The crystal structure of rösslerite MgHAsO4 · 7 H2O

Abstract

Abstract Rösslerite MgHAsO4 · 7H2O crystallizes in the monoclinic system with cell dimensions a = 6,687 ± 0,003, b = 25,73 ± 0.01, c = 11,531 ± 0,005 Å, β = 95° 7′ ± 5′. The space group is C2/c and there are eight molecules per unit cell. The structure has been solved from a Patterson synthesis and partially phased Fourier syntheses and least-squares refinement has been completed on three-dimensional data (2065 structure factors within the CuKα sphere). The final residual is 11.2%. The structure consists of AsO2(OH½)2 tetrahedra, Mg · 6H2O octahedra in two independent positions and isolated water molecules held together by hydrogen bonds. The position of the hydrogen bonds have been inferred from stereochemical and crystallographic evidence. The AsO2(OH½)2 tetrahedron is distorted, with a mean (uncorrected) arsenic–oxygen bond length of 1,691 ± 0,003 Å and the Mg · 6H2O octahedra are also distorted, with a mean (uncorrected) magnesium–water oxygen of 2,077 ± 0,003 Å. However in this case if the water oxygen is a receptor atom for a hydrogen bond the magnesium–water oxygen bond is lengthened. The thermal vibrations have been analyzed and the mean corrected bond lengths are arsenic–oxygen 1,695, magnesium–water oxygen 2,086 (assuming riding motion in each case).