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Horakite, ideally (Bi7O7OH)[(UO2)4(PO4)2(AsO4)2(OH)2]·3.5H2O, is a new uranyl mineral discovered on a specimen originating from Jachymov, Czech Republic (most probably from the Geister vein, Rovnost mine). It occurs as a su-pergene alteration mineral in association with phosphuranylite (overgrowing older metatorbernite–metazeunerite) in a quartz gangue with abundant tennantite. Horakite 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. Horakite is optically biaxial (+), alpha =approx 1.81, beta =approx 1.84, gamma =approx 1.88 (measured in white light); 2Vobs. is 78(1)°, 2Vcalc. is 83°; non-pleochroic. The optical orientation is X = b, Z =approx.c. 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. Horakite is monoclinic, C2/c, a = 21.374(2), b = 15.451(3), c = 12.168(2) A, beta = 122.26(1)° and V = 3398.1(10) A3, Z = 4. The eight strongest X-ray powder-diffraction lines are [dobs A(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 horakite, 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., horakite 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 Frantisek Horak (1882–1919), the mining engineer in Jachymov, and his grandson, Vladimir Horak (born 1964), an amateur mineralogist and expert on the mining history of the Jachymov ore district.

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