Ferroselite

Chemical formula: FeSe<sub>2</sub>

Ferroselite is a rare iron selenide, forming metallic-lustrous, tin-white to reddish granular aggregates and radial clusters.

## Characteristics Ferroselite is an iron selenide belonging to the marcasite group. It occurs rarely, most often in the form of granular or massive aggregates. It has also been observed as radial or columnar clusters. Its crystals, though extremely rare, are tabular or prismatic. Fresh surfaces of the mineral are characterized by a metallic luster and a color ranging from tin-white to reddish, but they dull and darken over time. ## Physical Properties Ferroselite is a brittle mineral with a Mohs hardness of 6 to 6.5. Its density is significant, approximately 7.2 g/cm³. The luster is distinctly metallic, and the mineral is completely opaque. It exhibits weak cleavage. ## Colors and Varieties This mineral does not form color varieties. Its typical color on a fresh fracture is tin-white, steel-gray, often with a reddish tint. Upon exposure to air, its surface dulls and may become covered with a darker tarnish. ## History and Name The name ferroselite refers to its chemical composition – its content of iron (Latin: *ferrum*) and selenium (*selenium*). It was first described in 1947 by J. W. Earley based on material from a uranium-vanadium deposit in Montrose County, Colorado, USA. ## Uses Due to its rarity, ferroselite has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals and for scientists.

Properties

Mohs hardness
6-6.5
Luster
Metallic
Streak
Black
Density
7.2
Cleavage
Poor on {101}
Fracture
Uneven
Transparency
Opaque
Crystal system
Orthorhombic

Diagnostic features

## Identification Ferroselite can be identified by its metallic luster, relatively high hardness and density, and characteristic tin-white or reddish color on a fresh fracture. An important feature is its tendency to dull in air. It occurs in a specific geological environment. ## Distinguishing from similar minerals It can be confused with other metallic sulfides, arsenides, or selenides, such as pyrite, marcasite, arsenopyrite, or löllingite. It differs from pyrite by its color (pyrite is brass-yellow) and crystal habit. It differs from marcasite by a slightly different color and stability (marcasite is more prone to decomposition). Accurate identification usually requires chemical analysis (EDS) or X-ray diffraction (XRD). ## Crystal forms Ferroselite crystals are extremely rare and appear as short prisms or tabular forms. It typically forms granular, massive aggregates or radial and columnar clusters embedded in the host rock.

Geological environment

## Genesis Ferroselite is a mineral of hydrothermal origin. It forms under reducing conditions, at low to medium temperatures. It is most commonly found in uranium-vanadium and selenium deposits hosted in sandstones. ## Mineral associations It often co-occurs with other selenides, such as clausthalite, eucairite, umangite, berzelianite, and klockmannite. It is also associated with uraninite, calcite, barite, and copper and iron sulfides. ## Localities The most important ferroselite localities worldwide include deposits on the Colorado Plateau in the USA (Colorado and Utah), where it was discovered. It is also known from Předbořice in the Czech Republic, the Trogtal deposit in the Harz Mountains in Germany, the Shinkolobwe mine in the Democratic Republic of Congo, and a deposit near Lake Athabasca in Saskatchewan, Canada.

Rarity

Rare

For collectors

## Quality criteria The most prized specimens by collectors are those with well-formed, even microscopic, crystals, which are extremely rare. Rich aggregates with a distinct radial structure or pure, massive portions of the mineral with a visible fresh, metallic surface are also highly valued. Association with other rare selenides is also important. ## Popular localities Classic and most sought-after specimens come from historical localities on the Colorado Plateau in the USA. Material from the Czech Republic and Germany is also valued in specialized collections.

Care and storage

## Cleaning Ferroselite specimens should only be dry-cleaned, using a soft brush to remove dust. Contact with water, and especially with chemicals, can accelerate the oxidation process and damage the mineral's surface. ## What to avoid Contact with water, acids, and other chemical agents must be strictly avoided. Ferroselite is sensitive to moisture and atmospheric oxygen, which cause it to dull and form dark tarnishes. It should not be heated or exposed to rapid temperature changes. ## Storage It is recommended to store specimens in airtight, dust-free containers (e.g., membrane boxes) or in dry cabinets with a desiccant to limit contact with air and moisture. Exposure to open air leads to gradual degradation of the mineral's surface.

External references

Sources

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