Tennantite-(Fe)

Chemical formula: Cu₁+₆(Cu₁+₄Fe₂+₂)As₃+₄S₂-₁₃

Tennantite-(Fe) is an iron-rich member of the tennantite group, a copper, iron, and arsenic sulfosalt, forming dark, metallic crystals.

## Characteristics Tennantite-(Fe) is a mineral from the tennantite group, belonging to an isomorphic series with tetrahedrite. As a copper, iron, and arsenic sulfosalt, it is an important ore mineral. Its crystals, crystallizing in the isometric system, most often take the form of tetrahedra, often modified by other faces. It also occurs as granular, massive, or disseminated aggregates within other minerals. Its surface can be dull or covered with a thin layer of oxidation products, giving it a darker, matte appearance. ## Physical Properties This mineral is characterized by a hardness in the range of 3.5-4 on the Mohs scale, making it relatively soft. It has a metallic luster, which can be dulled on weathered surfaces. It is opaque and brittle, with a density of approximately 4.62 g/cm³. It does not exhibit fluorescence under ultraviolet light. ## Colors and Varieties Tennantite-(Fe) is steel-gray to black in color. The streak, which is the color of the powdered mineral, is black with a reddish tint. As a member of the tetrahedrite group, it forms solid solutions with other minerals of this group, leading to significant variations in chemical composition. Tennantite-(Fe) is distinguished by the dominance of iron over zinc in its structure. ## History and Name The name "tennantite" comes from the English chemist Smithson Tennant (1761–1815). The suffix "-(Fe)" was added in 2019 by the International Mineralogical Association (IMA) as part of a revision of the tetrahedrite group nomenclature, to precisely identify this specific mineral as an iron-dominant variety. ## Uses Due to its copper content, tennantite-(Fe) can be a minor local source of this metal if it occurs in sufficiently large accumulations. It is also of interest to collectors, especially well-formed crystals from classic localities.

Properties

Mohs hardness: 3.5-4
Luster: Metallic
Streak: Black with a reddish tint
Density: 4.62
Cleavage: None
Fracture: Uneven
Transparency: Opaque
Crystal system: Cubic

Diagnostic features

## Identification Tennantite-(Fe) is identified by its steel-gray to black color, metallic luster, and characteristic tetrahedral crystals. An important diagnostic feature is the black streak with a reddish tint. It is relatively soft and brittle. ## Distinguishing from Similar Minerals It can be confused with other minerals of the tetrahedrite group, such as tetrahedrite-(Zn) or tennantite-(Zn). Final differentiation requires advanced chemical analysis (e.g., EDS) to determine the dominant elements. It differs from chalcopyrite by its darker color and crystal form, and from sphalerite (especially dark varieties) by the absence of distinct cleavage and a different crystal habit. ## Crystal Forms It crystallizes in the isometric system, primarily forming crystals in the shape of tetrahedra. Additional faces of other forms often appear on them, complicating their appearance. It is also found as granular, massive aggregates, and as inclusions in other sulfides.

Geological environment

## Genesis Tennantite-(Fe) is a hydrothermal mineral, forming in ore veins at low to medium formation temperatures. It occurs in polymetallic deposits, often associated with intrusions of igneous rocks. ## Mineral Associations It often co-occurs with other sulfides and sulfosalts, such as chalcopyrite, pyrite, sphalerite, galena, bornite, enargite, as well as with quartz, baryte, dolomite, and fluorite. ## Localities Significant occurrences of tennantite-(Fe) are found in the Magma Mine in Superior (Arizona, USA), in Butte (Montana, USA), in Tsumeb (Namibia), and also in various regions of Peru (e.g., Quiruvilca, Morococha), Mexico, and in the historic mines of Cornwall (United Kingdom).

Rarity

Not very common

For collectors

## Quality Criteria Specimens with sharp, well-formed, and undamaged crystals with a strong metallic luster are most valued by collectors. The presence of crystals on a contrasting matrix, for example, on white quartz or pink dolomite, enhances their appeal. Large, single crystals or impressive crystal groups are much more sought after than massive or granular aggregates. ## Popular Localities Specimens from classic, historic localities such as Tsumeb in Namibia, mines in Arizona (USA), or Peru, are particularly prized in the collector's market due to their quality and historical significance.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. Distilled water can be used, but prolonged soaking should be avoided. After wet cleaning, the specimen must be thoroughly dried. ## What to Avoid Contact with acids and other strong chemicals, which can react with sulfides leading to mineral damage, should be strictly avoided. Tennantite-(Fe) is brittle, so it must be protected from impacts and falls. Prolonged exposure to humid air can cause oxidation and dulling of its surface. ## Storage It is recommended to store specimens in a dry place, preferably in closed boxes or display cases, to limit access to moisture and dust. Avoid storing it with minerals that might chemically react with it.

Sources

IMA Mineral List (CNMNC)