The new mineral wilhelmkleinite has ideal chemical composition ZnFe ^3+^
~2~(OH)~2~(AsO~4~)~2~ and crystallizes monoclinic, space group P2~1~/n, Z = 2 with lattice parameters a = 6.631 ( 1) Å, b = 7.611 (1) Å, c = 7.377 ( 1) Å and β = 91.80( 1)°. The structure was solved using direct methods. Refinement led to a final R (F) value of 0.014 for 1092 symmetrically independant reflections ≥ 3σ( I).
The structure is characterized by chains of edge-linked [Fe(OH)~2~O~4~]^7-^ octahedra which are connected via common (OH)^-^ groups. The chains have composition [Fe(OH)O~4~]^6-^ and run parallel to [010]. Different chains are bridged by [AsO~4~]^3-^ tetrahedra in the directions of [100] and [001] and thus a three dimensional framework is formed. The Zn^2+^ ions are incorporated in cavities and are surrounded by six oxygen atoms in the form of a distorted octahedron.
Alternatively, one [ZnO~4~(OH)~2~] octahedron shares opposite faces with two [FeO~4~(OH)~2~] octahedra, to form a face sharing trimer trans-M~3~Φ12 (Φ =O 2-, OH^-^) of composition [ZnFe~2~(OH)~4~O~8~] . Different trimers are connected via the arsenate tetrahedra. The structure can be mapped onto a {3^6^} net.
Distances and angles in wilhelmkleinite are in good agreement with values observed in comparable compounds. The face sharing of the octahedra leads to a pronounced shortening of the common edges.
Wilhelmkleinite is closely related to the orthorhombic modification of CuFe~2~(OH)~2~(AsO~4~)~2~. The framework of [FeO6]^9-^ octahedra and [AsO~4~]^3-^ tetrahedra are nearly identical in the two compounds. However, the Zn ^2+^ (or Cu ^2+^ ions respectively) are incorporated into different vacancies and this leads to a doubling of two of the axes of CuFe~2~(OH)~2~(AsO~4~)~2~ with respect to wilhelmkleinite.