Abstract
The crystal structures of eight compounds with compositions A
~2~
Me
^2+^(__X__O~4~)~2~ · 2H~2~O (A = Na, X = S, Me
^2+^ = Mn, Cd; A = Na, X = Se, Me
^2+^ = Mn, Co, Ni, Zn, Cd; and A = K, X = Se, Me
^2+^ = Co) were investigated using single crystal X-ray CCD diffraction data. The Mn- and Cd-compounds crystallize in the monoclinic kröhnkite structure type [Na~2~Cu(SO~4~)~2~ · 2 H~2~O, space group __P__2~1~/c], the Co-, Ni-, and Zn-selenates in the closely related triclinic structure type of collinsite [Ca~2~Mg(PO~4~)~2~ · 2 H~2~O, space group __P__1̅]. Both types are built up from infinite Me(__X__O~4~)~2~(H~2~O)~2~ chains, composed of __Me__O~4~(H~2~O)~2~ octahedra alternating with each two __X__O~4~ 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.