Wilkmanite
Chemical formula: Ni₃Se₄
Wilkmanite is a rare nickel selenide, forming metallic, brownish-white grains in association with other selenides.
Properties
- Mohs hardness
- 2.5
- Color
- Yellow-gray
- Luster
- Metallic
- Streak
- Black
- Density
- 0
- Cleavage
- None
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Amateur identification of wilkmanite is practically impossible. It requires advanced laboratory methods, such as chemical composition analysis (EDS/WDS) and X-ray diffraction (XRD). Preliminary identification in a polished section under a microscope is based on its brownish-white color with a pinkish tint, anisotropy, and co-occurrence with other nickel and cobalt selenides. ## Distinguishing from similar minerals Wilkmanite is very similar to other selenides, such as berzelianite, umangite, or clausthalite, and especially to other nickel selenides like mäkinenite or sederholmite. Distinguishing it from these minerals is only possible through chemical and crystallographic analysis. ## Crystal forms This mineral forms irregular, anhedral grains and aggregates. Well-formed crystals have not been observed.
Geological environment
## Genesis Wilkmanite forms under hydrothermal conditions, in calcite-uraninite veins cutting metamorphic rocks, such as black shales. Its crystallization is associated with selenium mineralization processes at low to medium temperatures. ## Mineral associations This mineral co-occurs with other, often equally rare, selenides. Its typical associated minerals include: clausthalite, berzelianite, umangite, klockmannite, eucairite, uraninite, hematite, and calcite, in which it often forms inclusions. ## Localities The only confirmed and described occurrence of wilkmanite in the world is its type locality: the Kuusamo uranium deposit in the Kitka area, near the town of Oulanka in Finland.
Rarity
Extremely rare
For collectors
## Quality criteria The quality of a wilkmanite specimen is determined almost exclusively by its presence and identifiability. Since it occurs as microscopic grains, the most important criterion is the abundance of these grains in the rock matrix and their association with other rare selenides. Specimens where wilkmanite is visible (even under a microscope) and unambiguously identified are extremely valuable. ## Popular localities The only source of specimens is the type locality in Kuusamo, Finland. Material from this location is extremely difficult to obtain and practically does not appear on the collector's market.
Care and storage
## Cleaning Wilkmanite specimens, being extremely rare and usually microscopic, practically do not require cleaning. If absolutely necessary, compressed air can be used to remove dust. Contact with water and chemicals should be avoided. ## What to avoid All acids and chemical agents that may react with selenides should be avoided. The mineral is soft and brittle, so it should be protected from impacts, scratching, and vibrations. It should not be heated or subjected to ultrasound. ## Storage Specimens containing wilkmanite should be stored under stable conditions, in sealed "micromount" containers, to protect them from dust, moisture, and mechanical damage. It is best to keep them away from minerals that may oxidize, to avoid undesirable chemical reactions.