Chlorocalcite

Chemical formula: KCaCl₃

Potassium calcium chloride, occurring mainly as coatings and ephemeral efflorescences in volcanic environments.

## Characteristics Chlorocalcite is a rare halide, a potassium calcium chloride, forming primarily as a sublimation product in volcanic exhalations. It typically occurs as fine, colorless or white coatings, crusts, and efflorescences on lava. Less commonly, it is observed as small, tabular or pseudo-cubic crystals, which due to their nature are extremely delicate and ephemeral. ## Physical Properties This mineral is very soft, with a Mohs hardness of 2.5-3. It is hygroscopic, meaning it readily absorbs moisture from the air, leading to its dissolution. Crystals, if visible, exhibit a vitreous luster and are transparent. ## Colors and Varieties Chlorocalcite is most often colorless or white. Specimens from Vesuvius can be colored violet, which is a characteristic feature of this locality. No commercial or colored varieties are distinguished. ## History and Name The mineral's name, given in 1872, refers to its chemical composition – the presence of chlorine (chloro) and calcium (calcite). It was first described based on samples from Mount Vesuvius in Italy, which remains its type locality. ## Uses Chlorocalcite has no industrial applications. Its significance is purely scientific and as a collector's item, as a rare and specific product of volcanic activity.

Properties

Mohs hardness: 2.5-3
Luster: Vitreous
Density: 0
Cleavage: Cube-like in two directions, one better than the other.
Transparency: Transparent
Crystal system: Orthorhombic

Diagnostic features

## Identification Its occurrence as fragile coatings and small crystals near volcanic exhalations is a strong indicator. A key diagnostic feature is its extreme hygroscopicity – the mineral "sweats" and dissolves under the influence of atmospheric moisture. It reacts violently with a drop of water. ## Distinguishing from Similar Minerals It can be confused with other white, soluble volcanic halides, such as sylvite (KCl) or halite (NaCl). Sylvite has a bitter taste, and halite is salty, but taste testing is a highly unrecommended and dangerous method. Accurate differentiation usually requires chemical analysis. ## Crystal Forms It forms small, tabular crystals with a pseudo-cubic outline. However, it most often occurs as earthy, fibrous, or powdery coatings and crusts on volcanic rocks.

Geological environment

## Genesis Chlorocalcite is a mineral typical of volcanic environments. It forms by sublimation from hot volcanic gases (exhalations), depositing near fumarole vents on the lava surface. It is a high-temperature mineral, crystallizing directly from the gas phase. ## Mineral Associations It co-occurs with other halides and sulfates formed under similar conditions, such as halite (NaCl), sylvite (KCl), and also hematite (Fe₂O₃), which often forms in the vicinity of fumaroles. ## Localities The most important and historical locality is the Vesuvius-Somma volcanic complex in Italy. It has also been reported from active volcanoes, such as Tolbachik in Kamchatka (Russia), where well-formed crystals have been found.

Rarity

Rare

For collectors

## Quality Criteria For collectors, specimens with visible, well-formed crystals are most desirable, which is extremely rare. The abundance of the coating on a piece of host rock (matrix) and the intensity of the violet coloration in the case of Vesuvius specimens are also important. Due to the mineral's instability, the condition of the specimen is crucial. ## Popular Localities By far the most prized specimens come from the classic locality – Vesuvius in Italy. In recent years, interesting examples from the Tolbachik volcano in Kamchatka have also appeared on the market, often characterized by better crystal development.

Care and storage

## Cleaning Chlorocalcite specimens should generally not be cleaned. Contact with water, even distilled, will cause immediate dissolution. Dust can be removed very carefully using a soft brush or a stream of dry air. ## What to Avoid Contact with water and any moisture must be absolutely avoided. The mineral is highly hygroscopic and deliquescent – it absorbs water vapor from the surroundings and dissolves. It must be protected from changes in temperature and humidity. ## Storage Chlorocalcite requires storage in hermetic conditions. The best solution is a sealed container (e.g., a "perky box") with a desiccant (silica gel). Exposure to open air, especially in a humid climate, leads to rapid degradation of the specimen.