Argentotetrahedrite-(Fe)

Chemical formula: Ag₁+₆(Cu₁+₄Fe₂+₂)Sb₃+₄S₂-₁₃

A silver-bearing mineral from the tetrahedrite group, a member of the isomorphic series with tetrahedrite, freibergite, and kenoargentotetrahedrite-(Fe).

## Characteristics Argentotetrahedrite-(Fe) is a sulfosalt of silver, copper, iron, and antimony, belonging to the tetrahedrite group. It forms solid solutions (isomorphic series) with other minerals of this group, mainly with tetrahedrite, freibergite, and kenoargentotetrahedrite-(Fe). It rarely occurs in the form of well-formed crystals, which most often take the shape of tetrahedra. It is usually found as inclusions and granular aggregates within other ore minerals. It is opaque and has a metallic luster. ## Physical Properties This mineral is characterized by a hardness ranging from 3.5-4 on the Mohs scale. Its density is relatively high, approximately 5.13 g/cm³. It does not exhibit cleavage, and its fracture is uneven to subconchoidal. It is brittle. ## Colors and Varieties Argentotetrahedrite-(Fe) is steel-gray to black in color. In fresh exposures, its luster is strongly metallic, but it may tarnish over time. No distinct color or commercial varieties are recognized; identification relies on chemical analysis. ## History and Name The mineral's name refers to its chemical composition – the dominance of silver (Latin: *argentum*) and iron (Latin: *ferrum*) – and its association with the structural tetrahedrite group. It was formally described and approved by the International Mineralogical Association (IMA) in 2019 as part of the redefinition of the tetrahedrite group. It is a relatively newly defined mineral species, although material corresponding to its composition was previously known under more general names. ## Uses Due to its high silver content, argentotetrahedrite-(Fe) can be a locally important ore of this metal if it occurs in sufficiently large concentrations. For collectors, it is primarily of scientific and systematic interest as a member of the complex tetrahedrite group. Well-formed crystals are rare and sought after by specialists.

Properties

Mohs hardness: 3.5-4
Luster: Metallic
Streak: Black
Density: 5.13
Cleavage: None
Fracture: Uneven to subconchoidal
Transparency: Opaque
Crystal system: Cubic

Diagnostic features

## Identification Macroscopic identification of argentotetrahedrite-(Fe) is practically impossible without advanced analytical methods. Preliminary identification is based on its steel-gray to black color, metallic luster, lack of cleavage, and occurrence in association with other sulfides and sulfosalts of silver, lead, and copper. The characteristic tetrahedral crystal shape is distinctive if crystals are well-formed. Definitive identification requires chemical analysis (e.g., using an electron microprobe - EDS/WDS) to determine the Ag, Cu, Fe, and Zn ratios. ## Distinguishing from Similar Minerals Argentotetrahedrite-(Fe) is visually indistinguishable from other minerals of the tetrahedrite group, such as tetrahedrite, freibergite, argentotennantite-(Fe), or kenoargentotetrahedrite-(Fe). All these minerals have very similar appearance and physical properties. It can also be confused with tennantite, enargite, sphalerite (especially the dark variety marmatite), or chalcocite. Differentiation from these minerals often requires chemical tests or X-ray diffraction (XRD) analysis. ## Crystal Forms Crystals, though rare, most often take the form of tetrahedra, often with modifications of other faces. It usually occurs as irregular grains, massive aggregates, or as inclusions in other ore minerals such as galena, sphalerite, or chalcopyrite.

Geological environment

## Genesis Argentotetrahedrite-(Fe) is a hydrothermal mineral. It forms in low- to medium-temperature ore veins rich in silver, antimony, copper, and iron. It is a typical component of polymetallic Ag-Pb-Zn-Cu deposits. ## Mineral Associations It most commonly co-occurs with other minerals from the tetrahedrite group, as well as with galena, sphalerite, chalcopyrite, pyrite, arsenopyrite, acanthite, stephanite, pyrargyrite, proustite, and quartz, dolomite, siderite, and calcite. ## Localities As a newly defined mineral, its exact distribution is still being studied. Confirmed occurrences (often re-classifications of former "freibergites" or "silver-bearing tetrahedrites") include historical mining regions. It has been identified, among others, in mines around Freiberg in Saxony (Germany), in the Uchucchacua deposit (Peru), in mines of the Coeur d'Alene district in Idaho (USA), and in Příbram (Czech Republic).

Rarity

Rare

For collectors

## Quality Criteria The most valued specimens by collectors are those with well-formed, sharp-edged crystals exhibiting a strong, metallic luster. Crystals with an ideal tetrahedral form are particularly sought after. Specimens where argentotetrahedrite-(Fe) grows on a mineralogically contrasting matrix (e.g., on white quartz or pink dolomite) or forms attractive compositions with other rare silver sulfosalts are also highly prized. ## Popular Localities Although precise localities for collector-quality specimens of this specific mineral are still being verified after the group's redefinition, historically, the best specimens of "silver-bearing tetrahedrites" (some of which are argentotetrahedrite-(Fe)) came from classic European deposits such as Freiberg (Germany) and Příbram (Czech Republic). Currently, research material also comes from deposits in Peru (e.g., Uchucchacua).

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft brush or paintbrush to remove dust. Compressed air can be used. Due to its potential reactivity, contact with water, and especially with acids and detergents, should be avoided. ## What to Avoid All chemicals should be avoided, especially acids (e.g., hydrochloric acid, nitric acid), which can react violently with the mineral, destroying it and releasing toxic fumes. The mineral is susceptible to oxidation, so it should be protected from moisture and prolonged contact with air, which can cause tarnishing and the formation of secondary coatings. It should not be heated. ## Storage It is recommended to store specimens in dry conditions, preferably in closed, airtight containers or display cases with a desiccant. Protect from direct sunlight and dust. Due to its brittleness, vibrations and impacts should be avoided.

Sources

IMA Mineral List (CNMNC)