Fletcherite
Chemical formula: Cu<sup>2+</sup>Ni<sup>3+</sup><sub>2</sub>S<sup>2-</sup><sub>4</sub>
Fletcherite is a rare copper-nickel sulfide, forming tiny, metallic grains in sedimentary rocks.
Properties
- Mohs hardness
- 5
- Luster
- Metallic
- Streak
- Black
- Density
- 4.85
- Cleavage
- None
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Isometric
Diagnostic features
## Identification Identifying fletcherite in the field is practically impossible due to the microscopic size of the crystals. In a collection, it can be recognized as tiny, metallic, octahedral crystals embedded in sedimentary rock. Definitive identification requires advanced analytical methods, such as electron microprobe analysis (EDS/WDS) to confirm its chemical composition (presence of copper, nickel, and sulfur). ## Distinguishing from Similar Minerals Fletcherite can be confused with other small, metallic sulfides, such as pyrite, chalcopyrite, or other minerals from the thiospinel group (e.g., carrollite, siegenite). Distinguishing it from these without specialized equipment is impossible. Fletcherite is characterized by a specific chemical composition with a dominance of copper and nickel. ## Crystal Forms It forms very small, most often perfectly developed octahedral crystals. It also occurs as rounded or irregular grains disseminated in the host rock.
Geological environment
## Genesis Fletcherite forms under low-temperature hydrothermal conditions or during diagenetic processes in sedimentary rocks. Its formation is associated with the presence of organic matter and hydrogen sulfide in a reducing environment. It is most commonly found in shales and sandstones containing disseminated sulfide mineralization. ## Mineral Associations This mineral co-occurs with other sulfides, such as chalcopyrite, bornite, digenite, djurleite, covellite, pyrite, as well as with calcite and quartz. ## Localities The most important and classic locality (type locality) is the Fletcher Mine in Reynolds County, Missouri (USA), within the so-called Viburnum Trend. It has also been reported in the Buick Mine in the same region. Outside the USA, its occurrence has been confirmed in small quantities in Poland (KGHM, Lubin-Sieroszowice) and in Germany (Mansfeld).
Rarity
Very rare
For collectors
## Quality Criteria The collector's appeal of fletcherite is low for the average collector due to the microscopic size of its crystals. However, it is valued by collectors specializing in rare, systematic minerals, and micromounts. The value of a specimen depends on the abundance of crystals on the rock matrix, their sharpness, and size, although even the largest rarely reach 1 mm. The most desirable specimens are from the type locality (Fletcher Mine, USA). ## Popular Localities The only source of collector-grade specimens is the Viburnum Trend region in Missouri, USA, especially the Fletcher Mine.
Care and storage
## Cleaning Fletcherite specimens, due to their small size and occurrence within a rock matrix, usually do not require cleaning. If necessary, compressed air can be used to remove dust. Avoid contact with water and chemical agents. ## What to Avoid Avoid contact with acids and other chemicals that may react with sulfides. The mineral is stable, but it should be protected from moisture to prevent potential long-term oxidation. ## Storage Specimens should be stored in a dry place, preferably in a closed display box or cabinet, to protect them from dust and moisture. Due to the small size of the crystals, they are often stored in special micromount boxes.