Watanabeite

Chemical formula: Cu₁+₄(As₃+,Sb₃+)₂S₂-₅

Watanabeite is a rare, lead-gray copper, arsenic, and antimony sulfosalt mineral, forming tiny, metallic-lustered grains.

## Characteristics Watanabeite is a sulfosalt mineral, specifically a sulfide of copper, arsenic, and antimony. It occurs as anhedral (lacking well-formed crystal faces) grains up to 0.3 mm in size, often intergrown with other sulfosalts. Its appearance is inconspicuous – it has a lead-gray color and a metallic luster. It typically forms inclusions within other minerals, mainly luzonite and enargite. ## Physical Properties This mineral is characterized by a metallic luster and is opaque. Its Mohs hardness is approximately 3, and its density, calculated based on chemical composition and unit cell parameters, is 4.77 g/cm³. It exhibits no cleavage, and its fracture is uneven. ## Colors and Varieties Watanabeite has a consistent, lead-gray color. In reflected light under a polarizing microscope, it reveals shades from grayish-brown to slightly violet. No varieties have been distinguished. ## History and Name The mineral was discovered in the Teine mine in Hokkaido, Japan. Its name, approved by the IMA in 1991, honors Professor Takeo Watanabe (1907-1986), a distinguished Japanese geologist and petrologist from the University of Tokyo, who made significant contributions to the study of ore deposits in Japan. ## Uses Due to its extreme rarity and occurrence as microscopic grains, watanabeite has no industrial applications. It is solely an object of scientific interest and a collector's item for specialized micromineral collectors.

Properties

Mohs hardness: 3
Luster: Metallic
Streak: Black
Density: 4.77
Cleavage: None
Fracture: Uneven
Transparency: Opaque
Crystal system: Orthorhombic

Diagnostic features

## Identification Identification of watanabeite is possible only through advanced laboratory methods, such as electron microprobe analysis (EDS/WDS) for chemical composition and X-ray diffraction (XRD). Visually, it is indistinguishable from many other sulfosalts. ## Distinguishing from Similar Minerals Watanabeite is most commonly confused with tetrahedrite, tennantite, enargite, luzonite, and other co-occurring sulfosalts. It differs from them by subtle optical properties in reflected light (e.g., weak pleochroism) and, most importantly, by its chemical composition and crystal structure. ## Crystal Forms This mineral forms exclusively very fine, irregular, anhedral grains, usually smaller than 0.3 mm. No well-formed crystals have been observed.

Geological environment

## Genesis Watanabeite forms under hydrothermal conditions in ore veins rich in copper and arsenic. It is a product of crystallization at relatively low temperatures, appearing as one of the later minerals in the crystallization sequence. ## Mineral Associations This mineral most commonly occurs in association with luzonite, enargite, pyrite, tennantite, tetrahedrite, chalcopyrite, barite, and quartz. It often forms intergrowths with luzonite and enargite. ## Localities Beyond its type locality – the Teine mine in Hokkaido, Japan – watanabeite has been identified in several other locations worldwide. Confirmed occurrences include the Sasa mine in North Macedonia and the Uzelga deposit in the Southern Urals, Russia.

Rarity

Very rare

For collectors

## Quality Criteria The quality of a watanabeite specimen is assessed solely within the context of micromineralogy. The most important criterion is the ability to unequivocally identify the mineral grains against the matrix rock. The size of the grains, their abundance in the field of view, and aesthetic associations with other, more colorful minerals (e.g., enargite) enhance the scientific and collector value of the specimen. ## Popular Localities The most classic and sought-after material by specialized collectors is that originating from the type locality – the Teine mine in Japan.

Care and storage

## Cleaning Specimens containing watanabeite, which are typically microminerals within a matrix rock, should not be cleaned mechanically or chemically. The safest method for dust removal is using compressed air or a soft brush. ## What to Avoid Avoid contact with chemicals, acids, and detergents, which can react with sulfides and sulfosalts. The mineral is susceptible to oxidation, so it should be protected from moisture and sudden temperature changes. ## Storage Storing specimens in stable conditions, in a sealed, dry container (e.g., a "micromount" type box), is the best way to preserve them. Avoid direct exposure to sunlight and humid air.

Sources

IMA Mineral List (CNMNC)