Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Jiří Sejkora, Ian E. Grey, Anthony R. Kampf, Jakub Plášil, Pavel Škácha

Bouškaite, a new molybdenyl-hydrogensulfate mineral, (MoO₂)₂O(SO₃OH)₂(H₂O)₂·2H₂O, from the Lill mine, Příbram ore area, Czech Republic

Journal of Geosciences, volume 64 (2019), issue 3, 197 - 205

DOI: http://doi.org/10.3190/jgeosci.287

Bouškaite, (MoO₂)₂O(SO₃OH)₂(H₂O)₂·2H₂O is a new supergene post-mining mineral from the mine dump of the Lill shaft, the Březové hory ore district (Příbram), Czech Republic, associated with rhomboclase and X-ray amorphous Mo-rich blue minerals. It was found in weathered quartz gangue with disseminated tiny grains of pyrite and a black X-ray amorphous Mo sulphide (likely jordisite). It forms colorless to light beige aggregates of randomly or radially arranged fibers up to 7 mm in length on the surface of quartz gangue fragments. Individual crystals are very narrow blades flattened on {001} and elongated along [100]. Bouškaite has a white to light grey streak, vitreous luster, and does not fluoresce under either short- or long-wave ultraviolet light. Cleavage is perfect on {001}, the Mohs hardness is ~2, and the fracture is uneven. Aggregates of the mineral are very brittle; thin blades (fibers) are somewhat flexible. The measured density is 2.40(2) g/cm³; the calculated density is 2.38 g/cm³. Bouškaite is optically biaxial positive, the indices of refraction are α = 1.504(2), β = 1.605(2), γ = 1.705(3) and 2V_meas. = 82(1)°. The mineral is triclinic, space group P-1, a = 5.581(3), b = 9.572(1), c = 14.425(4) Å, α = 97.43(1)°, β = 100.05(2)°, γ = 89.96(1)°, V = 752.2(5) ų, Z = 2. The seven strongest lines in the X-ray powder diffraction pattern are [d (Å)/I(hkl)]: 14.154/35(001), 7.078/100(002), 5.440/9(10-1), 4.838/7(101), 4.720/56(003), 4.010/7(102) and 3.240/10(10-4). The chemical analyses by electron microprobe yielded CaO 0.08, SiO₂ 0.36, MoO₃ 53.59, SO₃ 29.43, H₂O_calc. 16.71, total 100.17 wt. %. The empirical formula on the basis of 17 anions per formula unit is (MoO₂)_2.00Ca_0.01O(SO₃OH)_1.98(SiO₄)_0.03(H₂O)₂·2H₂O. The unique crystal structure of bouškaite was solved from synchrotron single-crystal diffraction data and refined to R_obs = 0.092 for 499 reflections with I > 3σ(I). The structure of bouškaite is built from (MoO₂)₂O(SO₃OH)₂(H₂O)₂ chains, oriented parallel to [010] and linked to one another by hydrogen bonding to H₂O molecules, Ow3 and Ow4 lying between the adjacent chains. The chains contain triangular clusters consisting of two Mo-centered octahedra and a S1-centred tetrahedron. The triangular clusters are connected by corner-sharing with a S2-centred tetrahedron. The Raman spectrum of bouškaite with tentative band assignments is reported. The mineral is named in honor of the prominent Czech mineralogist and geochemist Prof. Dr. Vladimír Bouška (1933-2000) from Charles University, Prague.