Calcite

  The vaterite. Aragonite will change to calcite at 470°C, and vaterite is even less stable.

Properties

  Calcite 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 Iceland spar. Acute scalenohedral crystals are sometimes referred to as "dogtooth spar".

Single calcite crystals display an optical property called 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 1.5^[4].

Calcite can be either 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.

Natural occurrence

Calcite is often the primary constituent of the shells of marine organisms, e.g., plankton (such as algae, some sponges, brachiopoda, echinoderms, most bryozoa, and parts of the shells of some bivalves, such as oysters and rudists).

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

Calcite in Earth history

hardgrounds, and rapid early seafloor aragonite dissolution.^[5] The evolution of marine organisms with calcium carbonate shells may have been affected by the calcite and aragonite sea cycle.^[6]

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.

See also

• Monohydrocalcite, CaCO₃·H₂O
• Ikaite, CaCO₃·6H₂O
• List of minerals
• Lysocline
• Ocean acidification

References

1. ^ http://rruff.geo.arizona.edu/doclib/hom/calcite.pdf Mineral Data Publishers
2. ^ http://www.mindat.org/min-859.html Mindat
3. ^ http://webmineral.com/data/Calcite.shtml Webmineral data
4. ^ 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 (1998) 341-346. http://dx.doi.org/10.1016/S0040-6090(97)00843-2
5. ^ 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
6. ^ 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 information and images

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.