{{Infobox mineral | name = Hydrohalite | category = Halide mineral | boxwidth = | boxbgcolor = | image = | imagesize = | alt = | caption = | formula = {{chem2|NaCl*2H2O}} | IMAsymbol = Hhl<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}</ref> | molweight = | strunz = 3.BA.05 | dana = 9.1.2.1 | system = Monoclinic | class = Prismatic (2/m) <br/><small>(same H-M symbol)</small> | symmetry = ''P2''<sub>1</sub>/c | colour = Colourless or white | habit = | twinning = | cleavage = | fracture = | tenacity = | mohs = | luster = | streak = | diaphaneity = Transparent | gravity = | density = | polish = | opticalprop = | refractive = | birefringence = | pleochroism = | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence= | absorption = | melt = | fusibility = | diagnostic = | solubility = | other = | alteration = | references = }} '''Hydrohalite''' is a halide mineral that occurs in saturated halite brines at cold temperatures (below 0.1 °C) and is the most common form of hydrated sodium chloride. It was first described in 1847 from an occurrence in Dürrnberg, Austria.
==Physical properties== Hydrohalite has a high nucleation energy, it decomposes at 0.1°C, giving a salty brine and solid halite. thumb|left|upright=1.15|Phase diagram of water–NaCl mixture The cryohydric point of hydrohalite is at {{convert|-21.2|°C}}, solutions will normally need to be supercooled for crystals to form. Above this temperature, liquid water saturated with salt can exist in equilibrium with hydrohalite. Unlike halite, hydrohalite has a strong positive temperature coefficient of solubility.<ref name=brait>{{cite book|last1=Braitsch|first1=O.|title=Salt Deposits Their Origin and Composition|chapter=The Stability Conditions of Salt Minerals|date=1971|pages=42–44|doi=10.1007/978-3-642-65083-3_2|publisher=Springer|isbn=978-3-642-65085-7}}</ref> Under pressure, hydrohalite is stable between 7,900 and 11,600 atmospheres pressure. The decomposition point increases at the rate of 0.007K per atmosphere (for 1–1000 atmospheres),<ref name=brait/> reaching a maximum decomposition temperature is at 25.8°C around 9400 atmospheres. The decomposition temperature reduces again at higher pressures.<ref name=brait/>
==Occurrence== The type locality is the Hallein Salt Mine in Austria.<ref name=Mindat>Page ''Hydrohalite: Mineral information, data and localities'' on {{cite web | url=https://www.mindat.org/min-1975.html | title=mindat.org | publisher=Hudson Institute of Mineralogy | access-date=2025-04-01}}</ref>
===Ceres=== Hydrohalite was discovered on Ceres by ''Dawn'',<ref>De Sanctis, M.C., Ammannito, E., Raponi, A. et al. Fresh emplacement of hydrated sodium chloride on Ceres from ascending salty fluids. Nat Astron 4, 786–793 (2020). https://doi.org/10.1038/s41550-020-1138-8</ref> suggesting an early ocean, possibly surviving as a relict ocean.
==References== {{Reflist}} *[http://www.webmineral.com/data/Hydrohalite.shtml Hydrohalite Mineral Data]
Category:Sodium minerals Category:Chloride minerals Category:Dihydrate minerals Category:Monoclinic minerals Category:Minerals in space group 14 Category:Minerals described in 1847
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