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Doubly refracting Calcite from Iceberg claim, Dixon, New Mexico. This 35 pound (16 kg) crystal, on display at the National Museum of Natural History, is one of the largest single crystals in the United States.

The unit cell of calcite

The carbonate mineral calcite is a chemical or biochemical calcium carbonate corresponding to the formula CaCO₃ and is one of the most widely distributed minerals on the Earth's surface. 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.

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].

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

Calcite from Brushy Creek Mine, Missouri, USA.

Calcite from Middle Atlas, Morocco

Calcite from Montana, USA

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.

Wikisource has an original article from the 1911 Encyclopædia Britannica about:

Calcite

• Monohydrocalcite - CaCO₃.H₂O
• Ikaite - CaCO₃.6H₂O
• List of minerals
• Lysocline
• Ocean acidification

• Mineral Data Publishers - PDF
• Mindat
• Webmineral data
• Calcite information and images

• 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.

• Information about the largest and most famous calcite crystals

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Calcite