Weibullite
Chemical formula: Ag<sup>1+</sup><sub>0.3</sub>Pb<sup>2+</sup><sub>5.3</sub>Bi<sup>3+</sup><sub>8.3</sub>(S<sup>2-</sup>,Se<sup>2-</sup>)<sub>18</sub>
Weibullite is a rare lead, bismuth, and silver sulfoselenide, forming acicular, metallic inclusions in other minerals.
Properties
- Mohs hardness
- 2.5
- Luster
- Metallic
- Streak
- Black
- Density
- 6.9-7.13
- Cleavage
- Good on {010}
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Weibullite is recognized by its characteristic form – acicular or fibrous crystals with a metallic luster and steel-gray color. It often occurs as inclusions in calcite or quartz. Its softness and black streak are also important diagnostic features. However, final confirmation requires advanced methods such as chemical analysis (EDS) or X-ray diffraction (XRD). ## Distinguishing from Similar Minerals It can be confused with other acicular sulfosalts, such as jamesonite, boulangerite, or bismuthinite. Bismuthinite is usually more flexible. Jamesonit and boulangerite often form tangled, felt-like aggregates. Precise differentiation of these minerals based on visual characteristics is very difficult and often impossible without specialized studies. ## Crystal Forms Weibullite crystals are elongated, acicular, or prismatic, often with visible longitudinal striations. They usually form disordered or radial aggregate inclusions within the host rock.
Geological environment
## Genesis Weibullite is a hydrothermal mineral. It forms in ore veins, usually in association with other sulfides and bismuth and lead sulfosalts. Its formation is associated with metasomatic processes, as in the case of skarns at the type locality. ## Mineral Associations This mineral co-occurs with minerals such as cobaltite, galena, bismuthinite, chalcopyrite, quartz, and calcite. At the type locality (Falun, Sweden), it was found in association with cobaltite and chalcopyrite. ## Localities The most important and classic locality for weibullite is the Falun mine in the Dalarna region of Sweden. Other confirmed localities include the Baita Bihorului mine in Romania and Stan Terzian in North Macedonia.
Rarity
Very rare
For collectors
## Quality Criteria The most valued specimens by collectors are those with well-formed, sharp, and clearly visible weibullite needles, contrasting with a light background, e.g., white calcite or quartz. Rich, radial aggregates enhance their attractiveness. Precise locality information is also important, especially if the specimen comes from the type locality in Falun. ## Popular Localities Undoubtedly, the most sought-after and historically significant specimens come from the Falun mine in Sweden. Specimens from other localities, such as Romania, are also valued, but those from Sweden hold the greatest importance for collectors specializing in mineral systematics.
Care and storage
## Cleaning Weibullite specimens should be cleaned very carefully, preferably using compressed air to remove dust. Due to its softness, all mechanical contact, including brushing, should be avoided. If liquid is necessary, pure distilled water or isopropyl alcohol, applied with great gentleness, is permissible. ## What to Avoid Contact with acids and other aggressive chemicals that can damage the mineral should be avoided. As a sulfosalt, it may be sensitive to moisture and oxidation, so it should be protected from prolonged exposure to humid air. It is soft and brittle, so it must be protected from impacts and scratching. ## Storage Weibullite specimens, especially those with delicate, acicular crystals, are best stored in sealed "micromount" boxes, which protect them from dust, moisture, and mechanical damage. It is not sensitive to light, but stable environmental conditions are recommended.