Crystal structure refinements of legrandite, adamite, and paradamite: The complex structure and characteristic hydrogen bonding network of legrandite

Abstract

Crystal structures of legrandite [Zn₂AsO₄(OH)·H₂O; a = 12.8014(11), b = 7.9390(3), c = 10.2262(5) Å, β = 104.490(2)°; space group P2₁/c; Z = 8], adamite [Zn₂AsO₄(OH); a = 8.3428(11), b = 8.5664(10), c = 6.0769(8) Å; space group Pnnm; Z = 4], and paradamite [Zn₂AsO₄(OH); a = 5.8438(5), b = 6.7226(6), c = 5.6566(4) Å, α = 104.348(3), β = 92.320(3), γ = 76.683(3)°; space group P1; Z = 2] were investigated by single–crystal X–ray diffraction and were refined to the R₁ values of 0.0212, 0.0282, and 0.0270 using 2800, 588, and 1128 unique reflections with F_o >4σ(F_o), respectively. The chemical formula of legrandite is similar to that of adamite and paradamite, except for the presence of water molecules. In the structure of legrandite, the hydrogen atoms are distributed among the two hydroxyl and the two water molecule positions. On the basis of bond valence calculations, the hydrogen bonding in legrandite can be classified into three types: (1) one acceptor with linear normal hydrogen bonding (Type–A), (2) two acceptors with linear hydrogen bonding and one excess weak hydrogen bonding (Type–B), and (3) several acceptors with one linear hydrogen bondings and several weak hydrogen bondings by weak electrostatic interactions (Type–C). The variety of hydrogen bonding interactions provides structural stability to legrandite. The Zn3–O1 bond shows a remarkable distance of 2.341(2) Å, which is ascribed to the three–dimensional periodicity of the complex mineral structure. The local structures of adamite and paradamite violate a fundamental crystallographic law with respect to the cation coordination number and unit cell volume. The crystal structures of legrandite and paradamite are characterized by proton transfer tunnels running along the crystal axes.