Chemistry:Hydrohalite

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Hydrohalite
General
CategoryHalide mineral
Formula
(repeating unit)
NaCl·2H2O
Strunz classification3.BA.05
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupP21/c
Identification
ColourColourless or white
DiaphaneityTransparent

Hydrohalite is a mineral that occurs in saturated halite brines at cold temperatures (below 0.1 °C). It was first described in 1847 in Dürrnberg, Austria. It exists in cold weather.

Phase diagram of water–NaCl mixture

Hydrohalite has a high nucleation energy, and solutions will normally need to be supercooled for crystals to form. The cryohydric point is at −21.2 °C (−6.2 °F). Above this temperature, liquid water saturated with salt can exist in equilibrium with hydrohalite. Hydrohalite has a strong positive temperature coefficient of solubility, unlike halite.[2] Hydrohalite decomposes at 0.1°C, giving a salty brine and solid halite. 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).[2] The maximum decomposition temperature is at 25.8°C under 9400 atmospheres. Above this pressure the decomposition point goes down.[2]

Ceres

Hydrohalite was discovered on Ceres by Dawn,[3] suggesting an early ocean, possibly surviving as a relict ocean.

References

  1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine 85 (3): 291–320. doi:10.1180/mgm.2021.43. Bibcode2021MinM...85..291W. 
  2. 2.0 2.1 2.2 Braitsch, O. (1971). "The Stability Conditions of Salt Minerals". Salt Deposits Their Origin and Composition. Springer. pp. 42–44. doi:10.1007/978-3-642-65083-3_2. ISBN 978-3-642-65085-7. 
  3. 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