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Calcite

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Calcite

Calcite
A one-inch calcite rhomb that shows the double image refraction property
General
Category Carbonate mineral
Formula
(repeating unit)
CaCO3
Strunz classification 05.AB.05
Crystal symmetry Trigonal 32/m
Unit cell a = 4.9896(2) Å, c = 17.0610(11) Å; Z=6
Identification
Color Colorless or white, also gray, yellow, green,
Crystal habit Crystalline, granular, stalactitic, concretionary, massive, rhombohedral.
Crystal system Trigonal hexagonal scalenohedral (32/m), Space Group (R3 2/c)
Twinning Common by four twin laws
Cleavage Perfect on [1011] three directions with angle of 74° 55'[1]
Fracture Conchoidal
Tenacity Brittle
Mohs scale hardness 3 (defining mineral)
Luster Vitreous to pearly on cleavage surfaces
Streak White
Diaphaneity Transparent to translucent
Specific gravity 2.71
Optical properties Uniaxial (-)
Refractive index nω = 1.640–1.660
nε = 1.486
Birefringence δ = 0.154–0.174
Solubility Soluble in dilute acids
Other characteristics May fluoresce red, blue, yellow, and other colors under either SW and LW UV; phosphorescent
References [2][3][4]
Crystal structure of calcite

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380–470 °C,[5] and vaterite is even less stable.

Properties

Calcite crystals are 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 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 defining 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, orange, 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. 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".

Single calcite crystals display an optical property called birefringence (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 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.[6] 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.[7]

Calcite, like most carbonates, will dissolve with most forms of acid. Calcite can be either dissolved by groundwater or precipitated by groundwater, depending on several factors including the water temperature, 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. Ambient carbon dioxide, due to its acidity, has a slight solubilizing effect on calcite. 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 caves. On a landscape scale, continued dissolution of calcium carbonate-rich rocks can lead to the expansion and eventual collapse of cave systems, resulting in various forms of karst topography.

Use and applications

High-grade optical calcite was used in World War II for gun sights, specifically in bomb sights and anti-aircraft weaponry.[8] Also, experiments have been conducted to use calcite for a cloak of invisibility.[9] Microbiologically precipitated calcite has a wide range of applications, such as soil remediation, soil stabilization and concrete repair.

Natural occurrence

The largest documented single crystals of calcite originated from Iceland, measured 7×7×2 m and 6×6×3 m and weighed about 250 tons.[10][11]

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 marble. It also occurs as a vein mineral in deposits from hot springs, and it occurs in caverns as stalactites and stalagmites.

Lublinite is a fibrous, efflorescent form of calcite.[12]

Calcite may also be found in volcanic or mantle-derived rocks such as carbonatites, kimberlites, or rarely in peridotites.

Calcite is often the primary constituent of the Trilobites, which became extinct a quarter billion years ago, had unique compound eyes that used clear calcite crystals to form the lenses.[13]

Formation processes

Calcite forms from a poorly ordered precursor (amorphous calcium carbonate, ACC).[14] The crystallization process occurs in two stages; firstly, the ACC nanoparticles rapidly dehydrate and crystallize to form individual particles of vaterite; secondly, the vaterite transforms to calcite via a dissolution and reprecipitation mechanism with the reaction rate controlled by the surface area of calcite.[15] The second stage of the reaction is approximately 10 times slower than the first. However, the crystallization of calcite has been observed to be dependent on the starting pH and presence of Mg in solution.[16] A neutral starting pH during mixing promotes the direct transformation of ACC into calcite. Conversely, when ACC forms in a solution that starts with a basic initial pH, the transformation to calcite occurs via metastable vaterite, which forms via a spherulitic growth mechanism.[17] In a second stage this vaterite transforms to calcite via a surface-controlled dissolution and recrystallization mechanism. Mg 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 unstabilizes the structure of vaterite.

In Earth history

[20]

Gallery

See also

References

  1. ^ Dana, James Dwight; Klein, Cornelis and Hurlbut, Cornelius Searle (1985) Manual of Mineralogy, Wiley, p. 329, ISBN 0-471-80580-7
  2. ^ Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W. and Nichols, Monte C., ed. (2003). "Calcite". Handbook of Mineralogy (PDF). V (Borates, Carbonates, Sulfates). Chantilly, VA, US: Mineralogical Society of America.  
  3. ^ Calcite. Mindat.org
  4. ^ Calcite. Webmineral. com
  5. ^ Yoshioka S.; Kitano Y. (1985). "Transformation of aragonite to calcite through heating". Geochemical Journal 19: 24–249. 
  6. ^ Elert, Glenn. "Refraction". The Physics Hypertextbook. 
  7. ^ Thompson, D. W.; Devries, M. J.; Tiwald, T. E.; Woollam, J. A. (1998). "Determination of optical anisotropy in calcite from ultraviolet to mid-infrared by generalized ellipsometry". Thin Solid Films. 313-314: 341.  
  8. ^ "Borrego's calcite mine trail holds desert wonders". Retrieved 2011-06-03. 
  9. ^ Chen, Xianzhong; Luo, Yu; Zhang, Jingjing; Jiang, Kyle; Pendry, John B.; Zhang, Shuang (2011). "Macroscopic invisibility cloaking of visible light". Nature Communications 2 (2): 176.  
  10. ^ Rickwood, P. C. (1981). "The largest crystals". American Mineralogist 66: 885–907. 
  11. ^ "The giant crystal project site". Retrieved 2009-06-06. 
  12. ^ Lublinite. Mindat.org
  13. ^  
  14. ^ Rodriguez-Blanco, J. D.; Shaw, S.; Benning, L. G. (2008). "How to make 'stable' ACC: Protocol and preliminary structural characterization". Mineralogical Magazine 72: 283.  
  15. ^ Rodriguez-Blanco, J. D.; Shaw, S.; Benning, L. G. (2011). "The kinetics and mechanisms of amorphous calcium carbonate (ACC) crystallization to calcite, via vaterite". Nanoscale 3 (1): 265–71.  
  16. ^ Rodriguez-Blanco, J. D.; Shaw, S.; Bots, P.; Roncal-Herrero, T.; Benning, L. G. (2012). "The role of pH and Mg on the stability and crystallization of amorphous calcium carbonate". Journal of Alloys and Compounds 536: S477.  
  17. ^ Bots, P.; Benning, L. G.; Rodriguez-Blanco, J. D.; Roncal-Herrero, T.; Shaw, S. (2012). "Mechanistic Insights into the Crystallization of Amorphous Calcium Carbonate (ACC)". Crystal Growth & Design 12 (7): 3806.  
  18. ^ Porter, S. M. (2007). "Seawater Chemistry and Early Carbonate Biomineralization". Science 316 (5829): 1302.  
  19. ^ Palmer, Timothy; Wilson, Mark (2004). "Calcite precipitation and dissolution of biogenic aragonite in shallow Ordovician calcite seas". Lethaia 37 (4): 417.  
  20. ^ Harper, E.M.; Palmer, T.J.; Alphey, J.R. (1997). "Evolutionary response by bivalves to changing Phanerozoic sea-water chemistry". Geological Magazine 134: 403–407.  

Further reading

  • 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.
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