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The crystal structures of zincroselite and gaitite

¹ Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany
² Institut für Anorganische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany

^* E-mail: paul.keller{at}po.uni-stuttgart.de

The crystal structures of zincroselite and gaitite, two polymorphs of Ca₂Zn[AsO₄]₂·2H₂O from Tsumeb (Namibia), were determined from single-crystal X-ray intensity data. The type crystal of zincroselite, monoclinic, P2₁/c, a = 5.827(1), b = 12.899(3), c = 5.646(1) Å and ß = 107.69(3)°, Z = 2, was refined with 935 unique reflections to R1 = 2.61%. The mean bond distances are <As-O> = 1.690 Å, <Zn-(O, H₂O)> = 2.123 Å and <Ca-(O, H₂O)> = 2.526 Å. Gaitite, P, a = 5.899(1), b = 6.978(1) and c = 5.755(1) Å, = 97.41(3)°, ß = 109.08(3)° and = 108.09(3)°, Z = 1, was refined with 914 unique reflections to R1 = 1.77%. The mean bond distances are <As-O> = 1.687 Å, <Zn-(O, H₂O)> = 2.102 Å and <Ca-(O, H₂O)> = 2.525 Å. For both structures, the positions of all H atoms were isotropically refined and the hydrogen-bonding arrangement identified. The common structural unit of zincroselite and gaitite are [Ca₂Zn(H₂O)₂O₈(AsO₄)₂]^16- rods parallel to the c axis. The polymorphism of Ca₂Zn[AsO₄]₂·2H₂O referred to the glide plane c can easily be visualized with different rod-packings. Zincroselite and gaitite belong to the roselite and collinsite structure type, respectively.

Key-words: zincroselite, gaitite, Ca₂Zn[AsO₄]₂·2H₂O, crystal structures, polymorphism, Tsumeb, Namibia.

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