REFINEMENT OF THE CRYSTAL STRUCTURE OF SCHNEIDERHOHNITE

The crystal structure of schneiderhohnite, Fe2+Fe33+As53+O13, triclinic P (1) over bar, a 8.945(3), b 10.022(3),c 9.161(4) angstrom, alpha 62.942(5), beta 116.072(6), gamma 81.722(6)degrees, has been refined to an R1 value of 1.4%. The structure is very close to that reported by Hawthorne (1985), but detailed examination of the structural connectivity shows that schneiderhohnite is better described as a sheet structure than as a framework structure. Zig-zag chains of edge-sharing Fe(1), Fe(2), Fe(3) (=Fe3+) octahedra extend parallel to the c axis, and these chains are cross-linked into a corrugated sheet by tetramers of edge-sharing Fe(4) (=Fe3+) and Fe(5) (=Fe2+) octahedra. (As O-3+(3)) triangular pyramids decorate the surface of this sheet of octahedra as single (AsO3) groups, with all three short bonds linked to anion vertices of the octahedra, and as two crystallographically distinct [As2O5] dimers with four short bonds (two from each As (3+)) to octahedron vertices. The sheets are connected along [100] via the bridging anion of one of the two [As2O5] dimers that bonds to an Fe2+ octahedron of the adjacent sheet, imparting a strong sheet-like character to the structure and accounting for the perfect (100) cleavage.