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Calcite

Calcite is a Carbonate minerals|carbonate mineral and the most stable Polymorphism (materials science)|polymorph of calcium carbonate (calcium|Ca carbon|C oxygen|O₃). The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 470°C, and vaterite is even less stable.

Properties

Calcite crystals are Rhombohedral crystal system|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 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. It has a Mohs hardness of 3, a specific gravity of 2.71, and its luster is vitreous in crystallized varieties. Color is white or none, 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 Denmark|Danish scientist Rasmus Bartholin in 1669. At a wavelength of ~590 nm calcite has ordinary and extraordinary refractive index|refractive indices of 1.658 and 1.486, respectively. Between 190 and 1700 nm, the ordinary refractive index varies roughly between 1.6 and 1.4, while the extraordinary refractive index varies between 1.9 and 1.5Thompson, D.W. et al 1998. Determination of optical anisotropy in calcite from ultraviolet to mid-infrared by generalized ellipsometry. Thin Solid Films 313-314 (1998) 341-346. http://dx.doi.org/10.1016/S0040-6090(97)00843-2. Calcite, like most carbonates, will dissolve with most forms of acid. Calcite can be either solvation|dissolved by groundwater or precipitate|precipitated by groundwater, depending on several factors including the water temperature, acidity|pH, and dissolved ion concentrations. Although calcite is fairly insoluble in cold water, acidity can cause dissolution of calcite and release of carbon dioxide gas. 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 (fluid)|permeability of the rock, and if it continues for a long period of time may result in the formation of caverns, most notably the Snowy River Cave in Lincoln County, New Mexico.

Natural occurrence

Calcite is often the primary constituent of the Seashell|shells of Marine biology|marine organisms, e.g., plankton (such as coccoliths and planktic foraminifera), the hard parts of red algae, some sea sponge|sponges, brachiopoda, echinoderms, most bryozoa, and parts of the shells of some Bivalvia|bivalves, such as oysters and rudists). 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. Calcite is the primary mineral in Metamorphic rock|metamorphic marble. It also occurs as a Vein (geology)|vein mineral in deposits from hot springs, and it occurs in caverns as stalactites and stalactite|stalagmites. Calcite may also be found in volcano|volcanic or mantle (geology)|mantle-derived rocks such as carbonatites, kimberlites, or rarely in peridotites. Calcite is found in spectacular form in the Snowy River Cave of New Mexico as mentioned above, where microorganisms are credited with natural formations. Trilobites, which are now extinct, had unique compound eyes. They used clear calcite crystals to form the lenses of their eyes. Lublinite is a fibrous, efflorescent form of calcite.http://www.mindat.org/min-9512.html Mindat

Calcite in Earth history

Calcite seas existed in Earth history when the primary inorganic precipitate of calcium carbonate in marine waters was low-magnesium calcite (lmc), as opposed to the aragonite and high-magnesium calcite (hmc) precipitated today. Calcite seas alternated with aragonite seas over the Phanerozoic, being most prominent in the Ordovician and Jurassic. Petrography|Petrographic evidence for these calcite sea conditions consists of calcitic ooids, lmc cements, hardgrounds, and rapid early seafloor aragonite dissolution.Palmer, T.J. and Wilson, M.A. 2004. Calcite precipitation and dissolution of biogenic aragonite in shallow Ordovician calcite seas. Lethaia 37: 417-427. http://www.wooster.edu/geology/PalmerWilson05.pdf The evolution of marine organisms with calcium carbonate shells may have been affected by the calcite and aragonite sea cycle.Harper, E.M. , Palmer, T.J. and Alphey, J.R. 1997. Evolutionary response by bivalves to changing Phanerozoic sea-water chemistry. Geological Magazine 134: 403-407

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.