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/cm(3)
and can be corrected to 4.93(12) g/cm(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 (Pb0.89Sr0.12)(Sigma=1.01)
(U0.79+4U0.30+6)(Sigma=1.09) (Fe1.91+2Zn0.09)(Sigma=2.00)
(Ti11.80Fe3.44+2Fe2.33+3V0.19+5Mn0.08Al0.07)(Sigma=17.90)
[O-35.37(OH)(2.63)](Sigma=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 R $(3) over bar $, and
unit-cell parameters are varying from a = 10.576(3), c = 21.325(5)
angstrom (untreated crystal) to a = 10.4188(6), c = 20.942(1) angstrom
(800 degrees C treatment) and to a = 10.385(2), c = 20.900(7) angstrom
(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).
ne name cleusonite also applies to the previously described
``uranium-rich senaite'' from Alinci (Macedonia) and the
``plumbodavidite'' from Huanglongpu (China).