Tetrahedrite-(Fe)

Chemical formula: Cu₁+₆(Cu₁+₄Fe₂+₂)Sb₃+₄S₂-₁₃

A black, metallic mineral from the tetrahedrite group, an important ore of copper and silver, forming characteristic tetrahedral crystals.

## Characteristics Tetrahedrite-(Fe) is a sulfide mineral belonging to the tetrahedrite series, in which iron is the dominant divalent metal. Its name comes from the characteristic shape of its crystals – a tetrahedron. It usually occurs as well-formed, metallic-lustered crystals, ranging in color from steel-gray to black. It often forms granular aggregates or massive inclusions within other ore minerals. ## Physical Properties Tetrahedrite-(Fe) is a relatively brittle mineral. Its Mohs hardness is 3.5-4, and its density ranges from 4.9 to 5.1 g/cm³. The luster is strongly metallic, and on weathered surfaces, it can be dull. It is opaque. ## Colors and Varieties It is typically steel-gray to almost black. Thinner fragments may show a reddish translucence. Depending on impurities, especially silver, it can exhibit subtle changes in hue. Silver-rich varieties are sometimes called "freibergite," although this is often an imprecise term and refers to various silver-bearing members of the tetrahedrite group. ## History and Name The tetrahedrite group has long been known as an important ore of copper and silver. The name "tetrahedrite" was introduced by Wilhelm Haidinger in 1845 and refers to the typical crystal form (from Greek *tetra* - four and *hedra* - face). The suffix "-(Fe)" was added in 2019 by the International Mineralogical Association (IMA) as part of a nomenclature revision for the group, to precisely indicate iron as the dominant element in a specific structural site. ## Uses Tetrahedrite-(Fe) is an important ore mineral. It is a significant source of copper, and due to frequent silver impurities, it is also a valuable ore of this precious metal. Antimony, being a major component of the mineral, can also be recovered during its processing.

Properties

Mohs hardness: 3.5-4
Luster: Metallic
Streak: Black to reddish-brown
Density: 4.9-5.1
Cleavage: None
Fracture: Uneven, Sub-Conchoidal
Transparency: Opaque
Crystal system: Cubic

Diagnostic features

## Identification Key diagnostic features include the tetrahedral crystal shape, steel-gray to black color, strong metallic luster, and relatively low hardness. The streak is black or reddish-brown. The mineral is also noticeably heavier than non-metallic minerals of similar volume. ## Distinguishing from Similar Minerals Tetrahedrite-(Fe) can be confused with other members of the tetrahedrite group, such as tennantite (which usually has a lighter, more steel-like hue) or tetrahedrite-(Zn). Definitive distinction requires chemical analysis. It can also be confused with sphalerite, which, however, often exhibits better cleavage and has a resinous, rather than metallic, luster. It is distinguished from chalcopyrite by its color (chalcopyrite is brass-yellow) and crystal form. ## Crystal Forms It most commonly forms crystals in the shape of a tetrahedron, often with modifications of other faces, giving them a complex appearance. It also occurs as granular aggregates, massive forms, or as inclusions in other minerals.

Geological environment

## Genesis Tetrahedrite-(Fe) is a typical hydrothermal mineral, forming in low- to medium-temperature ore veins. It forms under a wide range of geological conditions, from polymetallic deposits to quartz-carbonate veins. ## Mineral Associations It often co-occurs with other sulfides and sulfosalts, such as chalcopyrite, pyrite, galena, sphalerite, bornite, as well as with quartz, barite, dolomite, and calcite. In silver-rich deposits, it is accompanied by acanthite, proustite, or pyrargyrite. ## Localities Significant deposits and localities for beautiful collector specimens include the Casapalca mine in Peru; Herodsfoot Mine in Cornwall (United Kingdom); Freiberg in Saxony (Germany); Příbram and Kutná Hora (Czech Republic); Baia Sprie and Cavnic (Romania), and numerous localities in the USA (e.g., Colorado and Montana).

Rarity

Not very common

For collectors

## Quality Criteria Most valued by collectors are specimens with sharp, well-formed crystals exhibiting a strong metallic luster. Ideal crystals are free from damage, have distinct, smooth faces, and reach significant sizes (above 1-2 cm). The attractiveness is enhanced by the contrasting placement of black tetrahedrite crystals on a light matrix, such as white quartz or pink dolomite. The presence of other, rarer associated minerals also increases the specimen's value. ## Popular Localities Specimens of the highest collector quality come from historical mines in Romania (Cavnic, Baia Sprie), Peru (Casapalca), Germany (Freiberg), and some regions of the USA. Romanian specimens are renowned for their large, sharp crystals on contrasting matrices.

Care and storage

## Cleaning Specimens should be cleaned very carefully, preferably dry using a soft brush or compressed air. If necessary, a damp cloth with distilled water can be used, followed by immediate and thorough drying of the specimen. Avoid immersion in water. ## What to Avoid Tetrahedrite-(Fe) is sensitive to acids and other chemicals. Prolonged exposure to moisture can lead to oxidation and the formation of a dull tarnish. Ultrasonic cleaners should be avoided, as they can damage brittle crystals. ## Storage Specimens should be stored in stable, dry conditions, away from direct sunlight. It is best to keep them in separate, padded boxes to prevent scratches and mechanical damage from contact with harder minerals.

Sources

IMA Mineral List (CNMNC)