Name | Calcite |
---|---|
Category | Carbonate mineral |
Caption | A one-inch Calcite Rhomb that shows the Double image refraction property. |
Formula | CaCO3 |
Strunz | 05.AB.05 |
Symmetry | Trigonal 2/m |
Unit cell | a = 4.9896(2) Å, c = 17.061(11) Å; Z=6 |
Color | Colorless or white, also gray, yellow, green, |
Habit | Crystalline, granular, stalactitic, concretionary, massive, rhombohedral. |
System | Trigonal hexagonal scalenohedral (2/m), Space Group (R 2/c) |
Twinning | Common by four twin laws |
Cleavage | Perfect on [101] three directions with angle of 74° 55' |
Fracture | Conchoidal |
Tenacity | Brittle |
Mohs | 3 (defining mineral) |
Luster | Vitreous to pearly on cleavage surfaces |
Refractive | nω = 1.640 – 1.660 nε = 1.486 |
Opticalprop | Uniaxial (-) |
Birefringence | δ = 0.154 – 0.174 |
Streak | White |
Gravity | 2.71 |
Solubility | Soluble in dilute acids |
Diaphaneity | Transparent to translucent |
Other | May fluoresce red, blue, yellow, and other colors under either SW and LW UV; phosphorescent |
References |
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. 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.
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 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.
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.
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, brachiopods, 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.
Category:Calcium minerals Category:Carbonate minerals Category:Limestone Category:Optical materials Category:Transparent materials Category:Calcite group Category:Cave geology Category:Trigonal minerals
be:Кальцый be-x-old:Кальц bs:Kalcit bg:Калцит ca:Calcita cs:Kalcit da:Kalk (mineral) de:Calcit et:Kaltsiit el:Ασβεστίτης es:Calcita eo:Kalcito eu:Kaltzita fa:کلسیت fr:Calcite gl:Calcita ko:방해석 hi:कैल्साइट hr:Kalcit id:Kalsit is:Kalsít it:Calcite he:קלציט lv:Kalcīts lt:Kalcitas hu:Kalcit nl:Calciet ja:方解石 no:Kalsitt pl:Kalcyt pt:Calcita ro:Calcit ru:Кальцит simple:Calcite sk:Kalcit sl:Kalcit sr:Калцит sh:Kalcit fi:Kalsiitti sv:Kalkspat ta:கால்சைட் th:แคลไซต์ tr:Kalsit uk:Кальцит vi:Canxit zh:方解石This text is licensed under the Creative Commons CC-BY-SA License. This text was originally published on Wikipedia and was developed by the Wikipedia community.
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