Editing Calcite (section)

== 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 [[Lustre (mineralogy)|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.<ref name="mindat"/>

=== Crystal habits ===
Calcite has numerous habits, representing combinations of over 1000 [[crystallographic form]]s.<ref name=HOM/>  Most common are [[scalenohedron|scalenohedra]], with faces in the hexagonal {{mset|2 1 1}} directions (morphological unit cell) or {2 1 4} directions (structural unit cell); and rhombohedral, with faces in the {{mset|1 0 1}} or {{mset|1 0 4}} directions (the most common cleavage plane).<ref name=Hazen/> Habits include acute to obtuse rhombohedra, tabular habits, [[Prism (geometry)|prisms]], or various [[scalenohedron|scalenohedra]]. Calcite exhibits several [[crystal twinning|twinning]] types that add to the observed habits. It may occur as fibrous, granular, lamellar, or compact. A fibrous, efflorescent habit is known as ''lublinite''.<ref>{{cite web|title=Lublinite|url=http://www.mindat.org/min-9512.html|website=mindat.org|access-date=6 May 2018}}</ref> Cleavage is usually in three directions parallel to the rhombohedron form. Its fracture is [[Conchoidal fracture|conchoidal]], 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 acid]]s. Rhombohedral faces are not chiral.<ref name=Hazen/><ref>{{Cite journal |last1=Jiang |first1=Wenge |last2=Pacella |first2=Michael S. |last3=Athanasiadou |first3=Dimitra |last4=Nelea |first4=Valentin |last5=Vali |first5=Hojatollah |last6=Hazen |first6=Robert M. |last7=Gray |first7=Jeffrey J. |last8=McKee |first8=Marc D. |date=2017-04-13 |title=Chiral acidic amino acids induce chiral hierarchical structure in calcium carbonate |journal=Nature Communications |language=en |volume=8 |issue=1 |pages=15066 |doi=10.1038/ncomms15066 |pmid=28406143 |pmc=5399303 |bibcode=2017NatCo...815066J |issn=2041-1723}}</ref>

<gallery class="center">
File:Estonian Museum of Natural History Specimen No 182279 photo (g28 g28-218 1 jpg).jpg|Rhombohedral calcite
File:Calcite jaune sur fluorine violette (USA).jpg|Scalenohedral calcite
File:Calcite, galène et pyrite (Dal'negorsk - Fédération de Russie).JPG|Prismatic calcite
File:Calcite et fluorine (USA).JPG|Prismatic calcite
File:Calcite 7.jpg|Stalactitic calcite
File:Estonian Museum of Natural History Specimen No 202078 photo (g27 g27-415 1 jpg).jpg|Hexagonal calcite
File:Calcite-241250.jpg|Dodecahedral calcite
File:Calcite - Galleria di Mezzolombardo (Trento) - Paolo Ferretti.jpg|Bipyramidal calcite
File:Muséum de Nantes - 352 - Calcite (Grenoble, Isère, France).jpg|Druse calcite
File:Natural History Museum 155 (8047048490).jpg|Twinned calcite
File:Calcite - Sassari, Sardegna, Italia 01.jpg|Globular calcite
File:Calcite (Cave-in-Rock Mining District, Illinois, USA) 3 (42590140555).jpg|Botryoidal calcite
</gallery>

=== Optical ===
[[File:Calcite-refraction-property.jpg|thumb|Photograph of calcite displaying the characteristic birefringence optical behaviour]]
[[File:Fluorescence in calcite.jpg|thumb|Demonstration of birefringence in calcite, using 445&nbsp;nm laser]]
Calcite is [[transparency and translucency|transparent]] to [[opacity (optics)|opaque]] and may occasionally show [[phosphorescence]] or [[fluorescence]]. A transparent variety called "[[Iceland spar]]" is used for optical purposes.<ref>{{cite journal |last1=Harstad |first1=A. O. |last2=Stipp |first2=S. L. S. |title=Calcite dissolution; effects of trace cations naturally present in Iceland spar calcites. |journal=Geochimica et Cosmochimica Acta |date=2007 |volume=71 |issue=1 |pages=56–70 |doi=10.1016/j.gca.2006.07.037 |bibcode=2007GeCoA..71...56H }}</ref> Acute [[bipyramid|scalenohedral]] crystals are sometimes referred to as "dogtooth spar" while the [[rhombohedron|rhombohedral]] form is sometimes referred to as "nailhead spar".<ref name=Sinkankas1964>{{cite book |last1=Sinkankas |first1=John |title=Mineralogy for amateurs. |date=1964 |publisher=Van Nostrand |location=Princeton, N.J. |isbn=0442276249 |pages=359–364}}</ref> The rhombohedral form may also have been the "[[sunstone (medieval)|sunstone]]" whose use by [[Viking]] navigators is mentioned in the [[Sagas of Icelanders|Icelandic Sagas]].<ref>{{cite journal |last1=Ropars |first1=Guy |last2=Lakshminarayanan |first2=Vasudevan |last3=Le Floch |first3=Albert |title=The sunstone and polarised skylight: ancient Viking navigational tools? |journal=[[Contemporary Physics]] |date=2 October 2014 |volume=55 |issue=4 |pages=302–317 |doi=10.1080/00107514.2014.929797 |bibcode=2014ConPh..55..302R |s2cid=119962347 }}</ref>

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 [[Denmark|Danish]] scientist [[Rasmus Bartholin]] in 1669. At a wavelength of about 590&nbsp;nm, calcite has ordinary and extraordinary [[refractive index|refractive indices]] of 1.658 and 1.486, respectively.<ref>{{cite journal |url=http://physics.info/refraction/ |title=Refraction |last=Elert|first=Glenn |journal=The Physics Hypertextbook |year=2021}}</ref> Between 190 and 1700&nbsp;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.<ref>{{cite journal |doi=10.1016/S0040-6090(97)00843-2| title = Determination of optical anisotropy in calcite from ultraviolet to mid-infrared by generalized ellipsometry| journal=Thin Solid Films| volume=313–314| issue=1–2| pages=341–346| year=1998| last1=Thompson| first1=D. W.| last2=Devries| first2=M. J.| last3=Tiwald| first3=T. E.| last4=Woollam| first4=J. A. |bibcode=1998TSF...313..341T}}</ref>

=== Thermoluminescence ===
Calcite has [[Thermoluminescence|thermoluminescent]] properties mainly due to manganese divalent ({{chem2|Mn(2+)}}).<ref name="Medlin">{{cite journal |last1=Medlin |first1=W. L. |title=Thermoluminescent properties of calcite. |journal=The Journal of Chemical Physics |date=1959 |volume=30 |issue=2 |pages=451–458 |doi=10.1063/1.1729973 |bibcode=1959JChPh..30..451M}}</ref> An experiment was conducted by adding activators such as ions of Mn, Fe, Co, Ni, Cu, Zn, Ag, Pb, and Bi to the calcite samples to observe whether they emitted heat or light. The results showed that adding ions ({{chem2|Cu+}}, {{chem2|Cu(2+)}}, {{chem2|Zn(2+)}}, {{chem2|Ag+}}, {{chem2|Bi(3+)}}, {{chem2|Fe(2+)}}, {{chem2|Fe(3+)}}, {{chem2|Co(2+)}}, {{chem2|Ni(2+)}}) did not react.<ref name="Medlin" />  However, a reaction occurred when both manganese and lead ions were present in calcite.<ref name="Medlin" />  By changing the temperature and observing the glow curve peaks, it was found that {{chem2|Pb(2+) }}and {{chem2|Mn(2+)}}acted as activators in the calcite lattice, but {{chem2|Pb(2+)}} was much less efficient than {{chem2|Mn(2+)}}.<ref name="Medlin" />

Measuring mineral thermoluminescence experiments usually use x-rays or gamma-rays to activate the sample and record the changes in glowing curves at a temperature of 700–7500&nbsp;K.<ref name="Medlin" /> Mineral thermoluminescence can form various glow curves of crystals under different conditions, such as temperature changes, because impurity ions or other crystal defects present in minerals supply luminescence centers and trapping levels.<ref name="Medlin" /> Observing these curve changes also can help infer geological correlation and age determination.<ref name="Medlin" />

=== Chemical ===
Calcite, like most carbonates, dissolves in acids by the following reaction
: {{chem2|CaCO3 + 2 H+ -> Ca(2+) + H2O + CO2}}

The carbon dioxide released by this reaction produces a characteristic effervescence when a calcite sample is treated with an acid.

Due to its acidity, carbon dioxide has a slight solubilizing effect on calcite. The overall reaction is
: {{chem2|CaCO3(s) + H2O + CO2(aq) -> Ca(2+)(aq) + 2HCO3-(aq)}}

If the amount of dissolved carbon dioxide drops, the reaction reverses to precipitate calcite. As a result, calcite can be either [[solvation|dissolved]] by groundwater or [[precipitate]]d by groundwater, depending on such factors as the water temperature, [[acidity|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 (fluid)|permeability]] of the rock, and if it continues for a long period of time, may result in the formation of [[cave]]s. Continued dissolution of calcium carbonate-rich formations can lead to the expansion and eventual collapse of cave systems, resulting in various forms of [[Karst|karst topography]].<ref>{{cite encyclopedia |last1=Wolfgang |first1=Dreybrodt |year=2004 |title=Dissolution: Carbonate rocks |encyclopedia=Encyclopedia of Caves and Karst Science |pages=295–298 |url=https://www.researchgate.net/publication/313171146 |access-date=26 December 2020}}</ref>

Calcite exhibits an unusual characteristic called retrograde solubility: it is less soluble in water as the temperature increases. Calcite is also more soluble at higher pressures.<ref>{{cite journal |last1=Sharp |first1=W. E. |last2=Kennedy |first2=G. C. |title=The System CaO-CO 2 -H 2 O in the Two-Phase Region Calcite + Aqueous Solution |journal=The Journal of Geology |date=March 1965 |volume=73 |issue=2 |pages=391–403 |doi=10.1086/627069|s2cid=100971186 }}</ref>

Pure calcite has the composition {{chem2|CaCO3}}. 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 (mineral)|dolomite]], {{chem2|MgCa(CO3)2}}.<ref>{{cite book |last1=Blatt |first1=Harvey |last2=Middleton |first2=Gerard |last3=Murray |first3=Raymond |title=Origin of sedimentary rocks |date=1980 |publisher=Prentice-Hall |location=Englewood Cliffs, N.J. |isbn=0136427103 |edition=2d |pages=448–449}}</ref> Calcite can also contain small quantities of [[iron]] and [[manganese]].<ref>{{cite journal |last1=Dromgoole |first1=Edward L. |last2=Walter |first2=Lynn M. |title=Iron and manganese incorporation into calcite: Effects of growth kinetics, temperature and solution chemistry |journal=Chemical Geology |date=February 1990 |volume=81 |issue=4 |pages=311–336 |doi=10.1016/0009-2541(90)90053-A|bibcode=1990ChGeo..81..311D }}</ref> Manganese may be responsible for the fluorescence of impure calcite, as may traces of organic compounds.<ref>{{cite journal |last1=Pedone |first1=Vicki A. |last2=Cercone |first2=Karen Rose |last3=Burruss |first3=R.C. |title=Activators of photoluminescence in calcite: evidence from high-resolution, laser-excited luminescence spectroscopy |journal=Chemical Geology |date=October 1990 |volume=88 |issue=1–2 |pages=183–190 |doi=10.1016/0009-2541(90)90112-K|bibcode=1990ChGeo..88..183P }}</ref>