Structural Refinement of Köttigite–Parasymplesite Solid Solution

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. obtained from the refinement is consistent with the The M1 site forms an isolated octahedron and the M2 rounded empirical formula, Fe1.62Zn1.38(AsO4)2 •8H2O, sites are centered in the edge–sharing double octahedra. obtained from the chemical analysis. After the least– The AsO4 tetrahedron links the M1 and M2 octahedra to square refinements without hydrogen atoms, the R index form a sheet with a symmetry of (010) mirror plane. Only (= Σ||Fo| − |Fc||/Σ|Fo|) dropped to 0.030 using anisotropic the H52 hydrogen of the H2O groups bonded to the M2 thermal displacement factors. The initial positions of the site ties the respective (010) complex sheets. The hydro- hydrogen atoms in the successive refinements were locat- gen bonding holds together between the adjacent single ed by difference Fourier syntheses (Fig. 2). The refined and double octahedral sites. In the structure, a network hydrogen atomic positions were consistent with those in of hydrogen bonds spreads two– and three–dimensionally. vivianite (Bartl, 1989). Here, it was assumed that all hy- The site occupancies for Zn and Fe in M1, drogen atoms in the model have the same isotropic ther- (Zn0.76Fe0.24), and M2, (Zn0.43Fe0.57), sites are apparently mal displacement parameters. The positional and aniso- different. The zinc ion prefers the M1 site. The average tropic thermal displacement parameters are given in inter–atomic distance for the M1 site, 2.134 Å, is longer Tables 3 and 4. The selected interatomic distances and than that of the M2 site, 2.101 Å. The difference in the angles are presented in Table 5. The crystal structure average distances between the M1 and M2 sites contains was illustrated using VESTA (Momma and Izumi, 2011). important crystallographic specificity. The M1 and M2 sites are coordinated by four and two water molecules, RESULTS AND DISCUSSION respectively. The M–O distance from the oxygen of a wa- ter molecule should be long because the chemical bond- Description of the structure ing and electrostatic interaction of the H2O molecule is weaker than that of an O2−–ion against cation. It is desir- The solid solutions with intermediate compositions be- able that the M1 site is larger. Moreover, the M2 site is tween the köttigite and parasymplesite end members located in the edge–sharing double octahedra and the M2– seem to be rarely present. Two intermediate solid solu- M2 cation repulsion is large across the shared edge. Then, tions from the Mitate Mine and Ojuela Mine samples the M2–M2 bond is elongated and the O2–O2 sharing show the similar compositional dependence (Fe/(Fe + edge is shortened following Pauling’s third rule. It is de- Zn) = 0.35–0.64) and occurrence (Fig. 1). The gradual sirable that the M2 site is smaller. The M2 site experien- compositional changes in Fe/Zn were common. It was ces significant geometrical restriction and its flexibility is not easy to find a good quality single crystal because relatively lower than that of the M1 site. On the contrary, the crystals are easy to cleave and deform. Zn, with smaller ionic radius, prefers larger M1 sites and The structure refinement of kottigite 367

Table 3. Atomic coordinates, cation site occupancy (SOF), Ueq and Uiso values of köttigite–parasymplesite solid solution from Ojuela Mine

Table 4. Anisotropic thermal displacement parameters for köttigite–parasymplesite solid–solution from Ojuela Mine

Table 5. Selected interatomic distances (Å)

The O4 and O5 are the oxide ions of water molecule

Fe, with larger ionic radius, prefers the smaller M2 sites. structural distortion. The difference in size between the The ionic radius (VIZn2+: 0.74 Å) for Zn is smaller than M1 and M2 sites is being reduced by the cation site pref- the radius (VIFe2+: 0.78 Å) of Fe2+. We interpreted this erence. A similar reverse distribution, which decreases unique cation site preference as a tendency to reduce the the structural distortion, is observed in spinel solid solu- 368 A. Yoshiasa, Y. Miyano, H. Isobe, K. Sugiyama, H. Arima, A. Nakatsuka, K. Momma and R. Miyawaki

Figure 3. Crystal structure of the köttigite–parasymplesite solid solu- tion and the isolated octahedral M1 site and one edge–sharing double octahedral M2 site. AsO4, M1, and M2 polyhedra form a complex sheet with mirror symmetry, paral- lel to (010). The M1 and M2 sites are coordinated by four and two water molecules, respectively. Col- or version of Figure 3 is available online from http://doi.org/10.2465/ jmps.151207.

Table 6. Bond valence sum for köttigite–parasymplesite solid solution

tions (Ito et al., 2000; Yoshiasa et al., 2010). cause of the M2–M2 cation repulsion across the O2–O2 Hill (1979) reported that the transition metals are sharing edge. The M2–O5 bond distance (2.094 Å), where randomly distributed over insular single and double O5 is the oxygen of the H2O groups, is shorter than the (edge–sharing) octahedral groups in the crustal structure of M2–O2 and M2–O3 bond distances for O2−, and the M2– köttigite, (Zn2.44Co0.42Ni0.14)(AsO4)2 •8H2O. On the con- O5 bond is much stronger than the M1–O4 bond for the trary, Wildner et al. (1996) observed strong preferences of H2O groups. This observation is important. It is shown Fe2+ and Mg2+ for the M2 site in the crystal structures of that the bond between the water molecule and the cation erythrite, Co(Co0.505Fe0.37Ni0.125)2(AsO4)2 •8H2O, and an- becomes stronger by the coordination environment. nabergite, (Ni0.99Mg0.01)(Ni0.825Mg0.175)2(AsO4)2 •8H2O, respectively. The present study confirmed the biased dis- Hydrogen bonding and proton conduction tribution of Fe2+ into the M2 site for the köttigite–para- symplesite solid solution. Wildner et al. (1996) noted We carried out the calculation of bond valences based on a characteristic structural feature of all vivianite–type ar- the method proposed by Donnay and Allman (1970). Ta- senates and phosphates; the average M2–O bond length ble 6 presents the results of bond valence sum calculation is significantly shorter than the respective mean M1–O for the intermediate members of the köttigite–parasym- distances. plesite solid–solution series. The intermediate minerals The O4 and O5 are the oxide ions of the H2O mole- of the köttigite–parasymplesite solid solution series have cule in the structures studied. In an isolated M1 octahe- four hydrogen bonds (Table 5). The ÐO–H…O angles are dron, the bonding distance of M1–O4 is significantly long- 154.74, 162.92, 154.26, and 174.31° for O5–H51…O3, er than M1–O1. On the other hand, in an edge–sharing M2 O5–H52…O4, O4–H41…O3, and O4–H42…O1, respec- octahedron, the bonding distance of M2–O2 is a little tively. All of them are classified as linear and normal hy- longer than M2–O5. The M2–O2 distance is elongated be- drogen bonds (Fig. 4). Only one of the four hydrogen The structure refinement of kottigite 369

Figure 4. Geometry around the hy- drogen atoms. Only the H52 hy- drogen links the respective com- plex sheets (left) and H41, H42, and H52 hydrogen atoms con- tribute to the bonding inside the (010) complex sheet (right). Color version of Figure 4 is available online from http://doi.org/10.2465/ jmps.151207. bonds, O5–H52…O4, links the respective complex sheets of the MO6 octahedra and AsO4 tetrahedra. In this hydro- REFERENCES gen bond, both the donor (O5) and acceptor (O4) are Bartl, H. (1989) Water of crystallization and its hydrogen–bonded oxygen atoms of the H2O groups. Dynamic exchange crosslinking in vivianite Fe (PO ) •8H O; a neutron diffrac- movements of hydrogen would happen frequently in 3 4 2 2 tion investigation. Zeitschrift für Analytische Chemie, 333, the H2O group. The remaining three hydrogen bonds 401–403. connect the internal O1 and O3 oxygen atoms within Donnay, G. and Allman, R. (1970) How to recognize O2−,OH− and – the complex sheet. The bond valence sum calculation H2O in crystal structures determined by X rays. American Mineralogist, 55, 1003–1015. (Table 6) showed that the H2O molecule of O4, which … Hill, R.J. (1979) The crystal structure of köttigite. American Min- has a greater bond valence (ca. 0.2 for a H O), is more eralogist, 64, 376–382. polarized than that of O5 (~ 0.14–0.16 for a H…O). Ito, T., Yoshiasa, A., Yamanaka, T., Nakatsuka, A. and Maekawa, The complex sheets parallel to (010) are connected H. (2000) Site preference of cation and structural variation in by only the O5–H52…O4 hydrogen bonds. This stacking MgAl2−xGaxO4 spinel solid solution. Zeitschrift für Anorgani- – pattern, maintained by the hydrogen bonds, can explain sche und Allgemeine Chemie, 626, 42 49. Momma, K. and Izumi, F. (2011) VESTA 3 for three–dimensional the perfect {010} cleavage of köttigite, which was point- visualization of crystal, volumetric and morphology data. ed out by Hill (1979). It is presumed that the exchange Journal of Applied Crystallography, 44, 1272–1276. movements of protons, H+, in this inter–sheet space are Mori, H. and Ito, T. (1950) The structure of vivianite and symlesite, significant. It can be seen that there are various proton Acta Crystallographica, 3, 1–6. conduction paths. The protons could be moving around Sakai, S., Yoshiasa, A., Sugiyama, K. and Miyawaki, R. (2009) Crystal structure and chemistry of conichalcite CaCu(AsO4) the polyhedra through vacancies. It can also be interpret- (OH). Journal of Mineralogical and Petrological Sciences, ed that the faster oxidation reaction, observed especially 104, 125–131. in vivianite as a color change, is caused by proton con- Sheldrick, G.M. (1997) SHELXL–97, Program for the Refinement duction. The reaction of Fe2+ +H+ → Fe3+ + vacancy is of Crystal Structure; University of Göttingen, Germany. 2+ Wildner, M., Giester, G., Lengauer, C.L. and McCammon, C.A. initiated when the Fe ion in one of the minerals in this – + (1996) Structure and crystal chemistry of vivianite type com- group is oxidized partially. The desorption of H would pounds: Crystal structures of erythrite and annabergite with a happen through this space parallel to (010). Mössbauer study of erythrite. European Journal of Mineralo- gy, 8,187–192. ACKNOWLEDGMENTS Yoshiasa, A., Ito, T., Sugiyama, K., Nakatsuka, A., Okube, M., Kurosawa, M. and Katsura, T. (2010) Peculiar site preference of B in MgAl2−xBxO4 (x = 0.0, 0.11, and 0.13) spinel under This study was performed under the auspices of Photon high–pressure and high–temperature. Zeitschrift für Anorgani- Factory (PAC No. 2012G540). sche und Allgemeine Chemie, 636, 472–475.

SUPPLEMENTARY MATERIALS Manuscript received December 4, 2015 Manuscript accepted May 26, 2016 Color versions of Figures 1, 3, and 4 are available online Published online September 24, 2016 from http://doi.org/10.2465/jmps.151207. Manuscript handled by Hiroki Okudera