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

Journal of Geosciences

Jiří Čejka^3¹  Jiří Sejkora^3²  Jakub Plášil^1³  Anthony R. Kampf^2⁴ 

[1] Department of Mineralogy and Petrology, National Museum, Cirkusová 1740, 193 00 Prague 9, Czech Republic^4;Exposition Boulevard, Los Angeles, CA 90007, USA^3;Institute of Physics, Academy of Sciences of the Czech Republic (ASCR) v.v.i, Na Slovance 2, 182 21 Prague 8, Czech Republic^1;Mineral Sciences Department, Natural History Museum of Los Angeles County, 900^2

关键词: Jáchymov;    crystal structure;    new mineral;    topology;    horákite;    uranyl arsenate;   

DOI  :  10.3190/jgeosci.267

学科分类：自动化工程

来源: Ceska Geologicka Spolecnost / Czech Geological Society

【 摘 要 】

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); 2V_obs. is 78(1)°, 2V_calc. is 83°; non-pleochroic. The optical orientation is X = b, Z ≈ c. Electron-microprobe analysis yielded the empirical formula (Bi_7.01Pb_0.14)O₇OH[(U_1.01O₂)₄(P_1.03O₄)₂(As_0.74Si_0.23O₄)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) ų, Z = 4. The eight strongest X-ray powder-diffraction lines are [d_obs 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.

【 授权许可】

CC BY