Abstract

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 β = 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, P1̄, a = 5.899(1), b = 6.978(1) and c = 5.755(1) Å, α = 97.41(3)°, β = 109.08(3)° and γ= 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.

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