Tachyhydrite

Chemical formula: CaMg<sub>2</sub>Cl<sub>6</sub>·12H<sub>2</sub>O

Tachyhydrite is a hydrated calcium and magnesium chloride, characterized by extreme hygroscopicity, causing it to rapidly dissolve upon contact with atmospheric moisture.

## Characteristics Tachyhydrite is a mineral from the halide group, chemically a hydrated calcium and magnesium chloride. Its most characteristic feature is strong hygroscopicity (deliquescence) – the ability to rapidly absorb moisture from the surroundings, leading to its quick dissolution and transition into a liquid state. For this reason, it is extremely difficult to preserve in collections. It most commonly occurs in compact, granular, or fibrous aggregates. Well-formed crystals are very rare and take the form of rhombohedra. ## Physical Properties This mineral is very soft, with a Mohs hardness of only 2. It is light, with a density of approximately 1.66 g/cm³. It has a vitreous luster, sometimes transitioning to resinous. It is transparent. It does not exhibit cleavage, and its fracture is conchoidal. It has a bitter taste. ## Colors and Varieties Tachyhydrite is usually colorless, white, waxy yellow, or honey yellow. It does not form color varieties or have special trade names. ## History and Name The mineral's name, given in 1856 by Carl Rammelsberg, comes from the Greek words *tachys* (ταχύς) meaning "swift" and *hydor* (ὕδωρ) meaning "water". This name is a direct reference to its unusual property – the rapid absorption of water and dissolution. ## Applications Due to its instability under atmospheric conditions, tachyhydrite has no industrial applications. It is solely an object of scientific interest and a rare, difficult-to-maintain collector's specimen.

Properties

Mohs hardness
2
Luster
Vitreous to Resinous
Streak
White
Density
1.66
Cleavage
None
Fracture
Conchoidal
Transparency
Transparent
Crystal system
Trigonal

Diagnostic features

## Identification The most important diagnostic feature of tachyhydrite is its extremely rapid deliquescence – when left in the air, it becomes moist and begins to dissolve within minutes. Other helpful identification features include very low hardness (2 on the Mohs scale), vitreous or resinous luster, and a bitter taste. Occurrence in paragenesis with other evaporite salts is also a strong indicator. ## Distinguishing from Similar Minerals Tachyhydrite can be confused with other soluble salt minerals, such as halite, sylvite, carnallite, or bischofite. - **Halite** and **sylvite** are not hygroscopic to such an extent and have excellent, cubic cleavage. - **Carnallite** and **bischofite** are also hygroscopic, but usually not as rapidly as tachyhydrite. Final differentiation requires chemical or crystallographic analysis. ## Crystal Forms Tachyhydrite crystals belong to the trigonal system and are extremely rare. They take the form of rhombohedra. It is much more commonly found in granular, fibrous aggregates or as massive, compact fillings.

Geological environment

## Genesis Tachyhydrite is an evaporite mineral. It forms in the final stages of seawater evaporation in closed sedimentary basins, under conditions of very high salinity and high concentrations of calcium and magnesium ions. It crystallizes from residual brines after the precipitation of halite and sylvite. ## Mineral Associations This mineral occurs in association with other evaporites. Its typical associated minerals are halite, carnallite, bischofite, sylvite, kainite, kieserite, and anhydrite. ## Localities The classic and first described locality of tachyhydrite is the potash salt deposits in the Stassfurt region of Saxony-Anhalt, Germany. Significant deposits and occurrences are also known from the Sergipe Basin in Brazil, from the vicinity of Pointe-Noire in Congo, as well as from Thailand and China. In all cases, these are deep-seated salt deposits.

Rarity

Rare

For collectors

## Quality Criteria For tachyhydrite, the main criterion for collector value is the mere existence of a stable, well-preserved specimen. Due to its nature, any specimen that has not dissolved is valuable. Extremely rare, well-formed crystals are highly sought after. The size and purity of the specimen are secondary to its state of preservation. ## Popular Localities Historically, the most important locality is Stassfurt, Germany, from which classic specimens originate. Currently, specimens from deep salt mines in Brazil (Sergipe basin) or Congo, where this mineral occurs in significant quantities but is rarely extracted in collector quality, may appear on the collector's market.

Care and storage

## Cleaning Tachyhydrite specimens must absolutely not be cleaned with water or any liquids, as this will cause their immediate and irreversible dissolution. Mechanical cleaning, e.g., with a brush, is theoretically possible but must be performed under extremely low humidity conditions (e.g., in a glove box). ## What to Avoid Any contact with moisture and air of standard humidity should be avoided. Even brief exposure to room conditions can initiate the deliquescence process. Heating is also not recommended, as it can lead to the loss of crystallization water and destruction of the mineral's structure. ## Storage The only effective storage methods involve complete isolation from moisture. The specimen must be kept in an hermetically sealed container (e.g., a sealed vial, a box with a gasket) along with a desiccant, such as fresh silica gel. Some collectors store tachyhydrite immersed in anhydrous paraffin oil.

External references

Sources

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