Avicennite

Chemical formula: Tl³⁺₂O₃

Avicennite is a very rare thallium oxide, forming small, black, metallic grains and coatings, found in the oxidation zones of hydrothermal deposits.

## Characteristics Avicennite is a thallium(III) oxide, occurring extremely rarely in nature. It typically forms very small, individual grains, less than a millimeter in size, as well as thin coatings and crusts on other minerals. Its crystals, observed only under laboratory conditions, are cubic. In nature, it is most often found as granular aggregates or earthy masses, strongly cemented with the surrounding rock (most commonly calcite or limonite). ## Physical Properties This mineral is characterized by a high density, approximately 8.9 g/cm³, which is palpable even in small samples. It has a hardness in the range of 2.5-3 on the Mohs scale, meaning it is relatively soft. It exhibits a strong, metallic luster and is completely opaque. It leaves a black or grayish-black streak. ## Colors and Varieties Avicennite has a uniform, black or grayish-black color. Due to its rarity and specific chemical composition, no color or commercial varieties are distinguished for it. ## History and Name The mineral was discovered in 1958 in a metal ore deposit in Dzhuzumli, Samarkand region, Uzbekistan. Its name commemorates the Persian scholar and philosopher Avicenna (Ibn Sina), who lived and worked in the territories of modern-day Uzbekistan in the 10th and 11th centuries. The discovery was made and described by the Soviet mineralogist K. N. Karpova. ## Uses Due to its extreme rarity, avicennite has no industrial applications. Its significance is purely scientific and collectible, being an object of desire for specialized collectors of rare minerals.

Properties

Mohs hardness: 1.5-2.5
Color: Greyish black with brownish black tint
Luster: Metallic
Streak: Greyish black, black with a brownish tint
Density: 8.9
Cleavage: None
Fracture: Irregular/Uneven,Hackly,Conchoidal
Transparency: Opaque
Crystal system: Isometric

Diagnostic features

## Identification Characteristic features of avicennite include its black color, strong metallic luster, black streak, and very high density. It occurs as fine grains or coatings, most often in association with calcite and iron oxides (limonite). Final identification requires advanced analytical methods, such as X-ray microanalysis (EDS/WDS). ## Distinguishing from Similar Minerals It can be confused with other black, metallic oxide minerals, such as magnetite, hematite, or pyrolusite. Avicennite differs from them by its significantly higher density, lack of magnetism (unlike magnetite), and characteristic co-occurrence with other thallium minerals in a specific geological environment. Its streak is black, whereas hematite has a reddish-brown streak. ## Crystal Forms Natural, well-formed crystals are practically nonexistent. This mineral primarily forms granular or earthy aggregates, or occurs as thin coatings and crusts on other minerals.

Geological environment

## Genesis Avicennite is a secondary mineral, formed in the oxidation zones of low-temperature, hydrothermal deposits rich in thallium. It forms as a result of the weathering of primary thallium-bearing sulfides and sulfosalts, under strongly oxidizing and alkaline conditions (high pH). ## Mineral Associations It most commonly co-occurs with calcite, dolomite, limonite (goethite), as well as other rare thallium minerals such as carlinite (Tl₂S), lorandite (TlAsS₂), or pierrotite (Tl(As,Sb)₅S₈). Its presence is a strong indicator of thallium mineralization in a given deposit. ## Localities The most important and classic occurrences of this mineral are in Uzbekistan (Dzhuzumli deposit, Samarkand region - type locality) and in the USA, Nevada (Carlin mine, Eureka County), where it occurs in Carlin-type gold deposits. Its presence has also been confirmed in China (La'erma deposit, Gansu province).

Rarity

Very rare

For collectors

## Quality Criteria The collector appeal of avicennite primarily depends on the abundance of its occurrence on the rock matrix. The most prized specimens are those where numerous, shiny, black grains or rich coatings are visible, contrasting with a lighter background (e.g., white calcite). Due to its microscopic nature, specimens with aggregates visible to the naked eye are rare. Analytical confirmation of the mineral's authenticity significantly increases its value. ## Popular Localities The vast majority of specimens available on the collector's market come from Carlin-type gold mines in Nevada (USA). Specimens from the type locality in Uzbekistan are extremely rare and practically unavailable commercially.

Care and storage

## Cleaning Avicennite specimens are generally stable, but due to their small size and fragility, mechanical cleaning is not recommended. If necessary, compressed air can be used to remove dust. Contact with water should be kept to a minimum. ## What to Avoid Avicennite, as a thallium compound, is a toxic substance. Inhaling dust and direct skin contact should be avoided. Hands should be thoroughly washed after any contact with the mineral. It should not be heated or exposed to acids, which can dissolve it, releasing toxic thallium compounds. ## Storage It is recommended to store specimens in tightly sealed, labeled containers or display cases, away from children and pets. The label should include a warning about the mineral's toxicity. Display should be in a closed cabinet.