Category Halide mineral Strunz classification 3.BA.05 Space group P21/c | Formula
(repeating unit) NaCl·2H2O Crystal system Monoclinic | |
Crystal class Prismatic (2/m)
(same H-M symbol) | ||
Hydrohalite is a mineral that occurs in saturated halite brines at cold temperatures (−5 °C). It was first described in 1847 in Salzburg, Austria.
Hydrohalite has a high nucleation energy, and solutions will normally need to be super cooled for crystals to form. The cryohydric point is at -21.2°C. 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. Hydrohalite melts at 0.1°C. This is an incongruent melting point, as a liquid salty water, of different composition and solid halite is formed on melting. Under pressure, hydrohalite is stable between 7,900 and 11,600 atmospheres pressure. The melting point increases at the rate of 0.007°K per atmosphere. (for 1-1000 atmospheres). The maximum melting temperatures is at 25.8°C under 9400 atmospheres. Above this pressure the melting point reduces.