Carobbiite

Chemical formula: KF

Synthetic potassium fluoride (KF), rarely occurring in nature as a product of volcanic sublimation.

## Characteristics Carobbite is a natural, crystalline form of potassium fluoride. It occurs extremely rarely, forming colorless or white, small cubic crystals or coatings. It typically forms as a product of volcanic gas sublimation in fumaroles. Due to its excellent solubility in water, it is an unstable mineral in humid environments and rarely preserved under natural conditions. ## Physical Properties Carobbite crystals exhibit a vitreous luster. This mineral is soft, with a Mohs hardness of 2.5. It is light, with a density similar to that of halite. Its most characteristic feature is its easy solubility in water and its salty, pungent taste. ## Colors and Varieties Carobbite is typically colorless or white. No colored or commercial varieties are distinguished. ## History and Name The mineral was named in honor of Guido Carobbi (1900-1983), an Italian professor of mineralogy and geochemistry at the University of Florence. It was first described in 1956 based on material found in eruption products of Vesuvius in Italy. ## Uses Natural carobbite has no industrial application due to its extreme rarity. It is solely an object of scientific and collecting interest. Its synthetic counterpart, potassium fluoride, is used in chemistry and metallurgy.

Properties

Mohs hardness: 2-2.5
Color: Colourless
Luster: Vitreous
Streak: White
Density: 2.505
Cleavage: On {001}.
Fracture: Conchoidal
Transparency: Translucent
Crystal system: Isometric

Diagnostic features

## Identification The key diagnostic feature of carobbite is its immediate and complete solubility in water. It occurs as small, colorless or white cubes with a vitreous luster, directly in the vicinity of volcanic fumaroles. Reaction to moisture (e.g., from breath) can cause the surface to become dull. ## Distinguishing from Similar Minerals Carobbite can be confused with halite (NaCl) or sylvite (KCl), which form similar cubic crystals and occur in similar environments. Halite and sylvite are also water-soluble, but carobbite dissolves much more easily. Final differentiation requires chemical analysis to confirm the presence of fluorine instead of chlorine. ## Crystal Forms Carobbite crystallizes in the isometric system, forming simple cubes {100}. It occurs as single, small crystals, as well as in granular aggregates and coatings on volcanic rocks.

Geological environment

## Genesis Carobbite is a mineral of volcanic origin. It forms by direct sublimation from hot gases (exhalations) in active fumaroles, at temperatures ranging from 100-200°C. It crystallizes on the surface of lava or volcanic tuff. ## Mineral Associations This mineral co-occurs with other volcanic sublimation products, such as sylvite (KCl), halite (NaCl), sal ammoniac (NH₄Cl), and malladrite (Na₂SiF₆). ## Localities The only confirmed and well-described locality, which is also the type locality, is Mount Vesuvius in Campania, Italy. Its occurrence has also been reported at the Hekla volcano in Iceland, but this requires further verification.

Rarity

Extremely rare

For collectors

## Quality Criteria The collector appeal of carobbite primarily depends on the presence of well-formed, sharp crystals with a visible cubic habit. Due to its extreme rarity, any confirmed specimen, even in the form of a coating, is valuable. Contrast with dark matrix rock (lava) enhances aesthetic qualities. However, the most important aspects are certainty of identification and accurate documentation of provenance. ## Popular Localities The only source of collector specimens is the historical type locality – Vesuvius in Italy. New finds are practically impossible to acquire, and historical specimens are mainly found in institutional collections and old private collections.

Care and storage

## Cleaning Carobbite is extremely soluble in water. **Never clean it with water or any liquids.** Mechanical cleaning is also risky due to its softness. The safest way to remove dust is with a soft, dry brush or a photographic blower. ## What to Avoid Contact with water, water vapor, and high humidity must be strictly avoided, as these can completely dissolve the specimen. It should also be protected from all chemicals. It is a soft mineral, susceptible to scratches and mechanical damage. ## Storage Carobbite specimens must be stored in airtight, sealed containers (e.g., "perky box" type) with a desiccant (e.g., silica gel). Store in a dry place, at a constant room temperature, away from sources of moisture.