Tennantite-(Hg)

Chemical formula: Cu₁+₆(Cu₁+₄Hg₂+₂)As₃+₄S₂-₁₃

A rare arsenical sulfosalt from the tetrahedrite group, distinguished by a significant mercury content and occurrence as metallic, black crystals.

## Characteristics Tennantite-(Hg) is the mercury analogue of tennantite, belonging to the tetrahedrite group. It forms isometric crystals, most often in the shape of tetrahedra, frequently modified by other faces, which gives them a complex, polyhedral appearance. It usually occurs as intergrown or overgrown crystals, as well as in granular aggregates and compact masses. It has a metallic luster and is opaque. Its color ranges from steel-gray to black, and on fresh fracture surfaces, it may exhibit a slightly oily luster. ## Physical Properties The mineral's hardness on the Mohs scale is 3.5-4.5, making it relatively soft and brittle. Its density is high, approximately 5.36 g/cm³, which is a direct consequence of the high content of the heavy element, mercury. It does not exhibit cleavage, and its fracture is conchoidal to uneven. It is opaque. ## Colors and Varieties This mineral has a consistent, dark color ranging from steel-gray to black. The name Tennantite-(Hg) indicates that it is a specific mineral species defined by the dominance of mercury in a particular position within the tennantite crystal structure. It does not form color varieties, and its chemical composition may show some variations in the content of other elements, such as iron, zinc, or silver, which can substitute for copper. ## History and Name The mineral's name refers to its chemical composition – it is tennantite with dominant mercury (symbol Hg). It was officially recognized as a new mineral species by the International Mineralogical Association (IMA) in 2018. The type locality (locus typicus) is the Quiruvilca mine in Santiago de Chuco Province, La Libertad Region, Peru. ## Uses Due to its rarity, Tennantite-(Hg) has no industrial applications. However, it is an object of scientific interest and a valued collector's mineral, sought after for its origin, rarity, and membership in the complex tetrahedrite group.

Properties

Mohs hardness: 3.5-4.5
Luster: Metallic
Streak: Black
Density: 5.36
Cleavage: None
Fracture: Conchoidal to uneven
Transparency: Opaque
Crystal system: Isometric

Diagnostic features

## Identification Identifying Tennantite-(Hg) based on visual characteristics is practically impossible. Its appearance is almost identical to other dark minerals from the tetrahedrite group, such as tennantite, tetrahedrite, or freibergite. The key diagnostic feature is its chemical composition, specifically the high mercury content, which can only be confirmed using advanced analytical methods, such as X-ray microanalysis (EDS/WDS). ## Distinguishing from Similar Minerals It is distinguished from tennantite by its dominant mercury content. It is distinguished from tetrahedrite by the dominance of arsenic over antimony. It differs from freibergite (a silver-rich variety of tetrahedrite) by the dominance of mercury and copper over silver. Visually, all these minerals can look the same – forming black, metallic crystals in the shape of tetrahedra. ## Crystal Forms The most typical form consists of well-formed, single or twinned crystals with a tetrahedral habit. Additional faces of other forms are often observed on them, complicating their shape. It also occurs as granular aggregates, irregular masses, and as inclusions in other minerals.

Geological environment

## Genesis Tennantite-(Hg) is a mineral of hydrothermal origin. It forms in ore veins that develop under medium to low-temperature conditions. It is a product of crystallization from solutions rich in copper, arsenic, sulfur, and, crucially, mercury. ## Mineral Associations At its type locality (Quiruvilca mine in Peru), it co-occurs with other sulfides and sulfosalts. The most common associated minerals include enargite, pyrite, sphalerite, galena, chalcopyrite, and other minerals from the tetrahedrite group. ## Localities The most important and well-documented locality worldwide is its type locality – the Quiruvilca mine in Peru, from which the best-studied specimens originate. Its occurrence has also been confirmed in several other places around the world, including Rudňany in Slovakia and the Toyoha mine in Japan, but specimens from these localities are much rarer on the collector's market.

Rarity

Rare

For collectors

## Quality Criteria The most highly valued specimens are those with well-formed, sharp-edged crystals with a strong, metallic luster. The size of the crystals is of great importance – even those a few millimeters in size are considered valuable. The attractiveness of a specimen is enhanced by its association with other minerals that contrast in color or form, such as quartz, pyrite, or enargite. The purity of the specimen, meaning the absence of mechanical damage, is also crucial. ## Popular Localities By far the most known and valued source of Tennantite-(Hg) specimens is the Quiruvilca mine in Peru. Specimens from this locality are considered reference material for this mineral species.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft brush to remove dust and loose debris. If necessary, distilled water can be used, followed by thorough drying of the specimen. Ultrasonic cleaners should be avoided, as they can cause cracks in the brittle mineral. ## What to Avoid The mineral is sensitive to strong acids and other chemicals. It should be protected from impacts and sudden temperature changes. Due to its arsenic and mercury content, it is recommended to wash hands after contact with the specimen and avoid inhaling dust during any processing. ## Storage Store in stable conditions, in a display case or cabinet, away from direct sunlight and heat sources. It is advisable to isolate it from minerals that could undergo chemical reactions, especially sulfides prone to oxidation.

Sources

IMA Mineral List (CNMNC)