Calcite

This 35-pound (16-kilogram) calcite crystal shows the property known as birefringence or double refraction. It was obtained from Iceberg claim, Dixon, New Mexico, and is on display at the National Museum of Natural History. It is one of the largest single crystals in the United States.

A model of the unit cell in crystals of calcite.

Calcite is a carbonate mineral and is among the most widely distributed minerals on the Earth's surface. Chemically, it is known as calcium carbonate and has the formula CaCO₃. Formed in various geological environments, it constitutes about 4 percent of the Earth's crust.

It occurs in many varieties.

It is perhaps best known for its optical property called birefringence or double refraction, because of which objects viewed through a clear piece of calcite appear doubled.

The name calcite is derived from the Greek word chalix, meaning "lime."

Occurrence

Calcite is a common constituent of sedimentary rocks, particularly limestone, and is the primary mineral in metamorphic marble. It also occurs as a vein mineral in deposits from hot springs, and it forms stalactites and stalagmites in caverns.

This mineral is often the primary constituent of the shells of marine organisms. It is found, for instance, in 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).

Characteristics

Calcite is usually white or colorless, but shades of gray, red, yellow, green, blue, violet, brown, or even black can occur when the mineral is charged with impurities. It is transparent to opaque and occasionally shows fluorescence or phosphorescence. It has a vitreous (glassy) luster in crystallized varieties. It has a Mohs hardness of 3 and a specific gravity of 2.71.

Crystals of calcite are hexagonal-rhombohedral, though actual calcite rhombohedrons are rare in nature. However, they show a remarkable variety of habits, including acute to obtuse rhombohedrons, tabular forms, prisms, or various scalenohedrons. Moreover, calcite exhibits several twinning types, adding to the variety of observed forms. It may occur as fibrous, granular, lamellar, or compact forms. Cleavage is usually in three directions parallel to the rhombohedron form. Its fracture is conchoidal, but difficult to obtain.

A beautifully transparent variety used for optical purposes comes from Iceland and is called Iceland spar. Acute scalenohedral crystals are sometimes referred to as dogtooth spar.

Single crystals of calcite display an optical property called birefringence or double refraction. This property involves the splitting of a ray of light into two rays when it passes through the crystal. Thus, objects viewed through a clear piece of calcite appear doubled in all their parts—a phenomenon first described by the Danish scientist Rasmus Bartholin in 1669.

• Calcite represents the stable form of calcium carbonate; aragonite will change to calcite at 470°C.

Gallery

• 

  Doubly refracting calcite. The crystal is 4 cm long (1.5 inches)

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  Calcite from Brushy Creek Mine, Missouri, USA.

• Calcite Morocco.jpg

  Calcite from Middle Atlas, Morocco

• Calcite09.jpg

  Calcite from Montana, USA

See also

• Birefringence
• Calcium
• Carbonate
• Limestone
• Marble
• Mineral

References

ISBN links support NWE through referral fees

• Farndon, John. 2006. The Practical Encyclopedia of Rocks & Minerals: How to Find, Identify, Collect and Maintain the World's best Specimens, with over 1000 Photographs and Artworks. London: Lorenz Books. ISBN 0754815412.

• Klein, Cornelis, and Barbara Dutrow. 2007. Manual of Mineral Science. 23rd ed. New York: John Wiley. ISBN 978-0471721574.

• Pellant, Chris. 2002. Rocks and Minerals. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 0789491060.

• Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. Rocks, Gems and Minerals. Rev. ed. New York: St. Martin's Press. ISBN 1582381321.

• Mindat.org. 2007. Calcite. Mindat.org. Retrieved April 10, 2007.

• Mineral Gallery. 2006. The Mineral Calcite. Amethyst Galleries. Retrieved April 10, 2007.

External links

• Calcite. Retrieved April 10, 2007.

• Calcite Mineral Data. Retrieved April 10, 2007.

• Calcite. (Giant crystals of calcite in Iceland.) Retrieved April 10, 2007.

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