Krautite, Mn(H₂O)(AsO₃OH): crystal structure, hydrogen bonding and relations with haidingerite and pharmacolite Available to Purchase

The crystal structure of synthetic krautite was solved (direct methods) and refined isotrop-ically (R = 0.112) in space group P2₁ with 2294 reflections measured on a single-crystal diffractometer, with MoKα radiation. The unit-cell parameters are: a = 8.012(2), b = 15.956(4), c = 6.801(2)A, β = 96.60(3)°; Z = 8. A strong P2₁/n pseudosymmetry appears in the structure, yet the average pseudo-center is slightly shifted from the correct position 1/4, 0, 1/4 in space group P2₁/n. The structure is built up by (010) layers of AsO₄ and MnO₆ coordination polyhedra. In each layer, MnO₆ octahedra share edges to form [101] chains, which sandwich the AsO₄ tetrahedra. Adjacent layers are related by the screw axis and are linked by four out of twelve independent hydrogen bonds; two of them are acidic and two are donated by water molecules. One of the latter hydrogen bonds is probably bifurcated, since the water molecule is involved in two contacts 2.92 and 3.08A long. Haidingerite (CaHAsO₄ · H₂O) and pharmacolite (CaHAsO₄ · 2H₂O) show similar structures formed by layers of coordination polyhedra connected by hydrogen bonds. For the single-layer topology, krautite resembles pharmacolite (chain-like structure) more than haidingerite (network-like structure), since the increase of coordination number from Mn²⁺ to Ca²⁺ is compensated by the increase of water content.