Calcite

Calcite

Estonian Museum of Natural History Specimen No 182279 photo (g28 g28-218 1 jpg)

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Calcite jaune sur fluorine violette (USA)

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Calcite, galène et pyrite (Dal'negorsk - Fédération de Russie)

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Calcite et fluorine (USA)

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, and vaterite is even less stable.

Properties

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. A fibrous, efflorescent form is known as calcareous sinter. Calcite is transparent to opaque and may occasionally show phosphorescence or fluorescence. A transparent variety called Iceland spar is used for optical purposes.

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 mineral form of calcium carbonate and 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.

Chemical Properties

Calcite reacts to acids by effervescing, producing carbon dioxide gas. It has a Mohs hardness of 3, its specific gravity is 2.71, and it has a vitreous luster. In addition to its uses in optical equipment, calcite has several other applications. It's a major component in cement and concrete, the primary base material in glass production, and is used in the manufacture of lime for agriculture and as an acid neutralizer in the chemical industry.

Cultural and Historical Uses

Calcite has been known and prized for thousands of years for its beauty and versatility. It has been used in the construction of temples and as a material for sculptures, especially during the Roman Empire. The ancient Egyptians carved many of their statues from calcite, appreciating its ease of working and polishability.

Environmental Significance

Calcite plays a crucial role in the carbon cycle. It acts as a carbon sink, locking away carbon dioxide in limestone and other carbonate rocks. Through geological processes, these rocks can release carbon dioxide back into the atmosphere or ocean, contributing to the long-term regulation of Earth's climate.

Gallery

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See Also

• Aragonite
• Vaterite
• Limestone
• Marble
• Carbonate minerals

References

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