Behounekite, U(SO4)2(H2O)4, from Jachymov (St Joachimsthal), Czech Republic: the first natural U4+ sulphate

Behounekite, orthorhombic U(SO4)(2)(H2O)(4), is the first natural sulphate of U4+. It was found in the Geschieber vein, Jachymov (St Joachimsthal) ore district, Western Bohemia, Czech Republic, crystallized on the altered surface of arsenic and associated with kaatialaite, arsenolite, claudetite, unnamed phase UM1997-20-AsO:HU and gypsum. Behounekite most commonly forms short-prismatic to tabular green crystals, rarely up to 0.5 mm long. The crystals have a strong vitreous lustre and a grey to greenish grey streak. They are brittle with an uneven fracture and have very good cleavage along 1100:. The Mohs hardness is about 2. The mineral is not fluorescent either in short- or long-wavelength UV radiation. Behounekite is moderately pleochroic, alpha similar to beta is pale emerald green and gamma is emerald green, and is optically biaxial (+) with alpha = 1.590(2), beta = 1.618(4), gamma = 1.659(2) (590 nm), 2V (calc.) = 81 degrees, birefringence 0.069. The empirical formula of behounekite (based on 12 0 atoms, from an average of five point analyses) is (U0.99Y0.03)(Sigma 1.02)(SO4)(1.97)(H2O)(4). The simplified formula is U(SO4)(2)(H2O)(4), which requires UO2 53.77, SO3 31.88, H2O 14.35, total 100.00 wt.%. Behounekite is. orthorhombic, space group Puma, a = 14.6464(3), b = 11.0786(3), c = 5.6910(14) angstrom, V = 923.43(4) angstrom(3), Z = 4, D-calc = 3.62 g cm(-3). The seven strongest diffraction peaks in the X-ray powder diffraction pattern are [d(obs) in angstrom (I) (hkl)]: 7.330 (100) (200), 6.112 (54) (210), 5.538 (21) (020), 4.787 (42) (111), 3.663 (17) (400), 3.478 (20) (410), 3.080 (41) (321). The crystal structure of behounekite has been solved by the charge-flipping method from single-crystal X-ray diffraction data and refined to R-1 = 2.10 % with a GOF = 1.51, based on 912 unique observed diffractions. The crystal structure consists of layers built up from [8]-coordinate uranium atoms and sulphate tetrahedra. The eight ligands include four oxygen atoms from the sulphate groups and four oxygen atoms from the H2O molecules. Each uranium coordination polyhedron is connected via sulphate tetrahedra with other uranium polyhedra and through hydrogen bonds to the apices of sulphate tetrahedra. The dominant features of the Raman and infrared spectra of behounekite are related to stretching vibrations of SO4 tetrahedra (similar to 1200-950 cm(-1)), O-H stretching modes (similar to 3400-3000 cm(-1)) and H-O-H bending modes (similar to 1650 cm(-1)). The mineral is named in honour of Frantisek Behounek, a well known Czech nuclear physicist.