Eckerite
Chemical formula: Ag¹⁺₂Cu¹⁺As³⁺S²⁻₃
Eckerite is a very rare silver, copper, and arsenic sulfosalt, forming tiny, metallic crystals with a lead-gray color.
Properties
- Mohs hardness
- 2.5-3
- Color
- Reddish-brown
- Luster
- Metallic
- Streak
- Dark orange-red
- Density
- 5.313
- Cleavage
- None
- Fracture
- Irregular/Uneven
- Transparency
- Transparent,Opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Identification of eckerite is possible only through advanced laboratory methods, such as chemical analysis using an electron microprobe (EDS/WDS) and X-ray diffraction (XRD). Macroscopic visual identification is impossible due to the microscopic size of the crystals. In electron microscope studies, it is distinguished by its characteristic chemical composition. ## Differentiation from Similar Minerals Eckerite occurs in paragenesis with other silver and copper sulfosalts, such as pearceite, polybasite, proustite, and samsonite. Distinguishing it from these minerals requires specialized chemical and structural analysis. Visually, it is practically indistinguishable from co-occurring, fine-grained, metallic sulfosalts. ## Crystal Forms Eckerite forms very small, elongated, platy or acicular crystals, reaching up to 200 micrometers (0.2 mm) in length and only a few micrometers in thickness. They occur as single, embedded crystals or form small, radial aggregates.
Geological environment
## Genesis Eckerite is a hydrothermal mineral. It forms in the final stages of crystallization in ore veins rich in silver, arsenic, and antimony. In its type locality (Clara mine), it crystallized in small vugs within barite. ## Mineral Associations This mineral co-occurs with other sulfosalts and sulfides. In its immediate vicinity, proustite, samsonite, stephanite, polybasite, pearceite, tennantite, pyrite, as well as barite and quartz have been identified. ## Localities The only confirmed locality of eckerite in the world is its type locality: the Clara mine, in the Rankach valley, near Oberwolfach in the Black Forest, in the German state of Baden-Württemberg. This is a historic fluorite and barite mine, known for the occurrence of over 400 different, often very rare minerals.
Rarity
Extremely rare
For collectors
## Quality Criteria The quality of an eckerite specimen is primarily assessed based on the richness and quantity of microcrystals on the rock matrix. The most valued specimens are those where well-formed, though still microscopic, crystals or their aggregates are visible, clearly standing out from the substrate. Association with other rare minerals, such as samsonite or proustite, is also important. ## Popular Localities The only source of eckerite specimens is the Clara mine in the Black Forest (Germany). The material comes from historical finds and is available in very limited quantities, mainly circulating among specialized micromount collectors.
Care and storage
## Cleaning Specimens containing eckerite should generally not be cleaned. The crystals are extremely small, fragile, and often embedded in a delicate matrix. Any attempt at mechanical (brush) or chemical cleaning risks irreversible damage to the mineral. Only careful dust removal using a photographic blower bulb is permissible. ## What to Avoid Avoid contact with all chemicals, acids, and detergents. As a sulfosalt, the mineral is susceptible to oxidation and chemical reactions. It should not be heated or exposed to prolonged moisture. ## Storage Eckerite specimens, being micromounts, should be stored exclusively in specialized, sealed boxes (micromount boxes) that protect them from dust, moisture, and mechanical damage. Shocks and vibrations should be avoided.