Wattevilleite

Chemical formula: Na₂Ca(S⁶⁺O₄)₂·₄H₂O

Wattevilleite is a rare, hydrated sodium and calcium sulfate, forming small, acicular crystals and efflorescences, typically found in salt mines.

## Characteristics Wattevilleite is a rare mineral from the sulfate group, chemically a hydrated sodium and calcium sulfate. It most often occurs as delicate, fibrous or acicular aggregates, forming efflorescences and crusts. Individual crystals are very small, elongated, and tend to form radial aggregates. Due to its fragility and water solubility, it is a difficult mineral to preserve in collections. ## Physical Properties This mineral is very soft and brittle. It is characterized by a vitreous luster. It is transparent to translucent. Its low hardness and perfect cleavage make it exceptionally delicate to the touch. ## Colors and Varieties Wattevilleite is typically colorless or white. No colored or commercial varieties are distinguished. ## History and Name The mineral was named in honor of Oscar de Watteville (1824-1901), a French director of science at the Ministry of Public Education. The mineral was first described in 1879 by Edouard Jannettaz based on samples from Hallstatt, Austria.

Properties

Mohs hardness: 2.5
Color: White
Luster: Vitreous
Streak: White
Density: 1.81
Cleavage: Perfect on {100}
Fracture: Uneven
Transparency: Translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification Characteristic features of wattevilleite include its occurrence as delicate, acicular or fibrous aggregates, vitreous luster, and its environment of formation – salt mines or evaporites. A key feature is its water solubility. ## Distinguishing from Similar Minerals It can be confused with other soluble sulfates such as mirabilite, thenardite, or gypsum. It is distinguished from gypsum by its significantly greater water solubility. Distinguishing it from other sodium sulfates often requires chemical analysis (XRD, EDS), as their physical properties are very similar. ## Crystal Forms It forms small, acicular or capillary crystals, often gathered into radial or tangled aggregates. It also occurs as crusts and efflorescences on other minerals.

Geological environment

## Genesis Wattevilleite is an evaporitic mineral. It forms by precipitation from solutions rich in sodium and calcium sulfates in drying salt basins. It is also found as a product of volcanic activity (fumaroles) or as a secondary mineral in mines. ## Mineral Associations It co-occurs with other evaporite minerals such as halite, thenardite, blödite, syngenite, gypsum, and mirabilite. ## Localities The most important localities for this mineral are Hallstatt, Austria (type locality), the Bex salt mine in Switzerland, and also some mines in Germany (e.g., in Lower Saxony). It is also known from several locations in the USA, including San Bernardino County, California.

Rarity

Rare

For collectors

## Quality Criteria The quality of a wattevilleite specimen primarily depends on the richness and aesthetics of its crystallization. Specimens with well-formed, abundant aggregates of acicular crystals, forming fluffy masses, are most valued. Association with other minerals can also enhance the visual appeal of the specimen. Due to its rarity, any well-preserved specimen has collector's value.

Care and storage

## Cleaning The mineral is water-soluble, so it must absolutely not be cleaned wet. To remove dust, a very soft brush or compressed air from a distance can be used very carefully. ## What to Avoid Avoid contact with water and any liquids that will dissolve it. It is sensitive to atmospheric moisture, which can lead to its slow degradation. It should be protected from temperature changes and mechanical damage. ## Storage Wattevilleite specimens must be stored in dry conditions. It is best to keep them in tightly sealed containers (e.g., "membrane boxes") or in display cases with a moisture absorber (silica gel).

External references

https://www.mindat.org/min-4249.html