The crystal structure of the new mineral wilhelmkleinite - ZnFe23+(OH)(2)(AsO4)(2)

The new mineral wilhelmkleinite has ideal chemical composition ZnFe32+(OH)(2)(AsO4)(2) and crystallizes monoclinic, space group P2(1)/n, Z = 2 with lattice parameters a = 6.631(1) A, b = 7.611(1) A, c = 7.377(1) A and beta = 91.80(1)degrees. The structure was solved using direct methods. Refinement led to a final R(F) value of 0.014 for 1092 symmetrically independant reflections greater than or equal to 3 sigma(I). The structure is characterized by chains of edge-lin;ed [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 [AsO4](3-) tetrahedra in the directions of [100] and [001] and thus a three dimensional framework is formed. The Zn2+ ions are incorporated in cavities and are surrounded by six oxygen atoms in the form of a distorted octahedron. Alternatively, one [ZnO4(OH)(2)] octahedron shares opposite faces with two [FeO4(OH)(2)] octahedra, to form a face sharing trimer trans-M(3)phi(12) (phi = O2-, OH-) Of Composition [ZnFe2(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 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-(OH)(2)(AsO4)(2). The framework of [FeO6](9-) octahedra and [AsO4](3-) tetrahedra are nearly identical in the two compounds. However, the Zn2+ (or Cu2+ ions respectively) are incorporated into different vacancies and this leads to a doubling of two of the axes of CuFe2(OH)(2)(AsO4)(2) with respect to wilhelmkleinite.