Allochalcoselite

Chemical formula: Cu₁+Cu₂+₅Pb₂+O₂(Se₄+O₃)₂Cl₅

Allochalcoselite is an extremely rare copper and lead selenite, forming microscopic, black crystals in volcanic fumarole environments.

## Characteristics Allochalcoselite is a complex selenite, chloride, and oxide of copper and lead. It occurs as very small, tabular or prismatic crystals, rarely exceeding 0.2 mm in length. It typically forms radiating aggregates, rosettes, or druses covering the rock surface. Its characteristic features are its black color and strong metallic luster. ## Physical Properties This mineral is opaque. Due to the microscopic size of its crystals, many of its physical properties, such as hardness or fracture, have not been thoroughly investigated. The density calculated based on its chemical composition and crystal structure is approximately 5.51 g/cm³. ## Colors and Varieties Allochalcoselite is monochromatic – it occurs exclusively in black. No color varieties or commercial forms are known. ## History and Name The mineral's name refers to its chemical and structural similarity to chalcoselite and the Greek word "allos" meaning "other," emphasizing that it is a distinct species. It was officially recognized by the International Mineralogical Association (IMA) in 2012. The type locality and only known occurrence is the fumaroles of Tolbachik volcano in Kamchatka, Russia. ## Applications Allochalcoselite has no industrial applications. Its significance is purely scientific and collectible, being an object of desire for specialists collecting rare minerals, microminerals, and specimens from specific localities (so-called "locality collecting").

Properties

Luster: Metallic
Streak: Black
Density: 5.51
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Identification of allochalcoselite is primarily based on its unique occurrence environment – it is a product of high-temperature volcanic exhalations. In the field or collection, it is recognized by its black color, metallic luster, and form of small, radiating crystalline aggregates. A key indicator is the locality – origin from the fumaroles of Tolbachik volcano. ## Differentiation from Similar Minerals It can be confused with other black, metallic minerals found in the same environment, such as tenorite. Due to the microscopic size of the crystals, definitive differentiation from similar, rare selenites (e.g., ilinskite) is only possible using advanced analytical methods, such as X-ray diffraction (XRD) or chemical microanalysis (EDS/WDS). ## Crystal Forms Crystals are most often tabular or short-prismatic. They form characteristic, radiating or star-shaped aggregates, as well as druses and crusts on porous volcanic substrate.

Geological environment

## Genesis Allochalcoselite is a fumarolic mineral. It crystallizes directly from hot volcanic gases (sublimates) at temperatures above 450°C. Its formation is associated with chemical reactions occurring in exhalations rich in copper, lead, selenium, and chlorine. ## Mineral Associations This mineral coexists with other rare products of volcanic sublimation. Its most common associations include: sophiite, ilinskite, burnsite, tenorite, hematite, and cotunnite. ## Localities The only confirmed locality of allochalcoselite in the world is its type locality: the "Arsenatnaya" and "Yadovitaya" fumaroles on the second cinder cone of the North Breakthrough of the Great Fissure Eruption of Tolbachik (1975-1976), Tolbachik volcano, Kamchatka, Russia.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of allochalcoselite specimens, being a typical micromineral, is assessed by different criteria than for larger minerals. Specimens with sharp, well-formed, and undamaged crystals forming aesthetic, radiating aggregates are most highly valued. The contrast with the matrix and the presence of other rare, well-identified associated minerals on the same rock fragment are also of great importance. ## Popular Localities The only source of collectible specimens is the Tolbachik volcanic complex in Kamchatka. Specimens from this locality are highly prized by specialized collectors due to their extreme rarity and unique origin.

Care and storage

## Cleaning Allochalcoselite specimens are extremely delicate and small. Wet cleaning is absolutely not recommended. To remove dust, only a photographic air blower or a can of compressed air should be used, applied from a distance and at low pressure. Any mechanical contact, even with a soft brush, risks irreversible damage to the microcrystals. ## What to Avoid As a fumarolic mineral, allochalcoselite may be sensitive to changes in humidity and temperature. Contact with any chemicals, acids, and detergents should be avoided. It must not be heated or exposed to direct sunlight. ## Storage Specimens should be stored in stable conditions, preferably in tightly sealed "micromount" boxes, which protect against dust, humidity, and mechanical damage. Display should be in closed display cases, away from sources of vibration.

Sources

IMA Mineral List (CNMNC)