Tennantite-(Cu)
Chemical formula: Cu¹⁺₆(Cu¹⁺₄Cu²⁺₂)As³⁺₄S²⁻₁₃
Tennantite-(Cu) is a rare mineral from the tennantite group, a copper arsenic sulfosalt, distinguished by the dominance of copper in its crystal structure.
Properties
- Mohs hardness
- 3.5-4
- Color
- Black
- Luster
- Metallic
- Streak
- Black
- Density
- 4.656
- Cleavage
- None
- Fracture
- Conchoidal
- Transparency
- Opaque
- Crystal system
- Isometric
Diagnostic features
## Identification Tennantite-(Cu) is recognized by its steel-gray to black color, metallic luster, and crystal form – most often tetrahedra. A characteristic feature is also the black streak with a reddish tint. Due to its similarity to other sulfosalts, definitive identification is possible almost exclusively through advanced analytical methods, such as chemical analysis (EDS/WDS) and X-ray diffraction (XRD). ## Differentiation from Similar Minerals Tennantite-(Cu) is visually indistinguishable from other minerals of the tennantite-tetrahedrite group, such as tennantite-(Zn), tennantite-(Fe), or tetrahedrite. All these minerals have a similar appearance, hardness, and luster. They differ only in the proportions of metals (Cu, Zn, Fe, Ag, Hg) in their structure, which can only be determined by laboratory analysis. ## Crystal Forms It most often forms well-developed crystals with a tetrahedral habit, often with visible modifications of the faces of other geometric forms. Crystals can reach several millimeters in size. It also occurs as granular aggregates and massive forms.
Geological environment
## Genesis Tennantite-(Cu) forms under hydrothermal conditions, in medium- to low-temperature ore veins. It is a product of crystallization from solutions rich in copper, arsenic, and sulfur. ## Mineral Associations This mineral co-occurs with other sulfides and sulfosalts. In its type locality (Jáchymov), it was found in association with chalcocite, digenite, covellite, chalcopyrite, bornite, tennantite-(Zn), native arsenic, native silver, uraninite, quartz, and calcite. ## Localities The type and, so far, only confirmed locality for tennantite-(Cu) is the Svornost mine in Jáchymov, Karlovy Vary Region, Czech Republic. This is a historic mining area, known for the occurrence of silver, cobalt, nickel, and uranium ores.
Rarity
Extremely rare
For collectors
## Quality Criteria The most valuable tennantite-(Cu) specimens are those with well-formed, sharp, and lustrous tetrahedral crystals, set on a contrasting rock matrix. Crystal size and the absence of mechanical damage significantly increase collectible value. Since it is an analytically defined mineral, specimens from the type locality (Jáchymov) with confirmed chemical composition are particularly prized. ## Popular Localities The only known and confirmed locality from which all studied and described specimens originate is the Svornost mine in Jáchymov, Czech Republic. Specimens from this locality are highly sought after by collectors specializing in mineral systematics and minerals from type localities.
Care and storage
## Cleaning Specimens should only be dry-cleaned, using a soft brush to remove dust. Contact with water, especially with detergents, is not recommended, as it can lead to slow oxidation and changes in the surface appearance. ## What to Avoid Avoid contact with chemicals, acids, and detergents. The mineral is sensitive to moisture and temperature changes. Prolonged exposure to humid air can cause oxidation and the formation of secondary minerals on the surface, leading to a loss of luster. ## Storage It is recommended to store specimens in stable conditions, in sealed, dry containers (e.g., plastic boxes with a gasket) or in display cases with controlled humidity. Avoid direct exposure to sunlight and keep away from heat sources.