Tellurobismuthite

Chemical formula: Bi<sup>3+</sup><sub>2</sub>Te<sup>2-</sup><sub>3</sub>

Tellurobismuthite is a rare mineral from the telluride group, a natural bismuth telluride, forming characteristic flakes with a metallic luster.

## Characteristics Tellurobismuthite is a mineral from the sulfosalt group, specifically a bismuth telluride. It usually occurs as platy, scaly, or tabular aggregates, less often as poorly formed, tabular crystals with a hexagonal outline. Its specimens have a tin-white or steel-gray color, often with a yellowish or brownish tarnish resulting from weathering. The mineral is opaque and possesses a characteristic metallic luster. ## Physical Properties Tellurobismuthite is a soft mineral, with a Mohs hardness of 1.5-2, meaning it can be scratched with a fingernail. It is also very dense – its specific gravity is approximately 7.86 g/cm³. It is flexible and sectile, and thin flakes are elastic. It exhibits perfect cleavage in one direction, which accounts for its tendency to form platy and scaly shapes. It possesses thermoelectric properties. ## Colors and Varieties This mineral does not show much color variation. Its fresh surface is tin-white to steel-gray. Under atmospheric conditions, it may become covered with a dull, yellowish, or iridescent tarnish. No colored varieties or trade names are known. ## History and Name The name "tellurobismuthite" was given to the mineral in 1863 by the American chemist and mineralogist Charles F. Chandler. It directly refers to the mineral's chemical composition, indicating the presence of tellurium (telluro-) and bismuth (-bismuthite). However, the mineral was known earlier and was first described based on samples from the Little Mildred mine in Colorado, USA. ## Uses Due to its rarity, tellurobismuthite has no industrial applications. However, it is an object of scientific interest as a bismuth telluride with potential thermoelectric properties. For collectors, it is a valuable and sought-after mineral due to its rarity and specific chemical composition.

Properties

Mohs hardness
1.5-2
Luster
Metallic
Streak
Lead-gray
Density
7.81-7.86
Cleavage
Perfect on {0001}
Fracture
Uneven
Transparency
Opaque
Crystal system
Trigonal

Diagnostic features

## Identification Tellurobismuthite can be identified by its tin-white color, strong metallic luster, very low hardness (1.5-2), and perfect cleavage, which causes it to break into thin, flexible flakes. It is very dense and heavy for its size. ## Distinguishing from Similar Minerals It can be confused with other bismuth tellurides (e.g., tetradymite, wehrlite) or other minerals with a metallic luster and platy habit, such as molybdenite or graphite. It differs from graphite by its significantly higher density and color. From molybdenite, it is lighter and more silvery. Accurate identification from other bismuth tellurides often requires chemical analysis (EDS/XRD), as their physical properties are very similar. ## Crystal Forms Crystals are rare, usually appearing as thin, hexagonal tablets. Most often, it occurs in platy, scaly, or granular aggregates, as well as inclusions in other minerals.

Geological environment

## Genesis Tellurobismuthite is a hydrothermal mineral, forming in medium- to high-temperature ore veins. It forms in environments rich in tellurium and bismuth, often in association with gold. ## Mineral Associations It often co-occurs with native gold, gold tellurides (e.g., calaverite, sylvanite), other bismuth tellurides (tetradymite), pyrite, chalcopyrite, galena, sphalerite, and quartz. ## Localities Key global localities include: the Little Mildred mine and others in the Boulder County area, Colorado (USA), where it was first described; the Boliden mine in Sweden; the Kalgoorlie region in Western Australia; the Orijärvi mine in Finland; Hunan Province in China; as well as the vicinity of Nagy-Börzsöny in Hungary and the Facebanya mine (Baia de Arieș) in Romania.

Rarity

Rare

For collectors

## Quality Criteria The most valuable specimens for collectors are those with well-formed, sharp crystals with a hexagonal outline, which is extremely rare. Rich, platy aggregates with a clean, shiny surface, free of oxidation, are also highly prized. Specimens where tellurobismuthite is clearly visible on the rock matrix or, rarely, co-occurs with visible native gold crystals, are particularly sought after. ## Popular Localities Classic and most valued specimens come from historical localities in Boulder County, Colorado, USA. Specimens from the Boliden mine in Sweden and from Kalgoorlie, Australia, are also known for their good quality.

Care and storage

## Cleaning Tellurobismuthite specimens should be cleaned with the utmost care due to their softness and perfect cleavage. It is best to use compressed air to remove dust. If necessary, a very soft brush can be used. Avoid water and any liquids, which can accelerate oxidation. ## What to Avoid Avoid contact with hard objects that can easily scratch or damage the delicate flakes of the mineral. The mineral is sensitive to acids. Prolonged exposure to humid air can cause tarnishing and dulling of the surface. Protect from sudden temperature changes. ## Storage Tellurobismuthite is best stored in closed, padded collector boxes to protect it from mechanical damage, dust, and moisture. Due to its softness, it should not be stored in contact with other, harder minerals.

External references

Sources

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