Kottigite, (Zn2.44Co0.42Ni0.14)(AsO4)2·8H2O, from the type locality (Schneeberg, East Germany) crystallizes in space group C2/m with a = 10.241(3), b = 13.405(3), с = 4.757(2)A, β = 105.21(2)°, and Z = 2. Isotypy with vivianite is confirmed: the hydrogen atoms have been located and the crystal structure refined using 812 Zr-filtered Mo data to an R value of 0.054 (Rω = 0.057). The transition metals are randomly distributed over insular single and double (edge-sharing) octahedral groups of O atoms and H2O molecules connected by AsO4 tetrahedra to form complex slabs parallel to (010). These sheets are held together by hydrogen bonding alone, thereby accounting for the perfect {010} cleavage of the mineral. Shortening of the shared edge relative to the unshared edges within the octahedral dimer (0.139A) is well within the rather wide range of values observed in other Zn compounds containing shared octahedral edges.

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