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Description and crystal structure of vajdakite, [(Mo⁶⁺O₂)₂(H₂O)₂ As³⁺₂O₅]· H₂O—A new mineral from Jáchymov, Czech Republic

Petr Ondru^1,*, Roman Skála¹, Ivana Císaová², Frantiek Veselovsk¹, Jií Frda¹ and Jií ejka³

¹ Czech Geological Survey, Klárov 3/131, P.O. Box 85, CZ-118 21 Prague 1, Czech Republic
² Faculty of Science, Charles University, Hlavova 2030, CZ-128 43 Prague 2, Czech Republic
³ National Museum, Václavské nám. 68, CZ-115 79 Prague 1, Czech Republic

Correspondence: ^* E-mail: ondrus{at}cgu.cz

Vajdakite, a new mineral from Jáchymov, NW Bohemia, Czech Republic, forms minute acicular, gray-green crystals associated with arsenolite, scorodite, parascorodite, kakite, annabergite, köttigite, pyrite, marcasite, nickelskutterudite, and löllingite. Microprobe analysis gave (in wt%): As = 27.72, Mo = 35.39, O = 36.66, total = 99.77. The simplified chemical formula is [(Mo⁶⁺O₂)₂(H₂O)₂ As³⁺ ₂O₅]· H₂O. The mineral is monoclinic, P2₁/c, a = 7.0515(6), b = 12.0908(9), c = 12.2190(14) Å, ß = 101.268(9)°, V = 1021.7(2) ų, Z = 4, D_meas = 3.50(2) g/cm³, and D_calc = 3.509 g/cm³. The strongest lines in the powder X-ray diffraction pattern d(I)(hkl) are: 6.046 (100)(020), 3.324 (59)(023), 6.915 (26)(100), 2.264 (19)(310), 3.457 (16)(200), 2.624 (15)(230), and 3.819 (10)(031). Vajdakite is optically positive, with X || b and Z ^ c = 12°; elongation is positive. Its birefrigence is 0.28, with 2V_calc = 35.1°, n = 1.757(2), n_ß = 1.778(2), and n = 2.04(1). The pleochroic scheme is X ~ Y = light greenish gray, and Z = yellowish gray. Crystal size varies between 0.1 to 0.5 mm. TG curve and IR spectra show that vajdakite contains two distinct types of water molecules. The crystal structure was solved by direct methods (MoK radiation) and refined using 1787 unique reflections to R = 0.0455, Rw = 0.1143. There are double chains built up by two individual chains with a sequence -O-As-O-Mo- interconnected by oxygen atoms from two triangular AsO₃ groups and two structurally non-equivalent MoO₅(H₂O) octahedra. The two vertex-sharing, triangular AsO₃ groups form an (As₂O₅)^4– diarsenite group. The first type of water molecule is not included in the coordination, but the second one is in octahedral coordination around Mo. The water molecules are linked by a complicated net of interlayer and intralayer hydrogen bonds.

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