Freboldite

Chemical formula: CoSe

Freboldite is a rare cobalt selenide, forming hexagonal crystals and granular aggregates with a metallic luster.

## Characteristics Freboldite is a rare mineral from the sulfide group, specifically a cobalt selenide. It occurs as small, hexagonal, tabular crystals, but most often forms granular or massive aggregates in association with other selenides. Its appearance is typically metallic, with a characteristic pinkish-violet or light pink hue on a fresh surface, which dulls over time. ## Physical Properties This mineral is characterized by a distinct metallic luster. It is opaque and relatively dense. Its hardness on the Mohs scale is approximately 2-2.5, making it a soft mineral, susceptible to scratching. ## Colors and Varieties Freboldite has a characteristic pinkish-violet, light pink, or brownish-pink color. No distinct varieties have been identified. ## History and Name The mineral's name honors the German geologist and paleontologist Georg Frebold (1891-1961), a professor at the University of Greifswald. Freboldite was first described in 1957 based on samples from the Trogtal and Eskeborner Stollen mines in the Harz Mountains, Germany, which remain its type locality. ## Applications Due to its extreme rarity, freboldite has no industrial applications. It is solely an object of scientific and collecting interest for specialized collectors of rare minerals.

Properties

Mohs hardness
2-2.5
Luster
Metallic
Streak
Black
Density
7.26
Cleavage
Good on {0001}
Fracture
Uneven
Transparency
Opaque
Crystal system
Hexagonal

Diagnostic features

## Identification Key diagnostic features of freboldite include its unique pinkish-violet color, metallic luster, and co-occurrence with other selenides. It is a soft mineral, which can be tested with a needle (it will leave a scratch). However, identification most often requires advanced methods, such as chemical composition analysis (EDS) or X-ray diffraction (XRD). ## Distinguishing from Similar Minerals Freboldite can be confused with other rare selenides of similar color, such as bornhardtite or trogtalite. Bornhardtite is usually more brownish, and trogtalite has a slightly different hue. Definitive differentiation is possible almost exclusively based on chemical analysis. It is distinguished from the much more common cobaltite by the lack of regular crystal forms and lower hardness. ## Crystal Forms Freboldite crystals are extremely rare. They take the form of small, hexagonal tablets. It occurs much more frequently as fine-grained, massive aggregates or as inclusions in other minerals, mainly clausthalite.

Geological environment

## Genesis Freboldite is a hydrothermal mineral. It forms at low to medium temperatures in ore veins rich in selenium. It crystallizes in a reducing environment, in the presence of cobalt and selenium, often replacing previously formed cobalt minerals. ## Mineral Associations This mineral almost always occurs in association with other selenides. Its most common associated minerals are clausthalite (PbSe), umangite (Cu₃Se₂), klockmannite (CuSe), berzelianite (Cu₂Se), trogtalite (CoSe₂), bornhardtite (Co₃Se₄), as well as hematite and calcite. ## Localities Freboldite is an extremely rare mineral, known from only a few localities worldwide. The most important occurrences, including the type locality, are in the St. Andreasberg area in the Harz Mountains, Germany (Trogtal, Eskeborn mines). Outside of Germany, its occurrences have been reported in Argentina (Sierra de Cacho, La Rioja), Bolivia (El Dragón mine, Potosí), and the Democratic Republic of Congo (Katanga Province).

Rarity

Extremely rare

For collectors

## Quality Criteria The most highly prized freboldite specimens are those that exhibit clear, rich aggregates of the mineral with an intense, pinkish-violet color, contrasting with the surrounding rock matrix (most often dark clausthalite). Due to the rarity of well-formed crystals, even microscopic but distinct hexagonal tablets are considered exceptional. The size and richness of the aggregate are key factors influencing value. ## Popular Localities By far the most classic and sought-after specimens by collectors come from historical localities in the Harz Mountains, Germany. Specimens from the Bolivian El Dragón mine, although discovered relatively recently, have also provided good quality material, often in interesting associations with other rare selenides.

Care and storage

## Cleaning Freboldite specimens should only be cleaned mechanically, using a soft brush to remove dust. The use of water or any liquids is not recommended, as the mineral may react and dull. ## What to Avoid Avoid contact with water, acids, chemical agents, and ultrasound. The mineral is soft and brittle, so it must be protected from impacts and scratches. Prolonged exposure to humid air can cause oxidation and color change. ## Storage It is recommended to store specimens in dry conditions, preferably in closed, padded display boxes or plastic cases, to limit air and moisture access and protect against mechanical damage.

Sources

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