Hexahydrite
Chemical formula: MgS<sup>6+</sup>O<sub>4</sub>·6H<sub>2</sub>O
Hexahydrite is a hydrated magnesium sulfate, forming white, fibrous efflorescences and crusts in dry environments, such as mines and caves.
Properties
- Mohs hardness
- 2-2.5
- Luster
- Vitreous, silky in fibrous aggregates
- Streak
- White
- Density
- 1.757
- Cleavage
- Perfect on {100}, good on {010}
- Fracture
- Conchoidal
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification The primary diagnostic feature of hexahydrite is its occurrence as white, fibrous efflorescences in dry places, such as mine walls or caves. Its low hardness, lightness, and water solubility are also characteristic. It has a salty-bitter taste. ## Distinguishing from Similar Minerals It is most easily confused with epsomite (MgSO₄·7H₂O), which is very similar visually and chemically. Hexahydrite crystallizes in the monoclinic system, while epsomite crystallizes in the orthorhombic system. In practice, differentiation without specialized tests (e.g., XRD) is very difficult. It can also be confused with other secondary sulfates, such as gypsum or mirabilite, but these differ in chemical composition and often crystal form. ## Crystal Forms Crystals are rare and appear as small needles or flattened tablets. Most commonly found are fibrous, hair-like, radial aggregates, as well as crusts and coatings.
Geological environment
## Genesis Hexahydrite is a secondary mineral, formed by the dehydration of epsomite under conditions of low air humidity. It forms as an evaporation product in drying salt lakes and deserts. It is also found as an efflorescence on mine walls (especially coal and sulfide mines) and caves, where it forms as a result of weathering of magnesium-rich rocks. ## Mineral Associations It most commonly co-occurs with epsomite, from which it directly forms. Other sulfates such as leonhardite, mirabilite, blödite, gypsum, and also halite may accompany it. ## Localities Key localities worldwide include the areas around Bonaparte and Clinton Lakes in British Columbia (Canada), where it was discovered. It also occurs in mines in the Dubník region in Slovakia, in Vesuvius in Italy, in coal mines in the Ruhr Basin (Germany), and in dry regions of Chile (Atacama Desert) and the United States (California, Nevada).
Rarity
Not very common
For collectors
## Quality Criteria For collectors, the most valuable hexahydrite specimens are well-formed, though always small, crystals. Rich, pure white fibrous aggregates covering a piece of matrix rock are also prized. Due to the mineral's instability, it is crucial that the specimen is well-preserved and shows no signs of dissolution or dehydration. Specimens with accurately documented localities are more highly valued. ## Popular Localities The most classic and highly valued specimens by collectors come from the original localities in British Columbia, Canada. Samples from Dubník in Slovakia and from the Italian volcano Vesuvius are also sought after.
Care and storage
## Cleaning Hexahydrite specimens should not be cleaned wet, as this mineral is water-soluble. To remove dust, a very soft brush can be used, or contaminants can be carefully blown off with compressed air from a safe distance. ## What to Avoid Contact with water and other liquids must be strictly avoided. The mineral is hygroscopic, so it should be protected from moist air, which can cause it to dissolve or transform into epsomite. High temperatures, which accelerate dehydration, should also be avoided. ## Storage Hexahydrite specimens require storage in dry conditions. It is best to place them in a sealed, airtight container (e.g., a "perky box") along with a moisture-absorbing agent, such as silica gel. They should be protected from mechanical damage due to their high brittleness.