Mercallite

Chemical formula: KHS<sup>6+</sup>O<sub>4</sub>

Mercallite is a rare, water-soluble potassium sulfate, forming tiny, colorless crystals and efflorescences in volcanic environments.

## Characteristics Mercallite is a potassium bisulfate, occurring as very small, tabular or acicular crystals, rarely exceeding 1 mm in length. It typically forms loose aggregates, crusts, and efflorescences on lava surfaces or in fumaroles. Due to its water solubility, it is an unstable mineral, found only in dry conditions. ## Physical Properties Mercallite crystals are very small and brittle. The mineral has a vitreous to pearly luster. It is completely water-soluble, which is its key physical and chemical property. ## Colors and Varieties Mercallite is colorless to white. No colored varieties are known. ## History and Name The mineral was first described in 1935 by Ferruccio Zambonini. The name comes from Giuseppe Mercalli (1850-1914), an Italian volcanologist and seismologist, director of the Vesuvius Observatory, and creator of the earthquake intensity scale. ## Uses Mercallite has no industrial application. It is solely an object of scientific and collecting interest.

Properties

Mohs hardness
2-2.5
Luster
Vitreous
Streak
White
Density
2.31
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Transparent
Crystal system
Orthorhombic

Diagnostic features

## Identification The key diagnostic feature of mercallite is its complete and rapid solubility in water. It occurs in a characteristic environment – as a product of volcanic exhalations. It forms very fine, colorless crystals in the form of coatings and crusts. ## Distinguishing from Similar Minerals It can be confused with other soluble sulfates and halides formed in fumaroles. The solubility test in a drop of water is the simplest distinguishing method. However, precise identification requires advanced methods, such as X-ray diffraction (XRD), due to the visual similarity of many fumarole products. ## Crystal Forms Mercallite forms very small, tabular crystals with a rhombic outline or elongated acicular crystals. It most often occurs as fine-grained, loose aggregates, coatings, and thin crusts.

Geological environment

## Genesis Mercallite is a secondary mineral, a product of volcanic activity. It forms by sublimation from hot volcanic gases (exhalations) in fumaroles, at temperatures from 100 to 250°C. It crystallizes directly from the gas phase on the surface of volcanic rocks and lava. ## Mineral Associations It co-occurs with other fumarole minerals such as misenite, native sulfur, sal ammoniac, mascagnite, and thenardite. ## Localities The type and main locality for mercallite are the active fumaroles of Mount Vesuvius in Italy, where it was first described. This is the only confirmed and well-documented occurrence of this mineral in the world.

Rarity

Very rare

For collectors

## Quality Criteria For mercallite, the most important criterion is the abundance and richness of its occurrence on a fragment of the host rock (matrix). Specimens with visible, though still very small, crystalline aggregates, rather than just a thin coating, are sought after. Due to its instability, a well-preserved, dry specimen is already valuable. ## Popular Localities The only source of collector specimens is Vesuvius in Italy. Specimens from historical collections from this locality are particularly prized.

Care and storage

## Cleaning The mineral must absolutely not be cleaned with water, as it dissolves instantly. Specimens can only be cleaned with compressed air or very carefully with a soft brush. ## What to Avoid Avoid contact with water, water vapor, and any moisture. Storage in a humid environment leads to the complete destruction of the specimen. Ultrasonic cleaners and all chemicals should also be avoided. ## Storage Mercallite specimens must be stored in airtight, sealed containers or "perky boxes," preferably with a desiccant (e.g., silica gel). They should be kept away from sources of moisture and sudden temperature changes.

Sources

Read more