Cleusonite, (Pb,Sr)(U (super 4+) ,U (super 6+) )(Fe (super 2+) ,Zn) (sub 2) (Ti,Fe (super 2+) ,Fe (super 3+) ) (sub 18) (O, OH) (sub 38) , a new mineral species of the crichtonite group from the western Swiss Alps
Cleusonite, (Pb,Sr)(U (super 4+) ,U (super 6+) )(Fe (super 2+) ,Zn) (sub 2) (Ti,Fe (super 2+) ,Fe (super 3+) ) (sub 18) (O, OH) (sub 38) , a new mineral species of the crichtonite group from the western Swiss Alps (in Micro- and mesoporous mineral phases, S. Merlino (editor))
European Journal of Mineralogy (December 2005) 17 (6): 933-942
Cleusonite, (Pb,Sr)(U (super 4+) ,U (super 6+) )(Fe (super 2+) ,Zn) (sub 2) (Ti,Fe (super 2+) ,Fe (super 3+) ) (sub 18) (O,OH) (sub 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/cm (super 3) and can be corrected to 4.93(12) g/cm (super 3) 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 (sub 0.89) Sr (sub 0.12) ) (sub Sigma =1.01) (U (super +4) (sub 0.79) U (super +6) (sub 0.30) ) (sub Sigma =1.09) (Fe (super +2) (sub 1.91) Zn (sub 0.09) ) (sub Sigma =2.00) (Ti (sub 11.80) Fe (super +2) (sub 3.44) Fe (super +3) (sub 2.33) V (super +5) (sub 0.19) Mn (sub 0.08) Al (sub 0.07) ) (sub Sigma =17.9 0 ) [O (sub 35.37) (OH) (sub 2.63) ] (sub Sigma =38) . Cations were measured by electron microprobe, the presence of structural (OH) was confirmed by infrared spectroscopy and the U (super 6+) /U (super 4+) and Fe (super 2+) /Fe (super 3+) 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) A (untreated crystal) to a = 10.4188(6), c = 20.942(1) A (800 degrees C treatment) and to a = 10.385(2), c = 20.900(7) A (1000 degrees 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).