Fischesserite
Chemical formula: Ag₃AuSe₂
Fischesserite is a rare gold and silver selenide, forming metallic, lead-gray grains and aggregates in hydrothermal veins.
Properties
- Mohs hardness
- 2
- Color
- Black
- Luster
- Metallic
- Streak
- Black
- Density
- 0
- Cleavage
- None
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Isometric
Diagnostic features
## Identification Identification of fischesserite is based on its metallic luster, lead-gray color, high density, and co-occurrence with other selenides. Its softness is also characteristic. Visual identification is very difficult and almost always requires advanced analytical methods, such as chemical composition analysis (EDS) or X-ray diffraction (XRD). ## Distinguishing from Similar Minerals Fischesserite can be confused with other rare selenides, such as naumannite (Ag₂Se), aguilarite (Ag₄SeS), or eucairite (CuAgSe). Distinguishing them without specialized equipment is practically impossible. From more common sulfides, like galena, it differs slightly in color and lack of distinct cleavage. ## Crystal Forms Fischesserite crystals are extremely rare, very small (up to 0.5 mm), and poorly formed. Most often, it occurs as anhedral (irregular) grains embedded in the host rock or as massive, granular aggregates.
Geological environment
## Genesis Fischesserite is a hydrothermal mineral. It forms at low to medium temperatures in calcite-selenide veins that cut metamorphic rocks, such as graphitic schists. It forms in an environment rich in selenium and poor in sulfur. ## Mineral Associations This mineral co-occurs with other selenides, which is crucial for its identification. It is most often associated with clausthalite, eucairite, tyrrellite, umangite, naumannite, aguilarite, as well as berzelianite, chameanite, uraninite, pyrite, hematite, and calcite. ## Localities The most important and classic locality (type locality) is Předbořice near Klatovy in the Czech Republic. Other confirmed localities include the De Lamar mine in Idaho (USA), the Tumiñico mine in Sierra de Cacho in Argentina, and the Lake Athabasca region in Saskatchewan, Canada.
Rarity
Very rare
For collectors
## Quality Criteria The collecting appeal of fischesserite does not depend on aesthetics but on its rarity and scientific value. The most prized specimens are those where the mineral forms the richest possible aggregates or rare, visible-to-the-naked-eye grains. The presence of well-identified associated minerals, confirming the paragenesis, is also crucial. The quality of a specimen is almost always linked to its precise analytical documentation. ## Popular Localities The most sought-after specimens by collectors are those from the type locality – Předbořice in the Czech Republic. Material from other confirmed localities, such as De Lamar in the USA or Tumiñico in Argentina, is equally valuable due to its extreme rarity.
Care and storage
## Cleaning Fischesserite specimens should generally not be cleaned mechanically or chemically. Any treatment can irreversibly damage delicate and rare aggregates. If absolutely necessary, compressed air can be used to remove dust. ## What to Avoid Avoid contact with chemicals, acids, and detergents. The mineral is sensitive to oxidation, so it should be protected from moisture and sudden temperature changes. It should not be washed in water or cleaned with ultrasonics. ## Storage Fischesserite specimens are best stored under stable conditions, in closed, airtight containers (e.g., "membrane boxes") or in boxes away from dust and moisture. Limiting air access will slow down oxidation processes and the formation of tarnish.