Tetrahedrite-(Cu)
Chemical formula: Cu¹⁺₆(Cu¹⁺₄Cu²⁺₂)Sb³⁺₄S²⁻₁₃
Tetrahedrite-(Cu) is a copper-rich variety of tetrahedrite, characterized by a steel-gray color and metallic luster.
Properties
- Mohs hardness
- 3.5-4
- Luster
- Metallic
- Streak
- black
- Density
- 5.029
- Cleavage
- indistinct cleavage.
- Fracture
- Conchoidal
- Transparency
- Opaque
- Crystal system
- Isometric
Diagnostic features
## Identification Tetrahedrite-(Cu) is most easily identified by its characteristic steel-gray color, metallic luster, and typical crystal form – tetrahedra. A black streak and relatively low hardness (about 3.5-4 on the Mohs scale) are also important diagnostic features. The density is noticeably high. ## Distinguishing from similar minerals This mineral is sometimes confused with other sulfides and sulfosalts of similar appearance, such as tennantite, sphalerite (marmatite variety), chalcopyrite, or galena. It can only be distinguished from tennantite (with which it forms a series) by chemical analysis. It differs from sphalerite by its lower hardness and black streak (sphalerite's streak is lighter). Galena has a similar luster but is softer, heavier, and exhibits perfect cleavage in three directions, forming cubic fragments. ## Crystal forms It most often forms crystals with a tetrahedral habit, often modified by other forms of the isometric system. It also occurs as granular, massive, and disseminated aggregates within the host rock.
Geological environment
## Genesis Tetrahedrite-(Cu) is a hydrothermal mineral, forming in low- to medium-temperature ore veins. It forms in polymetallic deposits, often associated with volcanic activity or magmatic intrusions. ## Mineral associations It frequently co-occurs with other sulfides and sulfosalts, such as chalcopyrite, pyrite, sphalerite, galena, tennantite, as well as with quartz, barite, dolomite, siderite, and calcite. ## Localities As a mineral of the tetrahedrite group, it is widely distributed. Important historical and classic localities for well-formed tetrahedrite crystals (in the broader sense) include Cavnic and Baia Sprie in Romania, Freiberg in Germany, the Příbram region in the Czech Republic, as well as numerous sites in Peru (e.g., Casapalca mine), Mexico, and the United States (e.g., in Colorado and Idaho). The type locality for the strictly defined tetrahedrite-(Cu) is the Bankov deposit in Slovakia.
Rarity
Not very common
For collectors
## Quality criteria Specimens with sharp, well-formed crystals and a strong, metallic luster are most valued by collectors. Large, undamaged tetrahedra, especially those set on a contrasting matrix (e.g., on white quartz or calcite), fetch the highest prices. Associations with other rare or colorful minerals also enhance their appeal. ## Popular localities Classic tetrahedrite specimens, prized in collections, come from historical European mining regions, such as Romania (Cavnic), Germany (Saxony), and the Czech Republic. Many excellent specimens, often with large crystals, are provided by mines in Peru and Mexico.
Care and storage
## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, but prolonged soaking should be avoided. After wet cleaning, the specimen must be thoroughly dried. ## What to avoid Avoid contact with acids and other aggressive chemicals that can damage the mineral's surface. Tetrahedrite-(Cu) is susceptible to oxidation, which can cause its metallic luster to tarnish. It should not be heated or exposed to sudden temperature changes. ## Storage It is recommended to store specimens in a dry place, preferably in closed boxes or display cases, to limit access to moisture and oxygen, which will slow down the oxidation process. It should be protected from scratching by harder minerals.