Calcite sur fluorine (USA) 1.JPG
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Calcite is a carbonate mineral and the most stable
polymorph Polymorphism, polymorphic, polymorph, polymorphous, or polymorphy may refer to: Computing * Polymorphism (computer science), the ability in programming to present the same programming interface for differing underlying forms * Ad hoc polymorphi ...
of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratch hardness comparison. Large calcite crystals are used in optical equipment, and limestone composed mostly of calcite has numerous uses. Other polymorphs of calcium carbonate are the minerals aragonite and vaterite. Aragonite will change to calcite over timescales of days or less at temperatures exceeding 300 °C, and vaterite is even less stable.


Etymology

Calcite is derived from the German ''Calcit'', a term from the 19th century that came from the Latin word for lime, ''calx'' (genitive calcis) with the suffix "-ite" used to name minerals. It is thus etymologically related to chalk. When applied by
archaeologists Archaeology or archeology is the scientific study of human activity through the recovery and analysis of material culture. The archaeological record consists of artifacts, architecture, biofacts or ecofacts, sites, and cultural landscap ...
and stone trade professionals, the term alabaster is used not just as in geology and mineralogy, where it is reserved for a variety of gypsum; but also for a similar-looking, translucent variety of fine-grained banded deposit of calcite.


Unit cell and Miller indices

In publications, two different sets of
Miller indices Miller indices form a notation system in crystallography for lattice planes in crystal (Bravais) lattices. In particular, a family of lattice planes of a given (direct) Bravais lattice is determined by three integers ''h'', ''k'', and '' ...
are used to describe directions in hexagonal and rhombohedral crystals, including calcite crystals: three Miller indices in the a_1, a_2, c directions, or four Bravais-Miller indices in the a_1,a_2,a_3,c directions, where i is redundant but useful in visualizing permutation symmetries. To add to the complications, there are also two definitions of unit cell for calcite. One, an older "morphological" unit cell, was inferred by measuring angles between faces of crystals, typically with a
goniometer A goniometer is an instrument that either measures an angle or allows an object to be rotated to a precise angular position. The term goniometry derives from two Greek words, γωνία (''gōnía'') 'angle' and μέτρον (''métron'') 'me ...
, and looking for the smallest numbers that fit. Later, a "structural" unit cell was determined using X-ray crystallography. The morphological unit cell is rhombohedral, having approximate dimensions and , while the structural unit cell is hexagonal (i.e. a
rhombic Rhombic may refer to: * Rhombus, a quadrilateral whose four sides all have the same length (often called a diamond) *Rhombic antenna, a broadband directional antenna most commonly used on shortwave frequencies * polyhedra formed from rhombuses, suc ...
prism), having approximate dimensions and . For the same orientation, must be multiplied by 4 to convert from morphological to structural units. As an example, calcite cleavage is given as "perfect on " in morphological coordinates and "perfect on " in structural units. In \ indices, these are and , respectively. Twinning, cleavage and crystal forms are often given in morphological units.


Properties

The diagnostic properties of calcite include a defining Mohs hardness of 3, a specific gravity of 2.71 and, in crystalline varieties, a vitreous luster. Color is white or none, though shades of gray, red, orange, yellow, green, blue, violet, brown, or even black can occur when the mineral is charged with impurities.


Crystal habits

Calcite has numerous habits, representing combinations of over 1000
crystallographic form A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
s. Most common are scalenohedra, with faces in the hexagonal directions (morphological unit cell) or directions (structural unit cell); and rhombohedral, with faces in the or directions (the most common cleavage plane). Habits include acute to obtuse rhombohedra, tabular habits, prisms, or various scalenohedra. Calcite exhibits several twinning types adding to the variety of observed habits. It may occur as fibrous, granular, lamellar, or compact. A fibrous, efflorescent habit is known as ''lublinite''. Cleavage is usually in three directions parallel to the rhombohedron form. Its fracture is
conchoidal Conchoidal fracture describes the way that brittle materials break or fracture when they do not follow any natural planes of separation. Mindat.org defines conchoidal fracture as follows: "a fracture with smooth, curved surfaces, typically sli ...
, but difficult to obtain. Scalenohedral faces are chiral and come in pairs with mirror-image symmetry; their growth can be influenced by interaction with chiral biomolecules such as L- and D- amino acids. Rhombohedral faces are not chiral.


Optical

Calcite is transparent to
opaque Opacity or opaque may refer to: * Impediments to (especially, visible) light: ** Opacities, absorption coefficients ** Opacity (optics), property or degree of blocking the transmission of light * Metaphors derived from literal optics: ** In lingui ...
and may occasionally show phosphorescence or fluorescence. A transparent variety called " Iceland spar" is used for optical purposes. Acute scalenohedral crystals are sometimes referred to as "dogtooth spar" while the rhombohedral form is sometimes referred to as "nailhead spar". The rhombohedral form may also have been the " sunstone" whose use by Viking navigators is mentioned in the Icelandic Sagas. Single calcite crystals display an optical property called
birefringence Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefring ...
(double refraction). This strong birefringence causes objects viewed through a clear piece of calcite to appear doubled. The birefringent effect (using calcite) was first described by the
Danish Danish may refer to: * Something of, from, or related to the country of Denmark People * A national or citizen of Denmark, also called a "Dane," see Demographics of Denmark * Culture of Denmark * Danish people or Danes, people with a Danish ance ...
scientist Rasmus Bartholin in 1669. At a wavelength of about 590 nm, calcite has ordinary and extraordinary refractive indices of 1.658 and 1.486, respectively. Between 190 and 1700 nm, the ordinary refractive index varies roughly between 1.9 and 1.5, while the extraordinary refractive index varies between 1.6 and 1.4.


Chemical

Calcite, like most carbonates, will dissolve in acids via the reaction :: The carbon dioxide released by this reaction produces a characteristic effervescence when dilute hydrochloric acid is dropped on a calcite sample. Ambient carbon dioxide, due to its acidity, has a slight solubilizing effect on calcite. The overall reaction is :: If the amount of dissolved carbon dioxide drops, the reaction reverses to precipitate calcite. As a result, calcite can be either dissolved by groundwater or precipitated by groundwater, depending on such factors as the water temperature, pH, and dissolved ion concentrations. When conditions are right for precipitation, calcite forms mineral coatings that cement rock grains together and 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 caves. Continued dissolution of calcium carbonate-rich formations can lead to the expansion and eventual collapse of cave systems, resulting in various forms of
karst topography Karst is a topography formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves. It has also been documented for more weathering-resistant ro ...
. Calcite exhibits an unusual characteristic called retrograde solubility in which it becomes less soluble in water as the temperature increases. Calcite is also more soluble at higher pressures. Pure calcite has the composition . However, the calcite in limestone often contains a few percent of magnesium. Calcite in limestone is divided into low-magnesium and high-magnesium calcite, with the dividing line placed at a composition of 4% magnesium. High-magnesium calcite retains the calcite mineral structure, which is distinct from that of
dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...
, . Calcite can also contain small quantities of iron and manganese. Manganese may be responsible for the fluorescence of impure calcite, as may traces of organic compounds.


Use and applications

Ancient Egyptians carved many items out of calcite, relating it to their goddess Bast, whose name contributed to the term alabaster because of the close association. Many other cultures have used the material for similar carved objects and applications. A transparent variety of calcite known as Iceland spar may have been used by Vikings for navigating on cloudy days. High-grade optical calcite was used in World War II for gun sights, specifically in bomb sights and anti-aircraft weaponry. It was used as a polarizer (in
Nicol prism A Nicol prism is a type of polarizer, an optical device made from calcite crystal used to produce and analyse plane polarized light. It is made in such a way that it eliminates one of the rays by total internal reflection, i.e. the ordinary ray ...
s) prior to the invention of
Polaroid Polaroid may refer to: * Polaroid Corporation, an American company known for its instant film and cameras * Polaroid camera, a brand of instant camera formerly produced by Polaroid Corporation * Polaroid film, instant film, and photographs * Polar ...
plates and still finds use in optical instruments. Also, experiments have been conducted to use calcite for a cloak of invisibility. Microbiologically precipitated calcite has a wide range of applications, such as soil remediation, soil stabilization and concrete repair. Calcite, obtained from an 80 kg sample of Carrara marble, is used as the
IAEA The International Atomic Energy Agency (IAEA) is an intergovernmental organization that seeks to promote the peaceful use of nuclear energy and to inhibit its use for any military purpose, including nuclear weapons. It was established in 1957 ...
-603 isotopic standard in
mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...
for the calibration of δ18O and δ13C.


Natural occurrence

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. It is the primary mineral in metamorphic marble. It also occurs in deposits from hot springs as a vein mineral; in caverns as
stalactite A stalactite (, ; from the Greek 'stalaktos' ('dripping') via ''stalassein'' ('to drip') is a mineral formation that hangs from the ceiling of caves, hot springs, or man-made structures such as bridges and mines. Any material that is soluble an ...
s and
stalagmite A stalagmite (, ; from the Greek , from , "dropping, trickling") is a type of rock formation that rises from the floor of a cave due to the accumulation of material deposited on the floor from ceiling drippings. Stalagmites are typically ...
s; and in volcanic or mantle-derived rocks such as carbonatites, kimberlites, or rarely in
peridotite Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high prop ...
s. Calcite is often the primary constituent of the shells of
marine organism Marine life, sea life, or ocean life is the aquatic plant, plants, aquatic animal, animals and other organisms that live in the seawater, salt water of seas or oceans, or the brackish water of coastal estuary, estuaries. At a fundamental leve ...
s, such as plankton (such as
coccolith Coccoliths are individual plates or scales of calcium carbonate formed by coccolithophores (single-celled phytoplankton such as ''Emiliania huxleyi'') and cover the cell surface arranged in the form of a spherical shell, called a ''coccosphere''. ...
s and planktic foraminifera), the hard parts of red
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...
, some sponges, brachiopods, echinoderms, some serpulids, most bryozoa, and parts of the shells of some
bivalves Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...
(such as oysters and rudists). Calcite is found in spectacular form in the
Snowy River Cave Snowy River Cave Passage is a cave passage within Fort Stanton Cave in Lincoln County, New Mexico, obtaining its name from a stream bed of white calcite. Geology The bed of Snowy River is covered with bright white calcite. Over time, ancient, ...
of New Mexico as mentioned above, where microorganisms are credited with natural formations. Trilobites, which became extinct a quarter billion years ago, had unique compound eyes that used clear calcite crystals to form the lenses. It also forms a substantial part of birds' eggshells, and the δC of the diet is reflected in the δC of the calcite of the shell. The largest documented single crystal of calcite originated from Iceland, measured and and weighed about 250 tons. Classic samples have been produced at
Madawaska Mine Madawaska Mine (previously known as Faraday Mine) is a decommissioned underground uranium mine in Faraday, near the town of Bancroft, Ontario, which produced 9 million pounds (4,082 tonnes) of U3O8 concentrate, at an average ore grade of 0.1074 ...
, near Bancroft, Ontario. Bedding parallel veins of fibrous calcite, often referred to in quarrying parlance as ''beef'', occur in dark organic rich mudstones and shales, these veins are formed by increasing fluid pressure during diagenesis.


Formation processes

Calcite formation can proceed by several pathways, from the classical
terrace ledge kink model In chemistry, the Terrace Ledge Kink model (TLK), which is also referred to as the Terrace Step Kink model (TSK), describes the thermodynamics of crystal surface formation and transformation, as well as the energetics of surface defect formation. I ...
to the crystallization of poorly ordered precursor phases like
amorphous calcium carbonate Amorphous calcium carbonate (ACC) is the amorphous and least stable polymorph of calcium carbonate. ACC is extremely unstable under normal conditions and is found naturally in taxa as wide-ranging as sea urchins, corals, mollusks, and foramini ...
(ACC) via an Ostwald ripening process, or via the agglomeration of nanocrystals. The crystallization of ACC can occur in two stages. First, the ACC nanoparticles rapidly dehydrate and crystallize to form individual particles of vaterite. Second, the vaterite transforms to calcite via a dissolution and reprecipitation mechanism, with the
reaction rate The reaction rate or rate of reaction is the speed at which a chemical reaction takes place, defined as proportional to the increase in the concentration of a product per unit time and to the decrease in the concentration of a reactant per unit ...
controlled by the
surface area The surface area of a solid object is a measure of the total area that the surface of the object occupies. The mathematical definition of surface area in the presence of curved surfaces is considerably more involved than the definition of arc ...
of a calcite crystal. The second stage of the reaction is approximately 10 times slower. However, crystallization of calcite has been observed to be dependent on the starting pH and concentration of magnesium in solution. A neutral starting pH during mixing promotes the direct transformation of ACC into calcite without a vaterite intermediate. But when ACC forms in a solution with a
basic BASIC (Beginners' All-purpose Symbolic Instruction Code) is a family of general-purpose, high-level programming languages designed for ease of use. The original version was created by John G. Kemeny and Thomas E. Kurtz at Dartmouth College ...
initial pH, the transformation to calcite occurs via metastable vaterite, following the pathway outlined above. Magnesium has a noteworthy effect on both the stability of ACC and its transformation to crystalline CaCO3, resulting in the formation of calcite directly from ACC, as this ion destabilizes the structure of vaterite. Calcite may form in the subsurface in response to microorganism activity, such as sulfate-dependent
anaerobic oxidation of methane Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: * Anaerobic adhesive, a bonding a ...
, where methane is oxidized and sulfate is reduced, leading to precipitation of calcite and pyrite from the produced bicarbonate and
sulfide Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds lar ...
. These processes can be traced by the specific carbon isotope composition of the calcites, which are extremely depleted in the 13C isotope, by as much as −125 per mil PDB13C).


In Earth history

Calcite sea A calcite sea is a sea in which low-magnesium calcite is the primary inorganic marine calcium carbonate precipitate. An aragonite sea is the alternate seawater chemistry in which aragonite and high-magnesium calcite are the primary inorganic ca ...
s existed in Earth's 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 sea An aragonite sea contains aragonite and high-magnesium calcite as the primary inorganic calcium carbonate precipitates. The chemical conditions of the seawater must be notably high in magnesium content relative to calcium (high Mg/Ca ratio) for ...
s over the
Phanerozoic The Phanerozoic Eon is the current geologic eon in the geologic time scale, and the one during which abundant animal and plant life has existed. It covers 538.8 million years to the present, and it began with the Cambrian Period, when anima ...
, being most prominent in the Ordovician and Jurassic periods. Lineages evolved to use whichever
morph Morph may refer to: Biology * Morph (zoology), a visual or behavioral difference between organisms of distinct populations in a species * Muller's morphs, a classification scheme for genetic mutations * "-morph", a suffix commonly used in tax ...
of calcium carbonate was favourable in the ocean at the time they became mineralised, and retained this mineralogy for the remainder of their evolutionary history. Petrographic evidence for these calcite sea conditions consists of calcitic ooids, lmc cements,
hardgrounds Carbonate hardgrounds are surfaces of synsedimentarily cemented carbonate layers that have been exposed on the seafloor (Wilson and Palmer, 1992). A hardground is essentially, then, a lithified seafloor. Ancient hardgrounds are found in limestone ...
, and rapid early seafloor aragonite dissolution. The evolution of marine organisms with calcium carbonate shells may have been affected by the calcite and aragonite sea cycle. Calcite is one of the minerals that has been shown to catalyze an important biological reaction, the
formose reaction The formose reaction, discovered by Aleksandr Butlerov in 1861, and hence also known as the Butlerov reaction, involves the formation of sugars from formaldehyde. The term formose is a portmanteau of formaldehyde and aldose. Reaction and mechanism ...
, and may have had a role in the origin of life. Interaction of its chiral surfaces (see
Form Form is the shape, visual appearance, or configuration of an object. In a wider sense, the form is the way something happens. Form also refers to: *Form (document), a document (printed or electronic) with spaces in which to write or enter data ...
) with
aspartic acid Aspartic acid (symbol Asp or D; the ionic form is known as aspartate), is an α-amino acid that is used in the biosynthesis of proteins. Like all other amino acids, it contains an amino group and a carboxylic acid. Its α-amino group is in the pro ...
molecules results in a slight bias in chirality; this is one possible mechanism for the origin of homochirality in living cells.


Gallery

File:Calcite-Mottramite-cktsu-45b.jpg, Calcite with mottramite File:Erbenochile eye.JPG, Trilobite eyes employed calcite File:CalciteEchinosphaerites.jpg, Calcite crystals inside a test of the
cystoid Cystoidea is a class of extinct crinozoan echinoderms, termed cystoids, that lived attached to the sea floor by stalks. They existed during the Paleozoic Era, in the Middle Ordovician and Silurian Periods, until their extinction in the Devoni ...
'' Echinosphaerites aurantium'' (Middle Ordovician, northeastern Estonia) File:Calcite-Dolomite-Gypsum-159389.jpg, Rhombohedrons of calcite that appear almost as books of petals, piled up 3-dimensionally on the
matrix Matrix most commonly refers to: * ''The Matrix'' (franchise), an American media franchise ** ''The Matrix'', a 1999 science-fiction action film ** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchis ...
File:Calcite-Hematite-Chalcopyrite-176263.jpg, Calcite crystal canted at an angle, with little balls of
hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...
and crystals of chalcopyrite both on its surface and included just inside the surface of the crystal File:GeopetalCarboniferousNV.jpg, Thin section of calcite crystals inside a recrystallized
bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...
shell in a biopelsparite File:OoidSurface01.jpg, Grainstone with calcite
ooids Ooids are small (commonly ≤2 mm in diameter), spheroidal, "coated" (layered) sedimentary grains, usually composed of calcium carbonate, but sometimes made up of iron- or phosphate-based minerals. Ooids usually form on the sea floor, m ...
and sparry calcite cement;
Carmel Formation The Carmel Formation is a geologic formation in the San Rafael Group that is spread across the U.S. states of Wyoming, Utah, Colorado, north east Arizona and New Mexico. Part of the Colorado Plateau, this formation was laid down in the Middle J ...
, Middle Jurassic, of southern Utah, USA. File:Calcite-Aragonite-Sulphur-69380.jpg, Several well formed milky white casts, made up of many small sharp calcite crystals, from the
sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
mines at Agrigento, Sicily File:Calcite-tch21c.jpg, Reddish rhombohedral calcite crystals from
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...
. Its red color is due to the presence of iron File:Calcite-75480.jpg, Cobaltoan, the cobalt-rich variety of calcite File:Calcite-114508.jpg, Sand calcites (calcites heavily included with desert sand) in South Dakota, USA File:RM463c-calcite-butterfly-twin.jpg, Calcite, butterfly twin, . José María Patoni,
San Juan del Río, Durango San Juan del Río, Durango is a town and seat of the municipality of San Juan del Río, in the state of Durango, north-western Mexico. As of 2010, the town had a population of 2,912. San Juan del Río was the birthplace of journalist and poet J ...
( Mexico)


See also

* Ikaite, CaCO3·6H2O * List of minerals *
Lysocline The lysocline is the depth in the ocean dependent upon the carbonate compensation depth (CCD), usually around 3.5 km, below which the rate of dissolution of calcite increases dramatically because of a pressure effect. While the lysocline is the up ...
*
Manganoan calcite Manganoan calcite or manganocalcite is a variety of calcite rich in manganese, which gives the mineral a pink color.Man ...
, (Ca,Mn)CO3 * Monohydrocalcite, CaCO3·H2O * Ocean acidification * Ulexite


References


Further reading

* {{Authority control Calcium minerals Carbonate minerals Limestone Optical materials Transparent materials Calcite group Cave minerals Trigonal minerals Minerals in space group 167 Evaporite Luminescent minerals Polymorphism (materials science)