Ab initio X-ray powder structure determination of parascorodite, Fe(H2O)2AsO4

The crystal structure of parascorodite, Fe(H2O) 2AsO4, has been solved ab-initio from powder data. The mineral is trigonal, with space group P3̄c1, Z = 6, a = 8.9232(2) Å, c = 9.9296 (3) Å. The Rietveld refinement was led up to RP = 4.5 %. The structure is built up by the same coordination polyhedra found in its dimorph scorodite, namely slightly distorted Fe-centred octahedra and As-centred tetrahedra, but their connection lead to quite different crystal structures for the two polymorphs. Parascorodite structure can be described in terms of mixed octahedral-tetrahedral "columns" including Fe2 and As polyhedra, running along c, similar to those found in ferrinatrite and kaatialaite. These columns are interconnected in a threedimensional framework by the sharing of the O1 oxygen between [AsO4] tetrahedra and Fe1 octahedra, and by the strong hydrogen bond OW ⋯ O2. © 2004 E. Schweizerbart'sche Verlagsbuchhandlung.