Crystal structures and crystal chemical relationships of krohnkite- and collinsite-type compounds Na2Me2+(XO4)2•2 H2O (X = S, Me = Mn, Cd; and X = Se, Me = Mn, Co, Ni, Zn, Cd) and K2Co(SeO4)2•2 H2O

The crystal structures of eight compounds with compositions A(2) Me2+ (XO4)(2) . 2H(2)O (A = Na, X = S, Me2+ = Mn, Cd; A = Na, X = Se, Me2+ = Mn, Co, Ni, Zn, Cd; and A = K, X = Se, Me2+ = Co) were investigated using single crystal X-ray CCD diffraction data. The Mn- and Cd-compounds crystallize in the monoclinic krohnkite structure type [Na2CU(SO4)(2) . 2 H2O, space group P2(1)/c], the Co-, Ni-, and Zn-selenates in the closely related triclinic structure type of collinsite [Ca2Mg(PO4)(2) . 2 H2O, space group P (1) over bar. Both types are built up from infinite Me(XO4)(2)(H2O)(2) chains, composed of MeO4(H2O)(2) octahedra alternating with each two XO4 tetrahedra by sharing corners. A-O bonds and hydrogen bonds link the chains to layers and further on to the three-dimensional structures. The influence of the ionic radii of the various A, Me, and X cations on the distances between these structural units is discussed. Special attention is paid to polyhedral distortions as well as to individual peculiarities of the alkaline coordinations and the hydrogen bonding systems. Thus, the crystal chemical relationships between the two structure types as well as to isotypic copper compounds, which are characterized by the strong octahedral Jahn-Teller distortions, are elucidated.