Selenium

Chemical formula: Se

Native selenium is a chemical element found in nature as rare, metallic gray or red crystals and coatings.

## Characteristics Native selenium is a naturally occurring, crystalline form of the element selenium. It forms small, acicular or capillary crystals, as well as coatings and granular aggregates. Its appearance varies – most commonly, forms with a metallic, gray luster are found, but under certain conditions, it forms a red, partially amorphous variety that eventually recrystallizes into the more stable, gray form. Specimens are usually small and brittle. ## Physical Properties Selenium is a soft mineral, with a hardness of 2 on the Mohs scale, meaning it can be scratched with a fingernail. It has a relatively high density of approximately 4.8 g/cm³. It exhibits a metallic luster and is opaque. It shows good cleavage in one direction. ## Colors and Varieties The most stable and common crystalline form is metallic gray to grayish-black. It also occurs in reddish hues, especially in forms that have rapidly precipitated from solutions or vapors. Red selenium is unstable and over time transforms into the gray trigonal variety. ## History and Name As an element, selenium was discovered in 1817 by Jöns Jacob Berzelius, who named it after the Greek word *selene* (σελήνη), meaning Moon, due to its chemical similarity to tellurium (named after the Latin *tellus* - Earth). As a mineral, it was formally described much later. ## Uses Native selenium is primarily of scientific and collector's interest. The element selenium, however, is an important semiconductor used in electronics, photocells, xerography, and as an additive in glass production and dietary supplements.

Properties

Mohs hardness: 2
Luster: Metallic
Streak: red
Density: 4.8
Cleavage: Good on <mi>{01_12}</mi>
Fracture: Uneven
Transparency: Opaque
Crystal system: Trigonal

Diagnostic features

## Identification Native selenium can be identified by its characteristic red streak, metallic luster (in the gray variety), and low hardness. It often forms small, acicular crystals or coatings. An important diagnostic feature, though requiring caution, is its behavior when heated – it melts at a low temperature and emits vapors with a sharp, unpleasant odor. ## Distinguishing from similar minerals Gray native selenium can be confused with other metallic minerals, such as stibnite or bismuthinite. It is distinguished from them by its red streak (stibnite has a grayish-black streak) and lower hardness. Red varieties may resemble cinnabar, but cinnabar is much denser and has a more intense red color. ## Crystal Forms It crystallizes in the trigonal system. Most often, it forms very fine, acicular or capillary crystals. It also occurs as coatings, crusts, granular aggregates, and spherical aggregates.

Geological environment

## Genesis Native selenium is a mineral of hydrothermal origin. It forms as a result of the condensation of volcanic vapors (fumaroles), where it precipitates directly from volcanic gases. It is also found in the oxidation zones of some ore deposits, especially those rich in selenides. It can also form in sedimentary environments under reducing conditions, often in association with uranium. ## Mineral Associations It often co-occurs with native sulfur, cinnabar, pyrite, marcasite, and various selenides, such as clausthalite or tiemannite. In volcanic environments, it is accompanied by sal ammoniac and other exhalation products. ## Localities Significant occurrences of native selenium are associated with volcanic activity. It is found in the fumaroles of Vesuvius and Etna in Italy. It also occurs in uranium deposits, e.g., in the United Verde mine in Jerome, Arizona (USA), from which well-formed crystals originate. Other known localities include the Pacajake mine in Bolivia, the vicinity of Předbořice in the Czech Republic, and Kladno in the Czech Republic, where it was found on burning mine dumps.

Rarity

Rare

For collectors

## Quality Criteria Specimens with well-formed, sharp crystals are most valued by collectors, which is a great rarity. An intense, metallic gray color and luster enhance its attractiveness. A contrasting combination with associated minerals, such as yellow sulfur or white calcite, is also desirable. Due to the small size of the crystals, micromount specimens are the most common form in collections. ## Popular Localities The best localities for native selenium are considered to be in the USA, especially the United Verde mine in Arizona, from which historical finds of crystals large for this mineral originated. Specimens from the Bolivian Pacajake mine are also valued. Classic, though usually microscopic, are specimens from the fumaroles of Vesuvius.

Care and storage

## Cleaning Selenium specimens are very delicate and brittle. They should only be cleaned using compressed air to remove dust. Any contact with water or chemicals is not recommended. ## What to avoid Avoid contact with water and chemical agents. Selenium is sensitive to elevated temperatures – it should not be heated, as it melts easily (at 221°C) and vaporizes, producing toxic fumes with a characteristic smell of decaying radish. It should be protected from shocks and impacts due to its high brittleness. ## Storage Native selenium specimens are best stored in closed, padded boxes or display cases to protect them from mechanical damage and dust. Due to the potential toxicity of its dust, it is recommended to keep it away from children and pets and to wash hands after handling a specimen.