Calcite

Calcite

Doubly refracting calcite from Iceberg claim, Dixon, New Mexico.
General
Category Carbonate mineral
Chemical formula CaCO3
Crystal symmetry Trigonal 32/m
Unit cell a = 4.9896(2) Å, c = 17.061(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
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 470 °C, and vaterite is even less stable.

Contents

Properties

Doubly terminated calcite crystal

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 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. A transparent variety called Iceland spar is used for optical purposes. Acute scalenohedral crystals are sometimes referred to as "dogtooth 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.[5] 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 1.5.[6]

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. 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, most notably the Snowy River Cave in Lincoln County, New Mexico.

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.[7][8]

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.

Calcite may also be found in volcanic or mantle-derived rocks such as carbonatites, kimberlites, or rarely in peridotites. Lublinite is a fibrous, efflorescent form of calcite.[9]

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 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.

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. Lineages evolved to use whichever morph 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.[10] Petrographic evidence for these calcite sea conditions consists of calcitic ooids, lmc cements, hardgrounds, and rapid early seafloor aragonite dissolution.[11] The evolution of marine organisms with calcium carbonate shells may have been affected by the calcite and aragonite sea cycle.[12]

Gallery

Fossil bivalve with calcite crystals.]]

Mississippian marble (made of calcite) in Big Cottonwood Canyon, Wasatch Mountains, Utah.

Thin section view of calcite crystals inside a recrystallized bivalve shell in a biopelsparite.

Calcite containing Niobium (giving it distinctive bluish color), from Medford Quarry, Maryland.

See also

References

  1. ↑ Klein, Cornelis and Cornelius S. Hurlbut, Jr., Manual of Mineralogy, Wiley, 20th, 1985, p. 329 ISBN 0-471-80580-7
  2. ↑ Mineral Data Publishers
  3. ↑ Calciate at Mindat
  4. ↑ Calcite at Webmineral
  5. ↑ Elert, Glenn. "Refraction". The Physics Hypertextbook. http://hypertextbook.com/physics/waves/refraction/. 
  6. ↑ Thompson, D.W. et al. (1998). "Determination of optical anisotropy in calcite from ultraviolet to mid-infrared by generalized ellipsometry". Thin Solid Films 313-314: 341–346. doi:10.1016/S0040-6090(97)00843-2. 
  7. ↑ P. C. Rickwood (1981). "The largest crystals". American Mineralogist 66: 885–907. http://www.minsocam.org/ammin/AM66/AM66_885.pdf. 
  8. ↑ "The giant crystal project site". http://giantcrystals.strahlen.org/europe/helgustadir.htm. Retrieved 2009-06-06. 
  9. ↑ Lublinite at Mindat
  10. ↑ Porter, S. M. (2007). "Seawater Chemistry and Early Carbonate Biomineralization". Science 316 (5829): 1302. doi:10.1126/science.1137284. PMID 17540895. 
  11. ↑ Palmer, Timothy; Wilson, Mark (2004). "Calcite precipitation and dissolution of biogenic aragonite in shallow Ordovician calcite seas". Lethaia 37: 417. doi:10.1080/00241160410002135. 
  12. ↑ 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

External links