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

Pierre Alain Wülser, Nicolas Meisser, Joël Brugger, Kurt Schenk, Stefan Ansermet, Michel Bonin, François Bussy

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Abstract

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 (Pb0.89 Sr0.12)∑=1.01(U+4 0.79 U+6 0.30)∑=1.09 (Fe+2 1.9Zn0.09)∑=2.00 (Ti11.80Fe+2 3.44Fe+3 2.33V+5 0.19Mn0.08 Al0.07)∑=17.90[O35.37 (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(l) 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).

Original languageEnglish
Pages (from-to)933-942
Number of pages10
JournalEuropean Journal of Mineralogy
Volume17
Issue number6
DOIs
Publication statusPublished - 1 Nov 2005
Externally publishedYes

Keywords

  • Cleusonite
  • Crichtonite group
  • Crystal morphology
  • New mineral
  • Swiss Alps
  • Uranium oxidation state

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