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Karen Friese, G. Adiwidjaja, K.-H. Klaska, P. B. Moore, J. Schlüter
The crystal structure of the new mineral wilhelmkleinite – ZnFe3+2(OH)2AsO4)2
The new mineral wilhelmkleinite has ideal chemical composition ZnFe 3+ 2(OH)2(AsO4)2 and crystallizes monoclinic, space group P21/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)2O4]7- octahedra which are connected via common (OH)- groups. The chains have composition [Fe(OH)O4]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-M3Φ12 (Φ =O 2-, OH-) of composition [ZnFe2(OH)4O8] . Different trimers are connected via the arsenate tetrahedra. The structure can be mapped onto a {36} 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 CuFe2(OH)2(AsO4)2. The framework of [FeO6]9- octahedra and [AsO4]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 CuFe2(OH)2(AsO4)2 with respect to wilhelmkleinite.
Zeitschrift für Kristallographie, Oldenbourg Wissenschaftsverlag
Print ISSN: 0044-2968
Volume: 215, 02/2000
Pages: 96