Hydrohalite

Chemical formula: NaCl·₂H₂O

Hydrohalite is hydrated sodium chloride, which forms at low temperatures from brines and is stable only below 0.1°C.

## Characteristics Hydrohalite is a mineral from the halide group, representing a hydrated form of sodium chloride (table salt). It occurs as colorless or white crystals, which typically have a needle-like, fibrous habit or form granular and massive aggregates. It is an unstable mineral under Earth's surface conditions – it melts at temperatures above 0.1°C, releasing water and leaving behind a white, powdery residue of halite. For this reason, natural specimens are extremely rare and difficult to preserve. ## Physical Properties Hydrohalite crystals are soft and brittle. The mineral is characterized by a vitreous luster and is transparent to translucent. Its density is significantly lower than that of halite, which is due to the presence of water in its crystal structure. ## Colors and Varieties Hydrohalite is usually colorless or white. No colored or commercial varieties are distinguished. ## History and Name The name hydrohalite refers to its chemical composition – "hydro" from water and "halite" from its anhydrous counterpart. It was first described by Carl Christian von Leon in 1826. The mineral was known earlier as a product of crystallization from brines at low temperatures. ## Applications Hydrohalite has no practical applications. Its significance is purely scientific, as an indicator of conditions prevailing during salt crystallization in low-temperature environments, such as some salt lakes or polar regions. It is also a subject of interest in cryology and planetology, as its presence is postulated on icy moons and Mars.

Properties

Mohs hardness: 1.5-2
Color: Colorless to white
Luster: Vitreous
Streak: White
Density: 0
Cleavage: Perfect on {110}
Fracture: Conchoidal
Transparency: Transparent
Crystal system: Monoclinic

Diagnostic features

## Identification The key diagnostic feature of hydrohalite is its instability at room temperature. Observing the mineral melting at a temperature slightly above 0°C, leaving a white halite residue, is unequivocal proof. Under refrigerated conditions, its needle-like crystals can be observed. It has a salty taste. ## Distinguishing from Similar Minerals - **Halite (rock salt):** Halite is stable at room temperature and forms cubic crystals. Hydrohalite melts, leaving halite behind. - **Ice (H₂O):** Ice melts at 0°C, leaving no residue. Hydrohalite melts at a similar temperature but leaves a salt residue. - **Other evaporites:** Most other salt minerals (e.g., sylvite, gypsum) are stable under normal conditions. ## Crystal Forms Hydrohalite most often forms elongated, needle-like or prismatic crystals. It also occurs as fibrous aggregates, radial aggregates, and crusts.

Geological environment

## Genesis Hydrohalite crystallizes from highly concentrated aqueous solutions of sodium chloride (brines) at temperatures below 0.1°C. It is a typical cryogenic mineral. It forms seasonally in some salt lakes in cold climates and polar regions, precipitating from seawater under ice. ## Mineral Associations It most commonly co-occurs with halite (as a product of its decomposition), ice, mirabilite, gypsum, and other evaporites formed at low temperatures. ## Localities Due to its instability, confirmed localities are few. It has been observed in salt lake sediments, including the Great Salt Lake in Utah (USA), in Siberia (Russia), and in sediments from the bottom of the Caspian Sea. Its presence has also been found as inclusions in quartz crystals from Austria and in sea ice in Antarctica and the Arctic.

Rarity

Very rare

For collectors

## Quality Criteria Hydrohalite specimens practically do not exist on the collector's market due to extreme storage requirements. Its scientific value is much greater than its collector's value. Theoretically, a well-formed, large, and transparent crystal preserved under refrigerated conditions would be an object of immense value to research institutions. ## Market Prices No data on the commercial market for this mineral. ## Popular Localities This mineral does not come from any "popular" localities in the collector's sense. Specimens for research are obtained during scientific expeditions to polar regions or salt lakes during winter periods.

Care and storage

## Cleaning Hydrohalite specimens are not cleaned due to their extreme instability. Any contact with water or heating above 0°C will cause its immediate decomposition. ## What to Avoid Temperatures above 0°C must be absolutely avoided. The mineral must be stored under refrigerated conditions. It is sensitive to changes in humidity and direct light, which can heat it. It should not be touched with bare hands, as body heat will cause it to melt. ## Storage The only effective storage is to keep the specimen in an airtight container in a freezer, at a constant sub-zero temperature. Display is practically impossible outside of specialized laboratory conditions.