Cleusonite
Chemical formula: Pb<sup>2+</sup>(U<sup>4+</sup>,U<sup>6+</sup>)Fe<sup>2+</sup><sub>2</sub>(Ti<sup>4+</sup>,Fe<sup>2+</sup>,Fe<sup>3+</sup>)<sub>18</sub>(O,OH)<sub>38</sub>
A rare, black oxide mineral from the crichtonite group, found in Alpine fissures and containing lead and uranium.
Properties
- Mohs hardness
- 7-7.5
- Luster
- Metallic
- Streak
- Black
- Density
- 4.75
- Cleavage
- None
- Fracture
- Conchoidal
- Transparency
- Opaque
- Crystal system
- Trigonal
Diagnostic features
## Identification Cleusonite is identified by its characteristic appearance: black, metallic-lustered crystals with a hexagonal outline. Key features also include its occurrence in a specific geological environment (Alpine fissures) and its association with characteristic minerals. Confirmation requires advanced analytical methods, such as chemical composition analysis (EDS/WDS). ## Distinguishing from similar minerals It can be confused with other black, metallic minerals from the crichtonite group, such as senaite, crichtonite, or davidite. Distinguishing it from these is practically impossible without specialized chemical analyses. Visually, it may also resemble ilmenite, hematite, or magnetite, but it differs from them in crystal habit and paragenesis. ## Crystal forms It most commonly forms thin, tabular crystals with a hexagonal outline, less often short, prismatic ones. Crystals are usually small, not exceeding a few millimeters.
Geological environment
## Genesis Cleusonite is a hydrothermal mineral. It forms in the late stage of crystallization in open Alpine-type fissures cutting gneisses and crystalline schists. It crystallizes under low-temperature conditions from solutions rich in titanium, iron, lead, and uranium. ## Mineral associations It most commonly co-occurs with quartz, adularia, chlorite, hematite, rutile, anatase, brookite, monazite-(Ce), xenotime-(Y), and synchysite-(Ce). At its type locality (Cleuson, Switzerland), it was found in association with hematite, quartz, and adularia. ## Localities This is a very rare mineral, known from only a few localities worldwide. Besides the type locality in Cleuson, Switzerland, its occurrence has also been confirmed in the Binn Valley (Binntal) in Switzerland and in the Ulyn-Khoy mountain area in Russian Yakutia.
Rarity
Very rare
For collectors
## Quality criteria The most valued specimens by collectors are those with well-formed, sharp, and undamaged crystals with a strong luster. Crystals set on a contrasting, light matrix, such as white quartz or adularia, are also highly prized. Due to their small size, the quality of the crystal itself, rather than the size of the entire specimen, is crucial. ## Popular localities The most known and valued specimens come from the type locality in Cleuson and from the Binntal valley in Switzerland. These are considered classic examples of this mineral.
Care and storage
## Cleaning Cleaning is recommended only with compressed air or a very soft brush to remove dust. Avoid contact with water and chemical agents, as its reactions to them are unknown. ## What to avoid As a mineral containing lead and uranium, avoid inhaling dust and direct skin contact. Washing hands after each contact with the specimen is mandatory. Protect it from chemicals, ultrasound, and sudden temperature changes. Due to potential light sensitivity, it should not be exposed to prolonged, intense illumination. ## Storage Cleusonite specimens should be stored in closed, stable containers (e.g., "perky boxes"), away from other minerals that it could scratch. Due to its weak radioactivity, it is recommended to store it away from areas of permanent human presence, in a well-ventilated room.