• Machovic, V.
• Novák, M.
• Majzlan, Juraj
• Hloušek, J.
• Čejka, J.
• Fejfarová, K.
• Sejkora, J.
• Škoda, R.
• Plášil, J.
• Talla, D.
• Dušek, M.

Abstract

<jats:title>Abstract</jats:title><jats:p>Bêhounekite, orthorhombic U(SO<jats:sub>4</jats:sub>)<jats:sub>2</jats:sub>(H<jats:sub>2</jats:sub>O)<jats:sub>4</jats:sub>, is the first natural sulphate of U<jats:sup>4+</jats:sup>. It was found in the Geschieber vein, Jáchymov (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. Bêhounekite 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 ﹛100﹜. The Mohs hardness is about 2. The mineral is not fluorescent either in short- or long-wavelength UV radiation. Bêhounekite is moderately pleochroic, α∼β is pale emerald green and γ is emerald green, and is optically biaxial (+) with<jats:italic>α =</jats:italic>1.590(2), β = 1.618(4), γ = 1.659(2) (590 nm), 2V (calc.) = 81°, birefringence 0.069. The empirical formula of bêhounekite (based on 12 O atoms, from an average of five point analyses) is (U<jats:sub>0.99</jats:sub>Y<jats:sub>0.03</jats:sub>)<jats:sub>Σ1.02</jats:sub>(SO<jats:sub>4</jats:sub>)<jats:sub>1.97</jats:sub>(H<jats:sub>2</jats:sub>O)<jats:sub>4</jats:sub>. The simplified formula is U(SO<jats:sub>4</jats:sub>)<jats:sub>2</jats:sub>(H<jats:sub>2</jats:sub>O)<jats:sub>4</jats:sub>, which requires UO<jats:sub>2</jats:sub>53.77. SO<jats:sub>3</jats:sub>31.88, H<jats:sub>2</jats:sub>O 14.35, total 100.00 wt.%. Bêhounekite is orthorhombic, space group<jats:italic>Pnma, a =</jats:italic>14.6464(3),<jats:italic>b =</jats:italic>11.0786(3), c = 5.6910(14) Å,<jats:italic>V =</jats:italic>923.43(4) Å<jats:sup>3</jats:sup>,<jats:italic>Z =</jats:italic>4, D<jats:sub>calc</jats:sub>= 3.62 g cm<jats:sup>–3</jats:sup>. The seven strongest diffraction peaks in the X-ray powder diffraction pattern are<jats:italic>[d<jats:sub>obs</jats:sub></jats:italic>in Å (I)<jats:italic>(hid)]:</jats:italic>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 bêhounekite has been solved by the charge-flipping method from single-crystal X-ray diffraction data and refined to<jats:italic>R<jats:sub>1</jats:sub>=</jats:italic>2.10 % with a<jats:italic>GOF =</jats:italic>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 H<jats:sub>2</jats:sub>O 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 bêhounekite are related to stretching vibrations of SO<jats:sub>4</jats:sub>tetrahedra (∼1200–950 cm<jats:sup>–1</jats:sup>), O-H stretching modes (∼3400–3000 cm<jats:sup>–1</jats:sup>) and H—O—H bending modes (∼1650 cm<jats:sup>–1</jats:sup>). The mineral is named in honour of František Bêhounek, a well known Czech nuclear physicist.</jats:p>

Topics

• impedance spectroscopy
• mineral
• surface
• phase
• x-ray diffraction
• Oxygen
• hardness
• Hydrogen
• space group
• Arsenic
• Uranium
• gypsum