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

摘要

K?ttigite and parasymplesite form a solid solution of Zn_(3?)_(x) ,Fe_(x) (AsO_(4))_(2)?8H_(2)O. The compositional variations in the k?ttigite–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_(1.62)Fe_(1.38)(AsO_(4))_(2)?8H_(2)O [monoclinic, space group C 2/m , a = 10.3417(13), b = 13.4837(16), c = 4.7756(5) ?, β =105. 306(4)°, V = 642.31(13) ?~(3), 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 Zn~(2+) ion prefers the larger M 1 site and the larger Fe~(2+) ion prefers the smaller M 2 site. This unique cation site preference reduces the structural distortions. The M 2–O5 bond distance, where O5 is the oxygen of the H_(2)O group, is shorter than that of M 2–O2 and –O3, in which the oxygen atoms form edge–sharing M 2_(2)O_(6)(H_(2)O)_(4) double octahedra. Only one hydrogen atom from the H_(2)O group, H52, connects the respective complex sheets consisting of M 2_(2)O_(6)(H_(2)O)_(4) double octahedra and AsO_(4) 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.