Editing Calcite (section)

=== 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>