Hydrohalite

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
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 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.[1] 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 0.007°K per atmosphere. (for 1–1000 atmospheres).[1] The maximum decomposition temperature is at 25.8°C under 9400 atmospheres. Above this pressure the decomposition point goes down.[1]

Ceres

Hydrohalite was discovered on Ceres by Dawn,[2] suggesting an early ocean, possibly surviving as a relict ocean.

References
  1. 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.
  2. 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
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