Vysokyite, U~(4+)[AsO_2(OH)_2]_4·4H_2O, a new mineral from Jachymov, Czech Republic

摘要

Vysokyite, U~(4+)[AsO_2(OH)_2]_4 (IMA 2012-067), was found growing on an altered surface of massive native As in the Geschieber vein, Jachymov ore district, Western Bohemia, Czech Republic. The new mineral was found in association with behounekite, stepite, kaatialaite, arsenolite, claudetite and gypsum. It forms extremely fibrous light-green crystals up to 8 mm long. Crystals have an alabaster lustre and a greenish-white to greyish streak. Vysokyite is brittle with uneven fracture and perfect cleavage along (100) and (001); the Mohs hardness is ~2. A density of 3.393 g/cm~3 was calculated using the empirical formula and unit-cell parameters obtained from a single-crystal diffraction experiment. Vysokyite is non-fluorescent under short or long wavelength UV radiation. It is colourless under the microscope, measured refractive indices are α' = 1.617(3), γ' = 1.654(3); the estimated optical orientation is α' ~ X, γ' ~ Z. The average of five spot wavelength dispersive spectroscopy (WDS) analyses is 29.44 UO_2, 1.03 SiO_2, 48.95 As_2O_5, 0.12 SO_3, 15.88 H_2O (calc.), total 95.42 wt.%. The empirical formula of vysokyite (based on 20 O a.p.f.u.) is U_(100)[AsO_2(OH)_2]_(3.90)(SiO_4)_(0.16) (SO_4)_(0.01)·4H_2O. The As—O—H and O—H vibrations dominate in the Raman spectrum. Vysokyite is triclinic, space group P1, with a = 10.749(2), b = 5.044(3), c = 19.1778(7) A, α = 89.872(15)°, β = 121.534(15)°, γ = 76.508(15)°, and V = 852.1(6) A~3, Z = 2 and D_(calc) = 3.34 g·cm~(-3). The strongest diffraction peaks in the X-ray powder diffraction pattern are [d_(obs) in A(I_(rel.))(hkl)]: 8.872(100)(100), 8.067(50)(002), 6.399(7)(103), 4.773(6)(104), 3.411(10)(302), 3.197(18)(313). The crystal structure of vysokyite was solved from single-crystal X-ray diffraction data by the charge-flipping method and refined to R_1 = 0.0595 based on 2718 unique observed reflection, and to wR_2 = 0.1160 for all 4173 unique reflections. The structure of vysokyite consists of UO_8 square antiprisms sharing all of their vertices with 8 As-tetrahedra to form infinite chains parallel to [010]. These chains are linked by hydrogen bonds involving terminal (OH) groups of the double-protonated As-tetrahedra and molecules of H_2O located between the chains. The new mineral is named in honour of Arnost Vysoky (1823—1872), the former chief of the Jachymov mines and smelters, chemist and metallurgist.