Selenojalpaite

Chemical formula: Ag₁+₃Cu₁+Se₂-₂

Selenojalpaite is a rare silver-copper selenide, forming tiny, metallic grains in association with other selenides.

## Characteristics Selenojalpaite is a rare mineral from the sulfide group, specifically a silver-copper selenide. It typically occurs as very fine, irregular grains or small aggregates embedded in other minerals. Its crystals, observed only under a microscope, have a tetragonal form. Visually, it is inconspicuous, possessing a metallic luster and a grayish-black color. ## Physical Properties This mineral is characterized by low hardness, approximately 2.5 on the Mohs scale, meaning it is soft and can be scratched with a fingernail. It has a relatively high density, typical for heavy metal minerals. It is opaque and exhibits a metallic luster. It is malleable and ductile. ## Colors and Varieties Selenojalpaite has a uniform, grayish-black color. On a fresh fracture, it may appear lighter, steel-gray, but it quickly tarnishes in air. There are no distinct color varieties or commercial forms. ## History and Name The mineral's name refers to its chemical composition (selenium) and its structural and chemical similarity to jalpaite, in which sulfur is replaced by selenium. It was first described in 1986 based on material from the Pacajake deposit in Bolivia. ## Uses Due to its extreme rarity, selenojalpaite has no industrial applications. It is solely an object of scientific and collecting interest for specialized collectors of rare minerals.

Properties

Mohs hardness: 2.5
Luster: Metallic
Streak: Black
Density: 7.08
Cleavage: None
Fracture: Uneven
Transparency: Opaque
Crystal system: Tetragonal

Diagnostic features

## Identification Amateur identification of selenojalpaite is practically impossible. The mineral occurs as fine, embedded grains with a grayish-black color and metallic luster. Definitive identification requires advanced analytical methods, such as X-ray microanalysis (EDS/WDS) to confirm chemical composition (presence of Ag, Cu, and Se) and X-ray diffraction (XRD) to determine crystal structure. ## Distinguishing from Similar Minerals It can be confused with many other fine-grained, gray ore minerals, such as alabandite, acanthite, chalcocite, and especially with other selenides (e.g., eucairite, berzelianite, umangite) or jalpaite. Differentiation without specialized equipment is impossible. ## Crystal Forms It forms very fine, irregular grains up to 0.2 mm in size. Poorly formed, prismatic crystals with a square cross-section, visible only under high magnification, are rarely observed.

Geological environment

## Genesis Selenojalpaite is a hydrothermal mineral. It forms at low to medium temperatures in ore veins rich in selenium, usually in association with other selenides of silver, copper, and lead. ## Mineral Associations It most commonly co-occurs with other selenides, such as umangite, eucairite, berzelianite, clausthalite, klockmannite, as well as with hematite, barite, calcite, and quartz. ## Localities The most important and well-documented selenojalpaite localities worldwide include: - Pacajake deposit in H. Dalence Province, Oruro Department, Bolivia (type locality). - Skrikerum mine in Valdemarsvik, Östergötland, Sweden. - Předbořice uranium deposit near Kovářov, Czech Republic. - Tumiñico mine in Sierra de Cacho, La Rioja Province, Argentina.

Rarity

Very rare

For collectors

## Quality Criteria The quality of a selenojalpaite specimen is determined not by its aesthetic appeal, but by the richness of its occurrence and the possibility of identification. The most valuable samples are those where the mineral grains are relatively large (for this species, meaning fractions of a millimeter) and occur in rich aggregates, preferably in association with other rare selenides. Analytical confirmation (e.g., an accompanying EDS result) significantly increases the value and credibility of the specimen. ## Popular Localities The most sought-after specimens by collectors come from the type locality, the Pacajake deposit in Bolivia, as well as from the classic selenide locality in Skrikerum, Sweden.

Care and storage

## Cleaning Due to its softness and rarity, mechanical cleaning is highly inadvisable. If absolutely necessary, compressed air can be used to remove dust. Contact with water and chemicals should be avoided. ## What to Avoid Avoid contact with acids and other chemicals that may damage it. The mineral is soft, so it must be protected from scratches and impacts. Prolonged exposure to humid air can cause its slow oxidation. ## Storage Selenojalpaite specimens, usually in the form of microscopic grains within the host rock, should be stored under stable, dry conditions. It is best to keep them in sealed "micromount" boxes to protect them from mechanical damage and dust.

Sources

IMA Mineral List (CNMNC)