Cleusonite, (Pb,Sr)(U4+,U6+) (Fe2+,Zn)2 (Ti,Fe2+,Fe3+)18 (O,OH)38, is a new member of the crichtonite group. 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. It occurs as black opaque cm-sized tabular crystals with a bright sub-metallic lustre. The crystals consist of multiple rhombohedra and hexagonal prisms that are generally twinned. Measured density is 4.74(4) g/cm3 and can be corrected to 4.93(12) g/cm3 for macroscopic swelling due to radiation damage; the calculated density varies from 5.02(6) (untreated) to 5.27(5) (heat-treated crystals); the difference is related to the cell swelling due to the metamictisation. The empirical formula for cleusonite from Cleuson is (Pb0.89Sr0.12)Σ = 1.01 (U+40.79U+60.30)Σ = 1.09 Fe+32.33V+50.19Mn0.08 Al0.07)Σ = 17.90 [O35.37(OH)2.63]Σ = 38. Cations were measured by electron microprobe, the presence of structural (OH) was confirmed by infrared spectroscopy and the U6+/U4+ and Fe2+/Fe3+ ratios were determined by X-ray photoelectron spectroscopy. Cleusonite is partly metamict, and untreated crystals only show three major X-ray diffraction peaks. Because of this radiation-damaged state, the mineral appears optically isotropic and shows a light-grey to white colour in reflected polarized light. Cleusonite is trigonal, space group R3, and unit-cell parameters are varying from a = 10.576(3), c = 21.325(5) Å (untreated crystal) to a = 10.4188(6), c = 20.942(1) Å (800°C treatment) and to a = 10.385(2), c = 20.900(7) Å (1000°C treatment). The three cells give a common axial ratio 2.01(1), which is identical to the measured morphological one 2.04(6). The name cleusonite also applies to the previously described “uranium-rich senaite” from Alinci (Macedonia) and the “plumbodavidite” from Huanglongpu (China).

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