Editing Gypsum (section)

==Physical properties==
[[File:Gypsum deformed cristal-MCG 7747-P4150901-black.jpg|thumb|left|Gypsum crystals are soft enough to bend under pressure of the hand. Sample on display at Musée cantonal de géologie de Lausanne.]]
Gypsum is moderately water-soluble (~2.0–2.5&nbsp;g/L at 25&nbsp;°C)<ref name="Bock_1961">{{Cite journal| last = Bock| first = E.| year = 1961| title = On the solubility of anhydrous calcium sulphate and of gypsum in concentrated solutions of sodium chloride at 25&nbsp;°C, 30&nbsp;°C, 40&nbsp;°C, and 50&nbsp;°C| journal = Canadian Journal of Chemistry| volume = 39| issue = 9| pages = 1746–1751 | doi = 10.1139/v61-228| df = dmy-all| doi-access = free}}</ref> and, in contrast to most other salts, it exhibits [[Solubility#Factors affecting solubility|retrograde solubility]], becoming less soluble at higher temperatures. When gypsum is heated in air it loses water and converts first to [[calcium sulfate hemihydrate]] ([[bassanite]], often simply called "plaster") and, if heated further, to anhydrous [[calcium sulfate]] ([[anhydrite]]).  As with [[anhydrite]], the solubility of gypsum in saline solutions and in [[brine]]s is also strongly dependent on [[sodium chloride]] (common table salt) concentration.<ref name="Bock_1961"/>

The structure of gypsum consists of layers of calcium (Ca<sup>2+</sup>) and sulfate ({{chem2|SO4(2-)}}) ions tightly bound together. These layers are bonded by sheets of [[anion water]] molecules via weaker [[hydrogen bond]]ing, which gives the crystal perfect cleavage along the sheets (in the {010} plane).<ref name=Cornelis/><ref>{{cite journal| doi= 10.1016/S0008-8846(01)00675-5| last1= Mandal| first1= Pradip K|year=2002|pages=313|volume=32|journal=Cement and Concrete Research|last2=Mandal|first2=Tanuj K| title= Anion water in gypsum (CaSO<sub>4</sub>·2H<sub>2</sub>O) and hemihydrate (CaSO<sub>4</sub>·1/2H<sub>2</sub>O)|issue=2}}</ref>