Geographic Minerals

Cleusonite: Properties and Occurrences

Cleusonite: Properties and Occurrences

Cleusonite is a member of the crichtonite group of minerals with a chemical formula of (Pb, Sr)(U4+, U6+)(Fe2+, Zn)(Ti, Fe2+, Fe3+)18(O, OH)38. It was found at two occurrences in greenschist facies metamorphosed gneissic series of the Mont Fort and Siviez-Mischabel Nappes in Valais, Switzerland (Cleuson and Bella Tolla summit), and named after the type locality.

This group of minerals contains approximately thirteen complex metal titanates. The structures of minerals of this group are complicated by frequent fine-scale twinning and metamictization due to radioactive elements. It is radioactive.

General Information

  • Category: Oxide minerals
  • Formula: (Pb, Sr)(U4+, U6+)(Fe2+, Zn)(Ti, Fe2+, Fe3+)18(O, OH)38
  • Crystal system: Trigonal
  • Crystal class: Rhombohedral (3)


Cleusonite is seen as a black opaque cm-sized tabular crystal with a bright sub-metallic luster. It does not have any cleavage and has a density of about 4.74(4) g/cm3. Its crystals are usually metamict black semimetallic prismatic and associated with quartz.

  • Color: Black
  • Crystal habit: Multiple rhombohedra and hexagonal prisms, metamict
  • Cleavage: None
  • Fracture: Conchoidal
  • Tenacity: Brittle
  • Mohs scale hardness: 6-7
  • Luster: Sub metallic
  • Streak: Black
  • Diaphaneity: Opaque
  • Specific gravity: 4.74 (synthetic), ~5.15 (calculated)

Geological occurrence

Cleusonite is found first in metamorphic rocks of the central Swiss Alps with the type locality being Oligocene-Miocene alpine cleft veins near Cleuson, Val de Nendaz, Valais and secondly in gneisses and crosscutting alpine veins near the Bella Tolla summit, also in Valais. In the Bella Tolla summit, it is found in the form of hematite-stained flattened aggregates with quartz, albite, baryte, chalcopyrite, uraninite, tennantite, pyrite, magnetite, cinnabar, and malachite.


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