Weissbergite

Chemical formula: Tl₁+Sb₃+S₂-₂

Weissbergite is a rare thallium antimony sulfide, forming characteristic platy crystals with a metallic luster and cherry-red internal reflections.

## Characteristics Weissbergite is a rare mineral from the sulfosalt group, chemically defined as a thallium antimony sulfide. Its most characteristic visual feature is its metallic luster and dark gray to black color, which, under strong light, reveals deep, cherry-red internal reflections. It typically occurs as thin, bladed or platy crystals, often forming fan-shaped or radial aggregates. The crystals can be flexible and plastic. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of just 1.5, meaning it can be scratched with a fingernail. It has a very high density, approximately 5.33 g/cm³, resulting from the presence of heavy elements – thallium and antimony. The luster is strongly metallic, and the mineral is opaque, with the exception of the thinnest fragments, which may appear translucent red. ## Colors and Varieties The dominant color of weissbergite is black or lead-gray. Characteristic are its intense, red internal reflections, especially visible on crystal edges or under strong, pinpoint illumination. No color varieties or commercial varieties are distinguished. ## History and Name Weissbergite was discovered in the Carlin mine in Eureka County, Nevada (USA). It was described and named in 1977 by Frank W. Dickson, Arthur S. Radtke, and James A. McCormack. The name honors George T. Weissberg, a New Zealand geochemist who first synthesized its chemical equivalent (TlSbS₂) in the laboratory in 1966, predicting its existence in nature. ## Uses Due to its extreme rarity, weissbergite has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals and scientists studying epithermal deposits.

Properties

Mohs hardness: 1.5
Luster: Metallic
Streak: Red
Density: 5.33
Cleavage: Perfect on {010}
Fracture: Micaceous
Transparency: Translucent in thin fragments, Opaque
Crystal system: Triclinic

Diagnostic features

## Identification Key diagnostic features of weissbergite include its black color with metallic luster, very low hardness (1.5 on the Mohs scale), and characteristic, intensely red internal reflections visible under strong light. Its crystals are often thin, bladed, and flexible. High density is also an important indicator. ## Distinguishing from Similar Minerals Weissbergite can be confused with other rare thallium sulfosalts, such as lorandite or pierrotite. Lorandite (TlAsS₂) has a similar red color but is more transparent and has perfect cleavage. Pierrotite ((Tl,Pb)₂(Sb,As)₁₀S₁₇) is usually black and opaque but rarely exhibits such intense red reflections. From more common minerals, like stibnite, it is distinguished by the presence of red reflections and lower hardness. ## Crystal Forms Weissbergite crystallizes in the triclinic system. It most often forms thin, flattened, bladed, or platy crystals. They frequently occur as radial, fan-shaped, or chaotically arranged aggregates, growing on other minerals, most commonly on realgar or calcite.

Geological environment

## Genesis Weissbergite is a low-temperature hydrothermal mineral. It forms in the final stages of crystallization in ore veins, in an environment rich in thallium, antimony, and sulfur, and poor in iron. It is typical of certain types of gold deposits, especially the Carlin type in Nevada. ## Mineral Associations This mineral most commonly co-occurs with realgar, orpiment, stibnite, getchellite, lorandite, pyrite, marcasite, quartz, and calcite. It often grows directly on realgar crystals. ## Localities The most important and classic locality for weissbergite is the Carlin mine in Eureka County, Nevada, USA, where it was discovered. Another significant locality is the Getchell gold and mercury deposit, also in Nevada. Outside the USA, its presence has been noted in mines in Peru (Quiruvilca), China (Zimudang gold deposit in Guizhou province), and North Macedonia (Allchar deposit).

Rarity

Very rare

For collectors

## Quality Criteria The most prized weissbergite specimens are those with well-formed, sharply terminated crystals exhibiting a distinct metallic luster. The intensity and visibility of the red internal reflections are key. The attractiveness of a specimen is significantly enhanced by rich crystal aggregates forming radial or fan-shaped habits. A contrasting association with accompanying minerals, especially red realgar or white calcite, is highly valued. Crystal size is also important, although large specimens are extremely rare. ## Popular Localities Specimens from the type locality – the Carlin mine in Nevada, USA – are by far the most sought after by collectors. They are characterized by classic crystal development and are historically most significant. Specimens from the Getchell mine (also Nevada) are also highly valued.

Care and storage

## Cleaning Weissbergite specimens should be cleaned with the utmost care. Due to its low hardness, it is best to use only compressed air to remove dust. Any mechanical contact, even with a soft brush, can scratch or damage the delicate crystals. The use of water is not recommended. ## What to Avoid Weissbergite is a mineral containing highly toxic thallium. It is absolutely essential to avoid inhaling dust and to wash hands after any contact with the specimen. It must not be heated or exposed to acids. It should be protected from moisture and temperature changes. It is sensitive to light, which can cause its slow decomposition, so prolonged exposure to strong lighting, especially sunlight, should be avoided. ## Storage Specimens should be stored in closed, stable containers (e.g., "perky boxes"), away from light and moisture, in a place with a constant temperature. Due to its toxicity, it should be kept out of reach of children and pets, and its label should include a clear warning about its thallium content.

Sources

IMA Mineral List (CNMNC)