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Cleusonite, (Pb,Sr) (U⁴⁺, U⁶⁺) (Fe²⁺, Zn)₂ (Ti,Fe²⁺,Fe³⁺)₁₈ (O, OH)₃₈, a new mineral species of the crichtonite group from the western Swiss Alps

¹ Musée Cantonal de Géologie, CH-1015 Lausanne, Switzerland
² Institut de Minéralogie et Géochimie, Université de Lausanne, CH-1015 Lausanne, Switzerland
³ Laboratoire de Cristallographie 1, EPFL, Dorigny, CH-1015 Lausanne, Switzerland
⁴ Cooperative Research Centre for Landscape Environments and Mineral Exploration (CRC-LEME) & School of Earth and Environmental Sciences, The University of Adelaide, North Terrace, Adelaide, AU-5000 Australia
⁵ Division of Mineralogy, South Australian Museum, North Terrace, Adelaide, AU-5000 Australia
⁶ Musée Cantonal d'Histoire Naturelle de Sion, CH-1950 Sion, Switzerland

^* Corresponding author, e-mail: pierre-alain{at}wulser.com

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 (Pb_0.89Sr_0.12) = 1.01 (U⁺⁴_0.79U⁺⁶_0.30) = 1.09 Fe⁺³_2.33V⁺⁵_0.19Mn_0.08 Al_0.07) = 17.90 [O_35.37(OH)_2.63] = 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).

Key-words: cleusonite, new mineral, Swiss Alps, crichtonite group, uranium oxidation state, crystal morphology.