Hydrohalite is a mineral brines at cold temperatures (below 0.1 °C). It was first described in 1847 in Dürrnberg. It exists in cold weather.
Hydrohalite has a high nucleation energy, and solutions will normally need to be supercooled 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 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). The maximum decomposition temperature is at 25.8°C under 9400 atmospheres. Above this pressure the decomposition point goes down.