Journal of


  (Formerly Journal of the Czech Geological Society)

Original paper

Jakub Plášil, Anthony R. Kampf, Jiří Sejkora, Jiří Čejka, Radek Škoda, Jaromír Tvrdý

Horákite, a new hydrated bismuth uranyl-arsenate-phosphate mineral from Jáchymov (Czech Republic) with a unique uranyl-anion topology

Journal of Geosciences, volume 63 (2018), issue 3, 265 - 276


Horákite, ideally (Bi7O7OH)[(UO2)4(PO4)2(AsO4)2(OH)2]·3.5H2O, is a new uranyl mineral discovered on a specimen originating from Jáchymov, Czech Republic (most probably from the Geister vein, Rovnost mine). It occurs as a supergene alteration mineral in association with phosphuranylite (overgrowing older metatorbernite-metazeunerite) in a quartz gangue with abundant tennantite. Horákite forms greenish-yellow to pale yellow prismatic crystals clustering to acicular aggregates, up to 1 mm across. Crystals are transparent to translucent with a vitreous luster. The mineral has a light yellow streak. Estimated Mohs’ hardness is ~2. The cleavage is perfect on {100}. The calculated density is 6.358 g/cm3. Horákite is optically biaxial (+), α ≈ 1.81, β ≈ 1.84, γ ≈ 1.88 (measured in white light); 2Vobs. is 78(1)°, 2Vcalc. is 83°; non-pleochroic. The optical orientation is X = b, Zc. Electron-microprobe analysis yielded the empirical formula (Bi7.01Pb0.14)O7OH[(U1.01O2)4(P1.03O4)2(As0.74Si0.23O4)2(OH)2]·3.5H2O based on 37.5 O apfu. Horákite is monoclinic, C2/c, a = 21.374(2), b = 15.451(3), c = 12.168(2) Å, β = 122.26(1)° and V = 3398.1(10) Å3, Z = 4. The eight strongest X-ray powder-diffraction lines are [dobs Å(I)(hkl)]: 11.77(100)(110), 6.21(23)(-202), 5.55(23)(310, -112), 4.19(27)(-331), 3.54(61)( 510, -423), 3.29(20)( 331), 3.14(58)(241, 023) and 3.02(98)(150, 113, -533, mult.). The crystal structure refinement of horákite, refined to R = 5.95 % for 1774 unique observed reflections, revealed a novel sheet structure. It consists of topologically unique [(UO2)4(PO4)2(AsO4)2(OH)2] sheets (i.e., horákite topology), and an interstitial {(Bi7O7OH)(H2O)3.5} complex. Sheets result from the polymerization of UO7 bipyramids by sharing edges to form tetrameric units; tetrahedrally coordinated sites are linked to the UO7 both monodentately (T1 to U1) and bidentately (T2 to U2). The mineral is named after František Horák (1882-1919), the mining engineer in Jáchymov, and his grandson, Vladimír Horák (born 1964), an amateur mineralogist and expert on the mining history of the Jáchymov ore district.

Journal of Geosciences, Published by © Czech Geological Society, with support from the Czech Geological Survey.
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