosphaerites aurantium (Middle Ordovician, northeastern Estonia).

• 

  Calcite rhomb from Iceberg claim, Dixon, New Mexico showing double refraction.

• 

  Mississippian marble (made of calcite) in Big Cottonwood Canyon, Wasatch Mountains, Utah.

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  Thin section view of calcite crystals inside a recrystallized bivalve shell in a biopelsparite.

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  Calcite containing niobium (giving it distinctive bluish color), from Medford Quarry, Maryland.

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  Nailhead spar calcite.

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  Reddish rhombohedral calcite crystals from China. Its red color is due to the presence of iron.

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  Calcite fluoresces pink under long wave ultraviolet light.

• 

  Calcite fluoresces blue under short wave ultraviolet light.

See also[link]

Wikisource has the text of the 1911 Encyclopædia Britannica article Calcite.

• Iceland Spar
• Ikaite, CaCO₃·6H₂O
• List of minerals
• Lysocline
• Manganoan Calcite, (Ca,Mn)CO₃
• Monohydrocalcite, CaCO₃·H₂O
• Ocean acidification
• Ulexite aka "TV rock", another mineral with an optical property often illustrated in the same way.
• Yule Marble‎

References[link]

Further reading[link]

• Schmittner Karl-Erich and Giresse Pierre, 1999. "Micro-environmental controls on biomineralization: superficial processes of apatite and calcite precipitation in Quaternary soils", Roussillon, France. Sedimentology 46/3: 463–476.

External links[link]

Wikimedia Commons has media related to: Calcite

• Calcite information and images