Hydrohalite

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.

Hydrohalite
General
Category	Halide mineral
Formula	NaCl·2H2O
IMA symbol	Hhl[1]
Strunz classification	3.BA.05
Dana classification	9.1.2.1
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/c
Identification
Colour	Colourless or white
Diaphaneity	Transparent

Physical properties

Hydrohalite has a high nucleation energy, it decomposes at 0.1°C, giving a salty brine and solid halite.

Phase diagram of water–NaCl mixture

The cryohydric point of hydrohalite is at −21.2 °C (−6.2 °F), 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.^[2] 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] reaching a maximum decomposition temperature is at 25.8°C around 9400 atmospheres. The decomposition temperature reduces again at higher pressures.^[2]

Occurrence

The type locality is the Hallein Salt Mine in Austria.^[3]

Ceres

Hydrohalite was discovered on Ceres by Dawn,^[4] suggesting an early ocean, possibly surviving as a relict ocean.