Structural refinement of köttigite-parasymplesite solid solution: Unique cation site occupancy and chemical bonding with water molecules

Köttigite and parasymplesite form a solid solution of Zn_3-x,Fe_x(AsO₄)₂ •8H₂O. The compositional variations in the köttigite-parasymplesite solid-solution system were determined by SEM/EDS with specimens from Mitate Mine, Miyazaki, Japan, and Ojuela Mine, Mapimi Durango, Mexico. Variations were observed in the direction perpendicular to the (010) plane in the continuous solid-solution system. A refinement of the crystal structure of Zn_1.62Fe_1.38(AsO₄)₂ •8H₂O [monoclinic, space group C2/m, a = 10.3417(13), b = 13.4837(16), c = 4.7756(5) Å, β =105. 306(4)°, V = 642.31(13) Å3, and Z = 4] converged into R = 0.0265 and S = 1.083 for 650 independent reflections in the single-crystal XRD data. The hydrogen bonds were described based on the hydrogen atom positions on the difference Fourier maps in reference to the bond valence calculations. The smaller Zn²⁺ ion prefers the larger M1 site and the larger Fe²⁺ ion prefers the smaller M2 site. This unique cation site preference reduces the structural distortions. The M2-O5 bond distance, where O5 is the oxygen of the H₂O group, is shorter than that of M2-O2 and -O3, in which the oxygen atoms form edge-sharing M2₂O₆(H₂O)₄ double octahedra. Only one hydrogen atom from the H₂O group, H52, connects the respective complex sheets consisting of M2₂O₆(H₂O)₄ double octahedra and AsO₄ tetrahedra. The space between the respective complex sheets is filled with hydrogen. It is presumed that the movement of proton in this space is the fastest.