Watkinsonite

Chemical formula: Pb₂+Cu₁+₂Bi₃+₄(Se₂-,S₂-)₈

Watkinsonite is a very rare copper, lead, and bismuth selenide, forming microscopic, metallic grains in hydrothermal veins.

## Characteristics Watkinsonite is a rare mineral from the sulfosalt group, specifically a complex copper, lead, and bismuth selenide, in which selenium can be partially replaced by sulfur. It occurs as very small, anhedral (irregular) grains or lamellar crystals, reaching sizes up to 0.2 mm. It typically forms intergrowths with other similar selenic minerals. In appearance, it is inconspicuous, with a gray or grayish-black color and a metallic luster. ## Physical Properties This mineral is very soft, with a Mohs hardness of only 2, meaning it can be scratched with a fingernail. It is characterized by a strong, metallic luster and is completely opaque. Despite the small size of its crystals, watkinsonite has a very high density, approximately 7.79 g/cm³, which is typical for minerals containing lead and bismuth. ## Colors and Varieties Watkinsonite is monochromatic – it occurs in shades of gray, from steel-gray to grayish-black. No colored varieties or commercial forms are known. ## History and Name The mineral was approved by the IMA in 1986, and its description was published in 1987. The name comes from David H. Watkinson (b. 1937), a professor of geology at Carleton University in Ottawa (Canada), in recognition of his contributions to the study of the mineralogy of the Tanco pegmatite in Canada, one of the localities where this mineral occurs. ## Uses Due to its extreme rarity and microscopic size, watkinsonite has no industrial applications. It is solely an object of scientific interest and is valued in advanced systematic and micromineral collections.

Properties

Mohs hardness: 2
Luster: Metallic
Streak: Black
Density: 7.79
Cleavage: Good on {010}
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Identifying watkinsonite based on visual features is practically impossible, even under a microscope. Its appearance is very similar to many other gray, metallic sulfosalts and selenides. Positive identification requires advanced analytical techniques, such as electron microprobe analysis (EDS/WDS) or X-ray diffraction (XRD). ## Distinguishing from Similar Minerals Watkinsonite can be confused with minerals with which it often coexists and forms intergrowths, such as laitakarite, weibullite, padmaite, bismuthinite, or galena. All these minerals have a similar gray color and metallic luster. Distinguishing them from each other is only possible based on precise chemical and structural analysis. ## Crystal Forms This mineral forms very small, anhedral (irregular, without developed faces) grains or lamellar and tabular crystals. It almost always occurs in the form of complex intergrowths with other minerals.

Geological environment

## Genesis Watkinsonite forms under hydrothermal conditions, in selenium-rich and sulfur-poor environments. It typically forms in calcite-quartz veins cutting various rock types. ## Mineral Associations This mineral coexists with other rare selenides, sulfides, and sulfosalts. Its most common associated minerals include: laitakarite, weibullite, wittite, padmaite, native bismuth, bismuthinite, galena, chalcopyrite, and pyrite. The gangue minerals are calcite and quartz. ## Localities The most important confirmed localities of this mineral worldwide are: - Eskeborn in the Harz Mountains, Saxony-Anhalt, Germany – this is the type locality. - Tanco Pegmatite at Bernic Lake, Manitoba, Canada. - Bytíz uranium deposit near Příbram in the Czech Republic.

Rarity

Very rare

For collectors

## Quality Criteria The quality of a watkinsonite specimen, being a micromineral, is assessed differently than for large crystals. The most important criterion is the abundance of the mineral on the matrix sample and the presence of well-defined, albeit microscopic, crystalline forms. Specimens with analytically confirmed identity, originating from the type locality or other classic occurrences, are highly valued. Association with other rare minerals also increases its value. ## Popular Localities Collector specimens come almost exclusively from a few known localities worldwide. The most prized samples come from the Eskeborn mine in Germany and the Tanco pegmatite in Canada, which have provided the best-studied materials.

Care and storage

## Cleaning Watkinsonite specimens are almost exclusively microminerals, which generally do not require cleaning. Any mechanical intervention (e.g., brushing) is highly risky and can damage delicate crystals. If absolutely necessary, the specimen can be carefully rinsed in distilled water and then dried non-contact with compressed air. ## What to Avoid The mineral is extremely soft (Mohs hardness 2) and susceptible to scratching. Avoid contact with harder objects and all chemicals, especially acids. As a selenide, it may be susceptible to slow oxidation, so it is recommended to avoid humid environments. ## Storage The safest storage method is to place the specimen in a sealed "micromount" box, which protects it from dust, moisture, and mechanical damage. Avoid exposure to direct sunlight and sudden temperature changes.

Sources

IMA Mineral List (CNMNC)