Eskebornite
Chemical formula: Cu<sup>1+</sup>Fe<sup>3+</sup>Se<sup>2-</sup><sub>2</sub>
Eskebornite is a rare copper iron selenosulfide, forming granular aggregates with a metallic luster and a brass-yellow color.
Properties
- Mohs hardness
- 3.5
- Luster
- Metallic
- Streak
- Black
- Density
- 5.7-5.8
- Cleavage
- Indistinct
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Tetragonal
Diagnostic features
## Identification Eskebornite is identified by its brass-yellow color, metallic luster, black streak, and occurrence in characteristic selenium parageneses. It is a brittle mineral with relatively low hardness. Field identification is very difficult and usually requires laboratory analysis (XRD, EDS) to distinguish it from more common sulfides. ## Distinguishing from Similar Minerals Eskebornite is visually very similar to chalcopyrite, from which it differs by a slightly darker, more brownish hue and a black streak (chalcopyrite has a greenish-black streak). It can also be confused with pyrite, which is significantly harder and has a brighter, straw-yellow color. It differs from bornite by the absence of characteristic iridescent tarnish colors. ## Crystal Forms Eskebornite crystals are extremely rare and small, usually taking the form of tetrahedra. Most often, it is found as irregular grains, granular aggregates, and inclusions in other selenide minerals.
Geological environment
## Genesis Eskebornite is a hydrothermal mineral. It forms in ore veins rich in selenium, under low to medium temperature conditions. Its formation is associated with metasomatic processes in carbonate rocks or shales. ## Mineral Associations This mineral often co-occurs with other selenides, such as clausthalite, umangite, berzelianite, tiemannite, and klockmannite. It is also accompanied by calcite, dolomite, ankerite, and less commonly chalcopyrite and uraninite. ## Localities The most important eskebornite localities worldwide include the historic type locality in the Harz region of Germany (Eskeborn, Trogtal). It is also known from the uranium mines in Předbořice and Petrovice in the Czech Republic, from Tuminico in Argentina, and from the Lake Athabasca region in Saskatchewan, Canada.
Rarity
Very rare
For collectors
## Quality Criteria The most valued specimens by collectors are those with clearly developed, even microscopic, crystals. Rich granular aggregates, clearly visible against the matrix rock (most often calcite or dolomite), are also highly prized. Paragenesis is key – specimens with rare, associated selenides (e.g., umangite) are significantly more valuable. Due to the mineral's instability, well-preserved specimens without visible signs of oxidation are rare. ## Popular Localities Classic and most sought-after specimens come from historical localities in the Harz Mountains in Germany. Material from the Czech Republic (Předbořice) has also yielded good quality samples, often in association with other selenides.
Care and storage
## Cleaning Eskebornite specimens should be cleaned only mechanically, using a soft brush to remove dust. Due to its reactivity, all contact with water and chemical agents should be avoided. ## What to Avoid Contact with water, moisture, and chemicals, which can lead to rapid oxidation and destruction of the mineral, must be strictly avoided. Eskebornite is sensitive to atmospheric changes, so it should not be exposed to direct air for extended periods. It should be protected from high temperatures. ## Storage It is recommended to store specimens in airtight, dust-free containers (e.g., "perky box" type) or zip-lock bags, preferably with a desiccant. Display should be in closed showcases, away from light and moisture sources.