Tennantite-(In)

Chemical formula: Cu¹⁺₆(Cu¹⁺₅In³⁺)As³⁺₄S²⁻₁₃

Tennantite-(In) is a rare, black mineral from the tennantite group, an arsenosulfide of copper and indium, distinguished by the presence of indium as a key component.

## Characteristics Tennantite-(In) is a mineral from the tennantite group, belonging to the sulfosalt class. Chemically, it is a complex copper arsenosulfide with a significant indium content, which is the key element defining this mineral. It occurs as fine, opaque grains with a black color and metallic luster. Due to its rarity and occurrence in small aggregates, well-formed crystals have not yet been described. ## Physical Properties This mineral is characterized by a hardness in the range of 3.5-4 on the Mohs scale. Its density, calculated based on the chemical formula and unit cell parameters, is 4.763 g/cm³. It has a metallic luster and a black streak. The fracture is conchoidal. It is completely opaque. ## Colors and Varieties Tennantite-(In) is a mineral of uniform, black color. No color or commercial varieties are distinguished. ## History and Name The mineral's name derives from its relation to common tennantite and from the dominant element in its composition – indium (In). It was officially recognized by the International Mineralogical Association (IMA) in 2021. The type locality (locus typicus), where this mineral was first identified, is the Pefka deposit near Alexandroupoli in Greece. ## Uses Due to its extreme rarity, tennantite-(In) has no industrial applications. It is solely an object of scientific interest and a collector's item for specialized collectors of rare minerals.

Properties

Mohs hardness
3.5-4
Luster
Metallic
Streak
black
Density
4.763
Fracture
Conchoidal
Transparency
Opaque
Crystal system
Isometric

Diagnostic features

## Identification Identifying tennantite-(In) based on visual characteristics is practically impossible. This mineral occurs as black, metallic grains, indistinguishable from other minerals of the tennantite-tetrahedrite group or similar sulfosalts. Definitive identification requires advanced analytical methods, such as X-ray microanalysis (EDS/WDS), to confirm the presence of indium as a key component. ## Distinguishing from Similar Minerals Tennantite-(In) is visually identical to other black, metallic sulfides and sulfosalts, such as tennantite, tetrahedrite, enargite, or sphalerite. Distinguishing it from these minerals is possible only through chemical analysis. ## Crystal Forms This mineral has so far been found only in the form of anhedral (unformed) grains, not exceeding a few tens of micrometers in size, forming aggregates within the host rock.

Geological environment

## Genesis Tennantite-(In) forms as a result of hydrothermal processes. In its type locality in Greece, it is associated with polymetallic Au-Ag-Te-Se-Pb-Bi mineralization, which is genetically linked to Miocene volcanism. It crystallizes under relatively low-temperature conditions from solutions rich in copper, arsenic, sulfur, and indium. ## Mineral Associations This mineral co-occurs with other sulfides and sulfosalts. In the Pefka deposit, it has been found in association with pyrite, chalcopyrite, galena, bismuthinite, enargite, luzonite, tennantite-(Zn), tennantite-(Fe), as well as tellurium and selenium minerals. ## Localities The only confirmed occurrence of tennantite-(In) in the world is its type locality – the Pefka deposit, located in the Thrace region, near the city of Alexandroupoli in Greece.

Rarity

Extremely rare

For collectors

## Quality Criteria As a microscopic mineral, tennantite-(In) is not evaluated according to typical collector criteria such as size, crystal form, or luster. Its value is purely scientific and systematic. For a specialized collector, the only criterion is the confirmation of the specimen's authenticity from the type locality through chemical analysis. The most valuable samples are those with the richest aggregates of this mineral's grains, analytically documented. ## Popular Localities The only source of specimens is the type locality in Greece (Pefka deposit).

Care and storage

## Cleaning Tennantite-(In) specimens are typically small and embedded in the host rock. Cleaning should be kept to an absolute minimum. If necessary, a soft brush can be used to remove dust. Avoid water and any chemical agents. ## What to Avoid As a sulfide, this mineral can be sensitive to moisture and oxidation, which may lead to its slow decomposition. Contact with chemicals, acids, and sudden temperature changes should be avoided. It should not be exposed to prolonged sunlight. ## Storage It is recommended to store specimens in a dry place, preferably in closed, airtight boxes away from light and moisture. The use of desiccants may be advisable to protect against degradation.

External references

Sources

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