Bornhardtite

Chemical formula: Co²⁺Co³⁺₂Se²⁻₄

Bornhardtite is a very rare cobalt selenide, distinguished by its characteristic pinkish-red color and metallic luster.

## Characteristics Bornhardtite is an extremely rare mineral from the sulfide group, specifically selenides, belonging to the linnaeite group. Chemically, it is a complex cobalt selenide. It forms very fine, granular aggregates, most often as coatings or inclusions in other minerals. Its most characteristic visual feature is its unique pinkish-red color, which can be helpful in identification. ## Physical Properties This mineral is opaque and exhibits a metallic luster. Its Mohs hardness is approximately 4. Due to its extreme rarity and occurrence as microscopic grains, many of its physical properties, such as density, have not been precisely determined. ## Colors and Varieties Bornhardtite occurs in a characteristic pinkish-red color. No color varieties or commercial varieties are known. ## History and Name The mineral is named in honor of Friedrich Bornhardt (1886-1946), a German mining engineer who made significant contributions to the study of ore deposits in the Harz Mountains. Bornhardtite was first described in 1955 from samples originating from the Trogtal quarries in Lower Saxony, Germany, which is its type locality. ## Uses Bornhardtite has no industrial application. Its significance is purely scientific and collectible, being a coveted object for specialized collectors of rare minerals.

Properties

Mohs hardness
4
Luster
Metallic
Density
0
Transparency
Opaque
Crystal system
Isometric

Diagnostic features

## Identification Amateur identification of bornhardtite is practically impossible. It requires advanced analytical techniques, such as chemical composition analysis (EDS) and X-ray diffraction (XRD). A preliminary indication may be its unique pinkish-red color, metallic luster, and co-occurrence with other selenides in a specific geological environment. ## Distinguishing from Similar Minerals Bornhardtite can be confused with other rare selenides with a reddish hue, such as trogtalite or clausthalite. Differentiation relies solely on laboratory analysis of chemical composition and crystal structure. ## Crystal Forms This mineral crystallizes in the isometric system. In nature, it occurs almost exclusively as very fine, anhedral (irregular) grains and aggregates, often forming coatings or inclusions in other minerals, mainly in clausthalite.

Geological environment

## Genesis Bornhardtite is a hydrothermal mineral. It forms in low-temperature calcite-selenide veins that cut metamorphic rocks, such as greywackes and shales. Its crystallization is associated with selenium-rich ore mineralization processes. ## Mineral Associations This mineral most often co-occurs with other selenides. Its typical associated minerals include clausthalite, trogtalite, hastite, as well as calcite, hematite, and umangite. It often forms very fine, intergrown aggregates with them. ## Localities Bornhardtite is an extremely rare mineral, known from only a few localities worldwide. The most important and type locality is the Trogtal quarries near Lautenthal in the Harz Mountains, Germany. Beyond this locality, its presence has also been reported in Eskaborn near Trier (Germany) and in the Tumiñico mine in Sierra de Cacho, Argentina.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a bornhardtite specimen is primarily assessed based on the richness and visibility of its aggregates. Since this mineral is microscopic, samples where it forms distinct, colorful coatings or aggregates, clearly visible under magnification, are most valued. Precise confirmation of the mineral's identification using analytical methods is also crucial. ## Popular Localities The only source of specimens of collectible significance is its type locality – the Trogtal quarries in the Harz Mountains, Germany. Specimens from this location, even microscopic ones, are extremely prized by specialized collectors.

Care and storage

## Cleaning Bornhardtite specimens, due to their microscopic size and occurrence as coatings, should generally not be cleaned mechanically or chemically. It is safest to leave them in their untouched state. If dust removal is necessary, a gentle stream of compressed air can be used. ## What to Avoid Avoid contact with all chemicals, acids, and detergents, which can damage both bornhardtite and its associated minerals. The mineral may be susceptible to oxidation, so it should be protected from moisture and sudden temperature changes. ## Storage Specimens containing bornhardtite are best stored under stable conditions, in closed, dry containers (e.g., "perky boxes"), away from direct sunlight and sources of contamination. An accurate locality label is crucial for preserving its scientific and collectible value.

External references

Sources

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