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

Abstract

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) ų, 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.