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Calcite

2007 Schools Wikipedia Selection. Related subjects: Mineralogy

   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.
   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 unit cell of calcite

   The carbonate mineral calcite is a chemical or biochemical calcium
   carbonate corresponding to the formula CaCO[3] 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.

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 calcite crystals 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 .

   Calcite can be either dissolved by groundwater or precipitated by
   groundwater, depending on several factors including the water
   temperature, pH, and dissolved ion concentrations. 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 caverns.

Gallery

   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

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.

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