| 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) (sameH-M symbol) |
| Space group | P21/c |
| Identification | |
| Colour | Colourless or white |
| Diaphaneity | Transparent |
Hydrohalite is ahalide mineral that occurs in saturatedhalite brines at cold temperatures (below 0.1 °C) and is the most common form of hydratedsodium chloride. It was first described in 1847 from an occurrence inDürrnberg,Austria.
Hydrohalite has a high nucleation energy, it decomposes at 0.1°C, giving a salty brine and solid halite.
Thecryohydric point of hydrohalite is at −21.2 °C (−6.2 °F), solutions will normally need to besupercooled for crystals to form. Above this temperature, liquid water saturated with salt can exist in equilibrium with hydrohalite. Unlikehalite, hydrohalite has a strong positive temperature coefficient ofsolubility.[2]Under pressure, hydrohalite is stable between 7,900 and 11,600 atmospheres pressure. The decomposition point increases at the rate of 0.007K peratmosphere (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]
The type locality is theHallein Salt Mine inAustria.[3]
Hydrohalite was discovered onCeres byDawn,[4] suggesting an early ocean, possibly surviving as arelict ocean.
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