Tetrahedrite-(Hg)

Chemical formula: Cu₁+₆(Cu₁+₄Hg₂+₂)Sb₃+₄S₂-₁₃

Tetrahedrite-(Hg) is a rare mineral from the tetrahedrite group, being a mercury-rich variety characterized by a distinctive metallic luster and steel-gray color.

## Characteristics Tetrahedrite-(Hg) is a copper, mercury, and antimony sulfosalt, belonging to the tetrahedrite group. It is the mercury analog of tetrahedrite-(Fe) and tetrahedrite-(Zn). It forms crystals with a typical tetrahedral shape for this group, from which its name is derived. It usually occurs as embedded, well-formed crystals, reaching several millimeters in size. It is also found in granular aggregates. Its color ranges from steel-gray to black, and on fresh fracture surfaces, it exhibits a strong, metallic luster. ## Physical Properties This mineral is characterized by a hardness of 3-4 on the Mohs scale, making it relatively soft. It is brittle, and its fracture is uneven to conchoidal. Its density is high, approximately 5.43 g/cm³, which is noticeable even in small specimens. It is an opaque mineral. ## Colors and Varieties Tetrahedrite-(Hg) occurs in uniform colors, from steel-gray to black. It does not form color varieties, and its chemical composition is strictly defined by the dominance of mercury in a specific position within the crystal structure. It is itself a specific variety within the broad tetrahedrite group. ## History and Name The name "tetrahedrite-(Hg)" refers to its dominant component (Hg - the symbol for mercury) and its association with the tetrahedrite group, alluding to the characteristic form of its crystals. The mineral was formally described and approved by the International Mineralogical Association (IMA) in 2019. It was separated as a distinct mineral species as part of the revision of the tetrahedrite group nomenclature. ## Applications Due to its rarity, tetrahedrite-(Hg) has no industrial applications. It is solely an object of scientific and collector interest.

Properties

Mohs hardness: 3-4
Luster: Metallic
Streak: Black
Density: 5.43
Cleavage: None
Fracture: Uneven to conchoidal
Transparency: Opaque
Crystal system: Cubic

Diagnostic features

## Identification Tetrahedrite-(Hg) can be identified by its characteristic tetrahedral crystal shape, steel-gray color, metallic luster, and high density. The streak is black. Identification is difficult without advanced analytical methods, such as chemical composition analysis (EDS/WDS), due to its strong resemblance to other minerals of the tetrahedrite group. ## Distinguishing from Similar Minerals It is practically impossible to distinguish it with the naked eye from other black sulfosalts of the tetrahedrite group, such as tetrahedrite-(Fe), tetrahedrite-(Zn), tennantite, or freibergite. All these minerals have a similar appearance, hardness, and luster. Certain differentiation requires specialized laboratory tests to determine the dominant element in the structure. ## Crystal Forms It most often forms single, well-developed crystals with a tetrahedral habit, often with modifications of other faces. Crystals are usually embedded in the host rock. It also occurs as granular aggregates.

Geological environment

## Genesis It is a hydrothermal mineral, forming in ore veins. It crystallizes under low to medium temperature conditions. Its crystallization is associated with the presence of solutions rich in copper, antimony, sulfur, and, crucially, mercury. ## Mineral Associations It most commonly co-occurs with other sulfides and sulfosalts. Typical associated minerals include quartz, dolomite, barite, chalcopyrite, sphalerite, galena, pyrite, and other minerals from the tetrahedrite group. ## Localities The most important and best-documented localities, from which reference specimens originate, are the Rožňava mine in Slovakia and the Gelnica mine, also in Slovakia. These are key locations for this mineral.

Rarity

Very rare

For collectors

## Quality Criteria The most prized specimens by collectors are those with sharp, well-formed crystals exhibiting a strong, metallic luster. The absence of mechanical damage is important, as is an attractive placement of crystals on the rock matrix, preferably accompanied by contrasting minerals such as white quartz or dolomite. Crystal size is also a significant factor – specimens with crystals exceeding several millimeters are considered exceptional. ## Popular Localities By far the most sought-after specimens come from classic localities in Slovakia, such as Rožňava and Gelnica. These are historical sites from which material for research and description of this mineral was obtained, giving them particular scientific and collector value.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft brush to remove dust. Compressed air can be used. Avoid any contact with water and chemical agents, which can cause reactions on the mineral's surface and lead to oxidation or damage. ## What to Avoid Absolutely avoid contact with water, acids, detergents, and other chemicals. The mineral is sensitive to moisture and atmospheric factors, which can cause it to tarnish and form secondary coatings. It should not be heated or exposed to sudden temperature changes. ## Storage It is recommended to store specimens in dry conditions, preferably in airtight containers (e.g., "perky boxes") away from moisture. Consider using desiccants. Display should be in closed showcases, protecting against dust and changes in air humidity.

Sources

IMA Mineral List (CNMNC)