Difference between revisions of "Calcite" - New World Encyclopedia
(claimed) |
|||
| Line 1: | Line 1: | ||
{{Claimed}} | {{Claimed}} | ||
| − | [[Image:Calcite-HUGE.jpg|thumb|right| | + | [[Image:Calcite-HUGE.jpg|thumb|right|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.]] |
| − | [[Image:Calcite-unit-cell-3D-vdW.png|thumb|right| | + | [[Image:Calcite-unit-cell-3D-vdW.png|thumb|right|A model of the [[Crystal structure#Unit cell|unit cell]] in crystals of '''calcite'''.]] |
| − | + | '''Calcite''' is a [[Carbonate minerals|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 CaCO<sub>3</sub>. | |
| − | ==Properties== | + | == Occurrence == |
| + | |||
| + | It is a common constituent of [[sedimentary rock]]s, [[limestone]] in particular. It is also the primary mineral in [[Metamorphic rock|metamorphic]] [[marble]]. It also occurs as a [[vein]] mineral in deposits from [[hot spring]]s, and also occurs in [[cavern]]s as [[stalactite]]s and stalagmites. Calcite is often the primary constituent of the shells of marine organisms, e.g., [[plankton]] (such as [[coccolith]]s and planktic [[foraminifera]]), the hard parts of red [[algae]], some [[sea sponge|sponge]]s, [[brachiopod]]a, [[echinoderm]]s, most [[bryozoa]], and parts of the shells of some [[Bivalvia|bivalves]], such as [[oyster]]s and rudists). Calcite represents the stable form of calcium carbonate; [[aragonite]] will change to calcite at 470°C. | ||
| + | |||
| + | == Properties == | ||
Calcite [[crystal]]s are [[Hexagonal (crystal system)|hexagonal]]-rhombohedral, though actual calcite rhombohedrons are rare as natural crystals. However, they show a remarkable variety of habits including acute to obtuse rhombohedrons, tabular forms, prisms, or various scalenohedrons. Calcite exhibits several [[Crystal twinning|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. | Calcite [[crystal]]s are [[Hexagonal (crystal system)|hexagonal]]-rhombohedral, though actual calcite rhombohedrons are rare as natural crystals. However, they show a remarkable variety of habits including acute to obtuse rhombohedrons, tabular forms, prisms, or various scalenohedrons. Calcite exhibits several [[Crystal twinning|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. | ||
Revision as of 21:00, 20 April 2007
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.
Occurrence
It is a common constituent of sedimentary rocks, limestone in particular. It is also the primary mineral in metamorphic marble. It also occurs as a vein mineral in deposits from hot springs, and also occurs in caverns as stalactites and stalagmites. 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, brachiopoda, echinoderms, most bryozoa, and parts of the shells of some bivalves, such as oysters and rudists). Calcite represents the stable form of calcium carbonate; aragonite will change to calcite at 470°C.
Properties
Calcite crystals are hexagonal-rhombohedral, though actual calcite rhombohedrons are rare as natural crystals. However, they show a remarkable variety of habits including acute to obtuse rhombohedrons, tabular forms, prisms, or various scalenohedrons. 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 Mohs hardness of 3, a specific gravity of 2.71, and its luster is vitreous in crystallized varieties. Colour is white or colourless, though shades of gray, red, 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. It is perhaps best known because of its power to produce strong double refraction of light, such that objects viewed through a clear piece of calcite appear doubled in all of their parts - a phenomenon first described by Rasmus Bartholin. A beautifully transparent variety used for optical purposes comes from Iceland, called Iceland spar. Acute scalenohedral crystals are sometimes referred to as "dogtooth spar."
Single crystals of calcite display an optical property called birefringence. 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 [1].
Gallery
Doubly refracting calcite. The crystal is 4 cm long (1.5 inches)
Calcite from Brushy Creek Mine, Missouri, USA.
- Calcite Morocco.jpg
Calcite from Middle Atlas, Morocco
- Calcite09.jpg
Calcite from Montana, USA
Calcite In Literature
A form of calcite, Iceland spar plays a critical role in the plot of Against the Day by Thomas Pynchon. The same form is referred to in The Amber Spyglass by Philip Pullman as it has very similar properties to a mineral found in that story.
See also
ReferencesISBN 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.
Credits
New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:
The history of this article since it was imported to New World Encyclopedia:
Note: Some restrictions may apply to use of individual images which are separately licensed.
