Bellidoite

Chemical formula: Cu<sub>2</sub>Se

Bellidoite is a rare copper selenide with a metallic luster, found in association with other selenides in the oxidation zones of uranium deposits.

## Characteristics Bellidoite is a rare mineral from the sulfide group, specifically a copper selenide. It occurs as small, anhedral (lacking crystal faces) grains, usually less than 0.5 mm in size, often forming rims around other minerals such as umangite. Its appearance is typically metallic, with a dark gray to black color and a characteristic luster. ## Physical Properties This mineral is characterized by low hardness, approximately 2 on the Mohs scale, meaning it is very soft. It has a metallic luster and is opaque. Its density is quite high, which is typical for heavy metal selenides. ## Colors and Varieties Bellidoite has a uniform, dark gray to black color. In reflected light under a microscope, its color is described as gray with a reddish or purplish tint. No color varieties or commercial varieties are known. ## History and Name The mineral was discovered in the Habří uranium mine, near the town of the same name in the Central Bohemian Region of the Czech Republic. It was described and approved as a new mineral in 1974. Its name honors Eleodoro Bellido Bravo (1920-2011), a Peruvian mining engineer and geologist, who was the director of the Peruvian Geological and Mining Service (Servicio de Geología y Minería del Perú).

Properties

Mohs hardness
2
Luster
Metallic
Streak
Black
Density
6.86
Cleavage
None
Fracture
Uneven
Transparency
Opaque
Crystal system
Tetragonal

Diagnostic features

## Identification Identification of bellidoite requires advanced laboratory techniques, such as microscopic analysis in reflected light and chemical analysis (e.g., using an electron microprobe - EDS/WDS) to confirm its composition (copper selenide). Its low hardness, metallic luster, and co-occurrence with other selenides are indicators, but do not allow for certain visual identification. ## Differentiation from Similar Minerals It can be confused with other dark, metallic selenides, such as klockmannite (CuSe) or umangite (Cu₃Se₂). Differentiation is primarily possible based on chemical analysis, which reveals a specific copper to selenium ratio (2:1 for bellidoite). In reflected light, it also shows slightly different hues than associated minerals. ## Crystal Forms Bellidoite forms irregular, anhedral grains and aggregates. It often occurs as thin rims or coatings on umangite crystals. No well-formed, euhedral crystals have been observed.

Geological environment

## Genesis Bellidoite is a hydrothermal mineral. It forms at low to medium temperatures in calcite veins cutting uranium-selenium deposits. It forms in oxidation zones, often as a result of alterations of other, earlier copper selenides. ## Mineral Associations This mineral co-occurs with other selenides, such as umangite, berzelianite, klockmannite, eskebornite, tyrrellite, as well as with uraninite, hematite, and calcite. ## Localities The most important and type locality (localitas typica) is the Habří uranium mine in the Czech Republic. Other confirmed localities include the Tumiñico deposit in Sierra de Cacho, Argentina, and the El Sharana mine in the Northern Territory, Australia.

Rarity

Very rare

For collectors

## Quality Criteria As a microscopic mineral, bellidoite is not evaluated according to typical criteria such as size or color. For collectors specializing in rare minerals (so-called "systematists") or "micromount" specimens, the greatest value lies in a specimen with analytically confirmed presence of bellidoite, preferably rich in this mineral and clearly visible under a microscope. Accurate documentation and provenance from one of the known localities are crucial. ## Popular Localities The most sought-after specimens come from the type locality - the Habří mine in the Czech Republic, as well as from the Tumiñico deposit in Argentina, which yields rich selenide associations.

Care and storage

## Cleaning Due to its extreme rarity and delicacy, bellidoite specimens should only be cleaned by specialists. They usually do not require cleaning, and any mechanical intervention (even with a soft brush) can damage the fine grains of the mineral. ## What to Avoid Avoid contact with chemicals, acids, and bases. The mineral is soft and susceptible to scratches. As a selenide, it may react in contact with certain chemical substances. It should be protected from moisture and sudden temperature changes. ## Storage Bellidoite specimens, usually in the form of microscopic grains within the host rock, should be stored under stable conditions, in sealed "micromount" containers, to protect them from dust, mechanical damage, and humidity changes. Avoid exposure to direct sunlight.

Sources

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