Journal of Mineralogical and Petrological Sciences, Volume 111, page 363–369, 2016 Structural refinement of köttigite–parasymplesite solid solution: Unique cation site occupancy and chemical bonding with water molecules Akira YOSHIASA*, Yumiko MIYANO*, Hiroshi ISOBE*, Kazumasa SUGIYAMA**, Hiroshi ARIMA**, † † Akihiko NAKATSUKA***, Koichi MOMMA and Ritsuro MIYAWAKI *Graduate School of Science and Technology, Kumamoto University, Kumamoto 860–8555, Japan **Institute for materials research, Tohoku University, Sendai 980–8577, Japan ***Graduate School of Science and Engineering, Yamaguchi University, Yamaguchi 755–8611, Japan †Department of Geology and Paleontology, National Museum of Nature and Science, Tsukuba 305–0005, Japan Köttigite and parasymplesite form a solid solution of Zn3−x,Fex(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 Zn1.62Fe1.38(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 con- sisting 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. Keywords: Kottigite solid–solution, Parasymplesite solid–solution, Unique site occupancy, Chemical bonding, Water molecule INTRODUCTION acterized by close chemistries, metaköttigite, and symple- site. The relation is the same as that between vivianite Köttigite, Zn3(AsO4)2 •8H2O, and parasymplesite, and metavivianite. They are not polymorphs, but are Fe3(AsO4)2 •8H2O, are zinc and ferrous arsenate minerals interchangeable by the following substitution reaction: 2+ 3+ − of monoclinic form, respectively. Both of them belong Fe +H2O ↔ Fe + (OH) . The substitution results to the vivianite group with the general formula of in a change in the symmetry of the monoclinic (C2/m) 2+ M 3 (TO4)2 •8H2O, where M = Mg, Mn, Fe, Co, Ni, Cu, and triclinic (P1) crystal structures. It is a part of signifi- and Zn and T = As and P. These arsenate minerals usually cances in mineralogy and proton super–ionic conduction occur as secondary minerals in the oxidation zones of to study the crystal chemistry in the vivianite group, es- some arsenic–rich hydrothermal base–metal mineral de- pecially the hydrogen bonding geometry. posits. Their prismatic crystals are often elongated along The crystal structures of vivianite and parasymple- [001] and flattened on {010}. They are individually char- site were determined by Mori and Ito (1950). However, doi:10.2465/jmps.151207 in the analyses of the isotropic thermal displacement pa- A. Yoshiasa, [email protected]–u.ac.jp Corresponding au- rameters, the positions of the hydrogen atoms in the crys- thor tal structures could not be determined. No refinement has 364 A. Yoshiasa, Y. Miyano, H. Isobe, K. Sugiyama, H. Arima, A. Nakatsuka, K. Momma and R. Miyawaki Table 1. Chemical compositions of kottigite–parasymplesite solid–solution from Ojuela Mine, Mapimi, Mexico, and Mitate Mine, Miyazaki, Japan been reported for the crystal structure of parasymplesite, determined by means of scanning electron microscopy whereas the crystal structure of vivianite has been refined (SEM; JEOL JSM–7001F; 15 kV, 1.0 nA) and energy in anisotropic mode to verify the positions of the hydro- dispersive spectroscopy (EDS; Oxford Inca Energy Sys- gen atoms (Bartl, 1989). The crystal structure of köttigite, tem). The following standard materials were employed: along with the hydrogen sites with anisotropic thermal Fe2O3 for Fe, ZnS for Zn, MnO for Mn, pure As for As, displacement parameters, was analyzed for the non–H and KTiPO5 for P. The intermediate members between atoms (Hill, 1979). Zn–free parasymplesite (Mori and köttigite and parasymplesite in the solid–solution system Ito, 1950) and Fe–free, Co– and Ni–bearing köttigite that consists of comparable amounts of Zn and Fe were (Hill, 1979) crystals were also examined. No crystallo- rarely found among the given specimens. Two intermedi- graphic examination has been carried out for the inter- ates, one from Mitate Mine, Miyazaki, Japan, (NSM– mediate members of the köttigite–parasymplesite solid M34107) and the other from Ojuela Mine, Mapimi, Du- solution. In this study, we will report the crystal chemis- rango, Mexico (Kumamoto University, C133–köttigite– try of the köttigite–parasymplesite solid solution. We re- parasymplesite–ss–201402), were selected after chemical fined the atomic coordinates of all the atoms, including analysis. The specimens had small amounts of P and Mn hydrogen. It is known that the arsenates show unique as substituents of As and (Fe,Zn), respectively (Table 1). crystallographic features, such as locally concentrated de- The empirical formulae derived from the mean chemi- formation and characteristic changes in the electron orbits cal compositions are Fe1.622Zn1.379Mn0.003[(As0.991P0.001 (Sakai et al., 2009). The present refinement of the occu- Si0.002)O4)]2 •7.674H2O (Ojuela) and Fe1.694Zn1.343 pancy parameters revealed a unique preference of Zn and [(As0.920Si0.012)O4)]2 •7.933H2O (Mitate). The backscat- Fe in the octahedral sites, which is a characteristic feature tering SEM images and line–scan profiles for Zn and of the crystal chemistry of Zn–Fe substitution in the viv- Fe on specimens from two different localities commonly ianite–type structure. We investigated the unique bonding showed compositional heterogeneity in the direction per- between the cation and water molecules in the köttigite– pendicular to the {010} cleavage (Fig. 1). It is presumed parasymplesite solid solution. that the Zn/(Zn+Fe) ratio changes with crystal growth. EXPERIMENTS Crystal structure analyses Samples and chemical composition Of the two specimens of the intermediate members of the köttigite–parasymplesite solid–solution system, a cleaved Several specimens of köttigite and parasymplesite were fragment of the Ojuela specimen gave X–ray diffraction selected and examined from the mineral collections of the spots clear enough for structural refinement. The qualities Japanese National Museum of Nature and Science and of the single crystal specimens were checked with a four– Kumamoto University. The chemical compositions were circle diffractometer at the BL–10A beam line of the Pho- The structure refinement of kottigite 365 Figure 1. SEM images for the köttig- ite–parasymplesite solid–solutions Mitate Mine, Miyazaki, Japan (A1) and Ojuela Mine, Mapimi Du- rango, Mexico (B1). Middle photo- graphs show the line analysis per- pendicular to the (010) plane. The counts (cps) for Zn (lower) and Fe (upper) atoms change in direction. The compositional variations were continuous. Similar changes were observed in both localities. Color version of Figure 1 is available online from http://doi.org/10.2465/ jmps.151207. Table 2. Experimental details and crystallographic data of to Factory, High Energy Accelerator Research Organiza- köttigite–parasymplesite solid solution from Ojuela Mine tion, KEK, Japan, using high–resolution monochromat- ized synchrotron X–ray radiation, and a single crystal from Ojuela Mine was chosen for further structural inves- tigations. The diffraction intensities and their angles were collected on a Rigaku RAPID diffractometer with a curved imaging plate detector at Tohoku University. The observed systematic absences on the diffraction intensities were consistent with the space group C2/m. The intensity of reflection was measured using graphite– monochromatized MoKα radiations. A total of 2608 re- flection intensities were collected. The data was corrected for the absorption effect as well as Lorentz and polariza- tion factors, and then averaged. The experimental details and crystallographic data of the single crystal from Ojue- la are summarized in Table 2. 366 A. Yoshiasa, Y. Miyano, H. Isobe, K. Sugiyama, H. Arima, A. Nakatsuka, K. Momma and R. Miyawaki Figure 2. Difference Fourier maps passing through the hydrogen and oxygen atoms of water molecules in the köttigite–parasymplesite sol- id solution after the anisotropic re- finements without hydrogen atom. The contour interval is 0.2 eÅ−3. Crosses indicate the refined posi- tions of H and O. The initial structural model was obtained by a direct The structure obtained in this refinement is shown in 5+ method. A total of 650 unique reflections with Fo > Figure 3. The structure has one As site and two distinct 2+ 2+ 4σ(Fo) were used for consecutive structural refinements M1 and M2 sites, which were occupied by Zn and Fe , with SHELXL97 (Sheldrick, 1997). The occupancy fac- respectively. The As site is a slightly distorted tetrahedral tors for Zn and Fe were refined in the M1 and M2 octa- AsO4 site, and the average As–O bond distance is 1.696 Å. hedral sites because the solid solutions are ideally made The octahedral M1 site is coordinated by two O2− and four only in the zinc and iron (Table 1). The chemical–struc- H2O groups with an average M1–O distance of 2.134 Å. 2− tural formula, (Zn0.76Fe0.24){Zn0.43Fe0.57}2(AsO4)2 •8H2O The octahedral M2 site is coordinated by four O and two where ( ) and { } designate M1 and M2 sites, respectively, H2O groups with an average M2–O distance of 2.101 Å.