Cadmoselite
Chemical formula: Cd<sup>2+</sup>Se<sup>2-</sup>
Cadmoselite is a rare cadmium selenide, forming black, metallic coatings and massive aggregates, found in the oxidation zones of uranium deposits.
Properties
- Mohs hardness
- 4
- Luster
- Metallic
- Streak
- Black
- Density
- 5.63
- Cleavage
- Perfect on {0001}
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Hexagonal
Diagnostic features
## Identification Identifying cadmoselite under amateur conditions is practically impossible. This mineral most often forms inconspicuous, black, metallic-lustered coatings on other minerals, especially uraninite. Identification requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis using an electron microprobe (EDS/WDS). ## Distinguishing from Similar Minerals It can be confused with many other black minerals with a metallic luster, such as uraninite, clausthalite, tiemannite, or other selenides. Differentiation without specialized equipment is not possible. Co-occurrence with uraninite in sandstones is an indicative, but not diagnostic, feature. ## Crystal Forms Macroscopic crystals are unknown. It occurs as microscopic, hexagonal plates and flakes, and most often forms very fine-grained, earthy, or compact aggregates and coatings.
Geological environment
## Genesis Cadmoselite is a mineral of low-temperature hydrothermal origin or forms through sedimentary processes. It forms under reducing conditions in sedimentary rocks, mainly in sandstones saturated with selenium- and cadmium-rich waters. Its formation is often associated with the presence of organic matter and uranium deposits. ## Mineral Associations It most often co-occurs with uraninite, clausthalite, tiemannite, pyrite, chalcopyrite, sphalerite, galena, as well as selenium minerals such as ferroselite and umangite. Calcite and organic matter (bitumens) are also often associated with it. ## Localities The most important localities worldwide include the uranium deposit in the Tuva region of Russia (type locality); the San Rafael Swell formation in Emery County, Utah, USA; the area around Předbořice in the Czech Republic; and the Niederschlema-Alberoda uranium deposit in Saxony, Germany.
Rarity
Very rare
For collectors
## Quality Criteria The collector appeal of cadmoselite is determined not by aesthetics, but by its extreme rarity and confirmed identity. The most prized specimens are those where cadmoselite forms visible to the naked eye, even if small, compact aggregates or thicker coatings on a well-defined associated mineral (e.g., on a calcite crystal). Reliable labeling with an analytically confirmed locality is crucial. ## Popular Localities From a collector's perspective, the best-known specimens, though still microscopic, come from the uranium deposits in the San Rafael Swell in Utah, USA. They are somewhat more accessible on the specialized market than material from the type locality in Russia.
Care and storage
## Cleaning Due to its fragility and occurrence as thin coatings, mechanical cleaning is highly inadvisable. If necessary, compressed air can be used to remove dust. Avoid contact with water and any chemicals. ## What to Avoid Cadmoselite is sensitive to acids. Avoid ultrasonics, steam cleaners, and sudden temperature changes. As a mineral containing cadmium and selenium, it is toxic – avoid inhaling dust and wash hands after any contact with a specimen. ## Storage Cadmoselite specimens should be stored under stable conditions, in a closed, dry "membrane box" container to prevent dusting and accidental damage. They should be protected from light and moisture. Store away from children and pets, with clear labeling regarding toxicity.