An update on the mineral-like Sr-containing transition metal arsenates

Abstract We report on the crystal structures of three novel synthetic SrM-arsenates (M = Ni and Fe3+), isostructural or structurally related to the minerals from tsumcorite, carminite and brackebuschite groups. They were synthesised under mild hydrothermal conditions and further characterised using single-crystal X-ray diffraction (SXRD), scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) and Raman spectroscopy. SXRD and SEM-EDS yielded formulae: (I) SrNi2(AsO4)2⋅2H2O, (II) Sr1.4Fe3+1.6(AsO4)2(OH)1.6 and (III) SrFe3+(AsO4)(AsO3OH). All three structures are built up of slightly distorted MO6 octahedra and AsO4 tetrahedra that are linked by Sr2+ with different coordination geometries and hydrogen bonds. I represent a basic structure type typical for tsumcorite-group minerals (space group C2/m) while II has a new intermediate structure between carminite, PbFe3+2(AsO4)2(OH)2 and arsenbrackebuschite, Pb2Fe3+(AsO4)2(OH) (s.g. Pm). III is triclinic and adopts a new structure-type (s.g. P$\bar{1}$). The structure of I is built up of infinite linear edge-sharing NiO4(OH2)2 octahedral chains, extending along [010] and linked by AsO4 tetrahedra, SrO8 polyhedra and hydrogen bonds. The structure of II is characterised by the carminite-like FeO4(OH)2 octahedral chains and Edshammar-polyhedral chains, which involves SrO11 coordination polyhedra similar to that of PbO11 in arsenbrackebuschite. Both chains in II are aligned parallel to the b axis of the monoclinic unit cell and connected together by the arsenate AsO4 tetrahedra, SrO8 polyhedra and the hydrogen-bonding network. The compound III has a new type of crystal structure based on the unusual corrugated octahedral–tetrahedral-quadruple chains. These are made up of a central double-sided chain linked to two single-sided chains into a quadruple chain extended along the a axis. The chains in III are built up of FeO6 octahedra and AsO4 tetrahedra further linked to each other by shared vertices. The quadruple chains are interconnected by additional AsO4 tetrahedra forming a heteropolyhedral 3D open framework. Strontium atoms are situated in the two channels. The structural connections to related minerals and inorganic compounds are discussed.


Introduction
Even though it is well known that arsenic is one of the most naturally abundant toxic metalloids, many crystal structures of arsenic oxosalts remain either undetermined or only poorly determined. This causes misunderstandings of their behaviour in various environmental media, such as soils, sediments and waters because it requires detailed knowledge of thermodynamic properties and, in particular, of formation conditions of arsenic phases and their respective crystal structures (O'Day, 2006;Drahota and Filippi, 2009). Furthermore, the crystal chemistry of transition-metal arsenates and phosphates reveals an interesting field of research for both mineralogists and solid-state researchers. Many natural or synthetic phosphates, arsenates and vanadates are of great interest due to their structural variability. In combination with the different oxidation states of some transition metals, there is a great variety of ternary and quaternary compounds. They can foster potential applications and original physical properties such as magnetism, heterogeneous catalysis, ion exchange, optics or thermal expansion (de Pedro et al., 2011;Đorđević et al., 2018 and references therein). In this context, the preparation, crystal structures, spectroscopic behaviour and some physical properties of numerous alkali-, alkali-earth and transition-metal arsenic oxosalts have been investigated (Đorđević and Karanović, 2018 and references therein).
All three SrM-arsenates adopt mixed tetrahedral-octahedral framework structures with different chains as the fundamental structural building units. The crystal-chemical features of these arsenates, their hydrogen-bonding schemes and their topological relation to the structurally-related minerals and inorganic compounds are discussed.
The mixtures were transferred into polytetrafluoroethylene vessels and filled with distilled water to 80% of their inner volume. The starting mixtures had pH values of 3 for I and 2 for II and III. Subsequently, the mixtures were enclosed in stainless steel autoclaves and heated under autogenous pressure under the following heating and cooling conditions: the mixtures were heated from room temperature to 220°C (4 h), held at that temperature for 48 h, and cooled to room temperature (96 h). After cooling the pH values were measured as 3 for I, and 1 for II and III. The reaction products were filtered and washed several times using distilled water.

Single-crystal X-ray diffraction
The intensity data for I and III were collected on a Nonius Kappa CCD single-crystal four-circle diffractometer (MoKα radiation, graphite monochromator), equipped with a 300 nm diameter capillary-optics collimator. The unit-cell parameters were determined with HKL SCALEPACK (Nonius, 2005(Nonius, -2007. A complete sphere of reciprocal space (w and ω scans) was measured. The intensity data were processed with the Nonius program suite DENZO-SMN (Nonius, 2005-2007 and corrected for absorption by the multi-scan method (Otwinowski et al., 2003). A singlecrystal of II was studied on a STOE StadiVari diffractometer with open Eulerian cradle, dual-beam source (sealed-tube and microfocus source) and Dectris PILATUS 300K pixel detector. Data collection for II was performed with X-AREA (Stoe, 2013), cell refinement was done using X-AREA (Stoe, 2013) and data reduction was performed with X-RED (Stoe, 2013). Single-crystal X-ray diffraction data were collected at ambient conditions, integrated and corrected for Lorentz and polarisation factors and absorption by scaling of partial multi-scans. The structures of I, II and III were solved by direct methods using SHELXT (Sheldrick, 2015a) and WinGX (Farrugia, 2012). The coordinates and the anisotropic displacement parameters for all non-hydrogen atoms were refined by full-matrix least-squares methods based on F 2 values by SHELXL (Sheldrick, 2015b).
For I anisotropic displacement parameters were allowed to vary for all atoms, except the hydrogen atoms. Hydrogen atoms from water molecules were located in the difference-Fourier map and refined using a riding model where U iso (H) = 1.5U eq (O) and with DFIX distance restraints for O-H = 0.85 Å and H···H = 1.37 Å.
For II the initially determined orthorhombic unit cell typical for the carminite structure type [a = 16.500(3), b = 7.5457 (15), c = 12.227(2) Å, V = 1522.4(5) Å 3 and Z = 8] gave high values for R-factors and weighting scheme parameters. The electron density observed was higher than desired at two of the O atom positions from the OH groups (O6 and O7), but lower at the Fe sites. Using the known model of carminite, the residual electron density could not explain such high values. Therefore, a new model was developed in space group P1, with the same lattice parameters. An additional symmetry check indicated that the smaller monoclinic cell [a = 9.0719(15), b = 12.227 (2), c = 7.5457(15) Å, b = 114.575 (6), V = 761.2(2) Å 3 and Z = 4] and the Pm space group is more appropriate and the unit cell was transformed using the matrix [½ ½ 0 / 0 0 1 / 0 1 0]. All atoms in the monoclinic unit cell are situated in general 2c positions excluding Sr21, Sr22, As11-As14, O21-O24, O31-O34 and mixed sites Sr61/O61-Sr64/ O64, which are in special positions 1a and 1b on the mirror planes (site symmetry m) at y = 0 and y = ½, respectively. The structure was refined as an inversion twin. The obtained Flack parameter x = 0.570(18) indicate equivolume inversion twins. The transformation to monoclinic symmetry gave more than satisfactory results. The final difference-Fourier map showed no unusual features or spurious peaks around the refined positions of the atoms. The highest peak (Δρ max = 0.65 e -Å -3 ) in the map was found 0.80 Å from O11. In order to find out how many octahedral vacancies are present, it was necessary to refine the site occupancies of four iron atoms, Fe11-Fe14, in the unit cell. The refinement of iron site occupancies shows that the Fe11 and Fe12 sites are more iron deficient than Fe13 and Fe14, which are almost fully occupied (63 and 97%, respectively). This explains the observed lower electron density at the iron position in the initially determined orthorhombic unit cell. Therefore, a simple constraint using free variables was applied: the same site occupancy was refined for Fe11 and Fe12 atoms and oxygens O61, O62 and O71 from hydroxyl groups bonding with them only, as their site occupancy factors (s.o.f.) should be equal. Similarly, the s.o.f. of oxygens O63, O64 and O72 from another hydroxyl groups should be equal to the s.o.f. of Fe13 and Fe14. The refined cation occupancies were 0.627(2) and 0.968 (2) for the first F11-O61-Fe11-O71-Fe12-O62-Fe12-O71-Fe11 and second F13-O63-Fe13-O72-Fe14-O64-Fe14-O72-Fe13 array, respectively. Conclusively, oxygen sites (O61, O62 and O71) and (O63, O64 and O72) in the first and second array are occupied by oxygen atoms from OH groups in approximately 63 and 97% of the unit cells, respectively. In the remaining 37 and 3%, they are occupied by 11-coordinated strontium atoms (Sr61, Sr62 and Sr71) and (Sr63, Sr64 and Sr72), respectively, forming six mixed Sr/O sites. Because strontium has more electrons than oxygen, the presence of the Sr atoms, even in very small quantities, make the electron density at oxygen sites greater than expected for oxygen atoms and explains the higher electron density at positions O6 and O7 in the initially determined orthorhombic unit cell. The atomic coordinates and atomic displacement parameters of the Sr and corresponding O atoms in mixed Sr/O sites were refined as identical.
In the latter cycles of refinement the presence of disordered Sr71 atoms was found, resulting in the appearance of two additional partly occupied Sr sites of very low occupancy (ca. 10% of available sites), denoted as Sr81 and Sr82 and separated from Sr71 by a distance of only 0.728 (6) and 0.697(5) Å, respectively. Similarly, the Sr72 position is also partly occupied and disordered, resulting in additional Sr83 and Sr84 atoms of very low occupancy (ca. 10% of available sites) at distances of 0.918 (8) and 0.891(9) Å, respectively. This finding suggests a statistical disorder and movement of Sr71 and Sr72 atoms between these Sr81-S82 and Sr83-Sr84 neighbouring sites, respectively. These movements are directed to the neighbouring iron-site vacancies. Hydrogen atoms of the OH groups (OH61, OH62, OH63, OH64, OH71 and OH72) were placed in geometrically calculated positions and refined using a riding model where U iso (H) = 1.5U eq (O) and DFIX distance restraints for O-H = 0.85 Å and H···A = 1.95 Å.  For III all non-hydrogen atoms excluding Sr2, which displays an extremely high degree of positional disorder, were initially refined anisotropically. The refinement in the lowest symmetry space group P1 did not provide an ordered structure. However, in space group P1 two disordered Sr2 atoms in the unit cell appear distributed among 12 positions. Two pairs of sites were related by the pseudo-inversion centre, but the remaining eight were not. This means that Sr2 is distributed among at least 10 positions in P 1, i.e. on 10 crystallographically independent sites Sr21-Sr30 with a multiplicity of two. Because Sr21-Sr30 atoms are spatially close they were refined with equal atomic displacement parameters. Their occupancies were refined using the free variables and SUMP instruction. The refined positions showed site occupancy factors from 0.225 (6) to 0.041 (4). Owing to the disorder of the Sr2 site, R parameters are large (R[F 2 > 2σ(F 2 )] = 0.06 wR(F 2 ) = 0.14). However, it can be assumed that the structure determined is correct, except possibly for details.
Hydrogen atoms from OH groups were placed in geometrically calculated positions and refined using the riding model with O-H = 0.82 Å and isotropic displacement parameters U iso (H) fixed to 1.5 times the U eq (O) value of the atoms they are linked to.
The O34-H34···H44-O44 contacts are short to host unit-occupancy H atoms. As a consequence, O34 and O44 atoms serve as both the hydrogen bond donor and acceptor, to each other, alternatively. The donor-acceptor O34···O44 distance is 2.832(17) Å and the H34···H44 distance is 1.4827(5) Å. Therefore the H34 and H44 atom position occupancy is a maximum of 50%, which is consistent with the charge balance.
Relevant information on crystal data, data collection and refinements are compiled in Table 1. Fractional atomic coordinates and equivalent isotropic displacement parameters for I-III are presented in Tables 2-4 and anisotropic displacement parameters for I-III are presented in Supplementary Tables S1-S3. Selected bond lengths for non-H atoms are listed in Table 5 and Tables  S4-S6. Bond valence analysis obtained using VALIST software (Wills, 2010) are presented in Tables S7-S9. Interatomic distances and angles involving H atoms are given in Tables 6-8. All drawings were plotted using ATOMS (Dowty, 2000) and VESTA (Momma and Izumi, 2011). The crystallographic information files and Supplementary Tables S1-S9 have been deposited with the Principal Editor of Mineralogical Magazine and are available as Supplementary material (see below).

Elemental Analysis
Elemental analysis was performed on the hydrothermally synthesised crystals with a JEOL JSM-6610 LV scanning electron microscope (SEM) connected to an Oxford INCA Energy 350 EDX analysis unit. The single crystals were mounted with conducting carbon onto Al sample holders. The sample holder was then coated with a thin layer of carbon. An accelerating voltage of 20 kV and accumulation time of 20 s were used for the chemical analysis. As a qualitative measure, energy dispersive spectroscopy confirmed the presence of each reported element in the title compounds. Semi-quantitative chemical analyses gave the presence of strontium in the range of 17.0-17.4 at.%, arsenic in the range 28. 8-29.3 at% and nickel in the range 17.8-18.9 at% for I, strontium in the range of 26. 8-27.4 at.%, arsenic in the range 28. 3-28.4 at.% and iron in the range 12.9-15.2 at.% for II and Table 2. Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 ) for I. (4) 0.034* Table 3. Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 ) for II.

Raman spectroscopy
To obtain further information on the anion groups, Raman spectra were acquired. Non-oriented single-crystal Raman spectra of I-III on the randomly oriented single-crystals were collected using a Horiba LabRam HR Evolution system equipped with a Si-based, Peltier-cooled CCD detector in the spectral range from 4000 to 60 cm -1 . The 633 nm excitation line of a He-Ne laser was focused with a 50× objective on the randomly oriented single crystals of I-III. The samples spectra were acquired with a nominal exposure time of 10 s (confocal mode, Olympus 1800 lines/mm, 1.5 μm lateral resolutions and ∼3 μm depth resolution).
Between these layers are situated 8-coordinated (6+2) Sr 2+ cations. The mutually isolated SrO 8 coordination polyhedra can be described as a somewhat distorted orthorhombic prism or dicapped slightly elongated octahedron, where the four symmetry equivalent O3 oxygen atoms are located in the equatorial plane and two O4 are at the vertices. In addition, two opposite triangular faces are capped by two O2 atoms at longer distances. The Sr-O distances are in the range 2.573(3)-2.976(5) Å and the average value is 2.684 Å (Table S4), which is in agreement with the value of 2.66 Å calculated from the sum (1.26+1.4 = 2.66 Å) of effective ionic radii for Sr 2+ with coordination number (CN) = 8 (Shannon, 1976).
The other way of describing the crystals structure of I are two kind of slabs positioned alternately along the [001] direction (Fig. 2b). The SrO 8 polyhedra share two edges and four vertices with six adjacent AsO 4 tetrahedra forming the first slab situated between z at approximately -0.33 and 0.33. The thickness of this slab is approximately double the thickness of the second slab which hosts Ni1O 4 (OH 2 ) 2 octahedra chains combined with intralayer hydrogen bonds. Its boundaries lie between z at approximately 0.33 and 0.67.
Bond-valence calculations (Brown and Altermatt, 1985;Brese and O'Keeffe, 1991) show that the Sr-O, Ni-O and As-O bond lengths are consistent with the presence of Sr 2+ , Ni 2+ , As 5+ and Table 4. Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 ) for III.
Crystal structure of Sr 1.4 Fe 1.6 (AsO 4 ) 2 (OH) 1.6 (II) Compound II is only partly (ca. 80%) isostructural with carminite, Pb 2+ Fe 3+ 2 (AsO 4 ) 2 (OH) 2 , which means that there are regions where the carminite-type structure is maintained in the form of octahedral Fe(O,OH) 6 chains (Fig. 3a). In contrast to carminite, the structure of II contains four partially vacant Fe and six mixed Sr/O sites in which 11-coordinated Sr 2+ and oxygen atoms from OH groups statistically share the same position. Therefore, in addition to edge-sharing octahedral In the second row, the chain is formed by similar edge-shared Fe13 2 (O,OH) 10 and Fe14 2 (O,OH) 10 octahedral dimers (Fig. 3a). All octahedral dimers share an edge along mirror planes at y = 0 and 0.5 in the first and second row, respectively. According to the refinement results, the volume occupied by the Fe11 2 (O,OH) 10 -Fe12 2 (O,OH) 10 chain is 63% and then by the Fe13 2 (O,OH) 10 -Fe14 2 (O,OH) 10 chain is 97%. This means that the Edshammar polyhedral SrO 11 -chains occupy the first and second row by 37 and 3%, respectively.
Four iron cations Fe11-Fe14, situated at general positions on the pseudoinversion centre, are coordinated by the four oxygen anions belonging to the AsO 4 tetrahedra and two oxygen anions belonging to the hydroxyl groups. The Fe(O,OH) 6 dimeric octahedra are distorted showing one shorter, four typical and one longer Fe-O bonds close to the calculated value from effective ionic radii of 6-coordinated Fe 3+ cations (0.65 + 1.4 = 2.05 Å; Shannon, 1976). Oxygen from the common edge of the dimer and the oxygen from the hydroxyl group bridging two adjacent dimers in the octahedral chain form the longest and the shortest Fe-O interatomic distances, respectively. The average <Fe 3+ -O> bond distances are similar and very close to the calculated value: 2.021, 2.015, 2.019 and 2.022 Å for Fe11(O,OH) 6 , Fe12(O,OH) 6 , Fe13(O,OH) 6 ] and Fe14(O,OH) 6 octahedra, respectively ( Table 5).

D-H···A
The partial occupancy of mixed sites Sr71/O71 and Sr72/O72 by Sr are accompanied by the appearance of extra atoms (Fig. 3e) at distances less than 1 Å from Sr71 (Sr81 and Sr82) and Sr72 (Sr83 and Sr84). The four additional 11-coordinated Sr81-Sr84 atoms are of very low occupancy (ca. 10% of available sites) and are surrounded by oxygen atoms in a similar way, like Sr71 and Sr72 atoms (Table S5). Some displacement ellipsoids of these oxygen atoms have a small problematic form indicating a disorder in this part of structure.
The difference-Fourier map did not reveal the positions of the H atoms. This was expected considering their amount, as well as the substitutional and positional disorder of Sr. Therefore the hydrogen-bonding system is complex. However, bond-valence calculations (Brown andAltermatt, 1985, Brese andO'Keeffe, 1991) show that the Sr-O, Fe-O and As-O bond lengths are consistent with the presence of 8-coordinated and 11-coordinated Sr 2+ , 6-coordinated Fe 3+ , 4-coordinated As 5+ and O 2-(Σν ij = 1.988(1), 2.100(7) and 2.126(9) vu for Sr11, Sr21 and Sr22, Σν ij = 3.072 (3) (2) and 5.083(1) vu for As11-As14, As21 and As22). The bond valence sums Σν ij for O61-O64, O71 and O72 without the contributions of H are close to 1, while that of O31-O34 are near 1.6 vu indicating that they are involved in hydrogen bonding (Table S8). Except O23 and O24, the calculated bond valence sums for the other oxygen atoms were ≈ 2 indicating that they hardly participate in hydrogen-bonding interactions. The small undersaturation of O23 and O24 can be the result of a positional and substitutional disorder, i.e. the displacements of O23 and O24 oxygen atoms due to the substitutions of differently sized and charged ions (Sr72 and O72) and their amounts in the coordination sphere as well as the positional disorder of Sr72.

Crystal structure of SrFe(AsO 4 )(AsO 3 OH) (III).
In contrast to I and II, in the structure of III the FeO 6 octahedra are completely isolated from one another by the surrounding arsenate tetrahedra. This arrangement produces unique metal arsenate octahedral-tetrahedral-quadruple chains, which are composed of one central double and two peripheral single chains extended along the a axis (Fig. 4a). The central double chain is built up of alternating corner-sharing Fe1O 6 octahedra and As1O 4 tetrahedra. The two sides of the double chain are 'out of phase' meaning that Fe1O 6 octahedra in one side are arranged in a line with, and are linked to, As1O 4 tetrahedra on the other side. The peripheral single chains are made up of alternating corner-sharing Fe2O 6 octahedra and As2O 4 tetrahedra, which are also 'out of phase' with regard to the central double chain. These octahedral-tetrahedral-quadruple chains are centred around the inversion centre at (0,0,0) with one single and one half of a double chain on each side (Fig. 4b). The chains are further linked by As3O 4 tetrahedra to form a slab parallel to the (010) plane. A slab is placed between the relative heights, y, from approximately -0.35 to 0.35 (Fig. 4b). The threedimensional structure is formed by interconnected slabs through bridging As4O 4 tetrahedra and hydrogen bonds. The adjacent slabs are linked additionally to each other via bonds to Sr 2+ cations, which are accommodated inside the channels of the hetero-polyhedral three-dimensional open framework.
Strontium cations are also linked by sharing O and OH groups. The Sr1 atoms are located inside the small channels extending nearly along [010]. The channels are centred on the inversion centre at 0.5, 0.5, 0.5 and are bounded by octahedral-tetrahedral chains with As3O 4 and As4O 4 tetrahedra. By sharing a common two symmetry-equivalent oxygen atom O23, two 9-coordinated Sr1 form a Sr1 2 O 16 dimer. The nine Sr-O distances range from 2.473(10) to 3.120(9) Å.
The Sr2 atoms inside the large channels along [100] are highly mobile in the crystal lattice. They appear distributed among ten positions in the channels. These Sr21-Sr30 positions are too close to be occupied simultaneously and therefore Sr2 atoms are actually distributed randomly over ten pairs of symmetry-equivalent sites. The site occupancy factors of the four most occupied positions are refined to 0.210 (6), 0.225(6), 0.140(6) and 0.113 (6). The other positions have less than 10% occupancy.
As expected, the As-OH bond distances are slightly elongated in comparison to the other As-O bonds in tetrahedra. One OH group (O24H24) acts as a hydrogen-bond donor to the oxygen O44 from the other OH group (D···A distance is 3.114(16) Å and D-H···A angle is 153°). Another two hydrogen bonds are bifurcated. The H-bond donor O34 is bound to two H-bond acceptors, O43 and O44, at D···A distances of 2.631(16) and 2.833(15) Å, respectively. Both are bent with D-H···A angles of 137 and 128°. Similarly, the other H-bond donor O44 is bound to two H-bond acceptors, O34 and O43, at the comparable D···A distances of 2.833 (15) and 3.123(14) Å, respectively, which are also bent (D-H···A angles are 152 and 125°). Because hydrogen atoms H34 and H44 were found in general positions at a short distance of 1.48 Å their occupancy factors were fixed at 0.5. All these hydrogen bonds are of average to weak strength (Table 8).

Raman spectroscopy
In all SrM-arsenate hydrates (M = Ni 2+ and Fe 3+ ) presented in this study, Raman bands in the 1000-600 cm -1 region are assigned to the symmetric and antisymmetric stretching modes of the (AsO 4 ) 3and/or (AsO 3 OH) 2groups, whereas internal bending vibrations of these tetrahedra are observed below 550 cm -1 , were they are partially overlaid by various external and lattice modes.
Raman spectra of III reflect the complexity of the crystal structure. The considerably large number of bands, which cannot be certainly specified, are caused by the four crystallographically different AsO 4 tetrahedra (of which three are protonated AsO 3 OH groups). Spectral data on orthoarsenates that have been published previously are, so far, somewhat incomplete and not in good agreement with each other. Therefore, attempts to compare with them failed. However, the distinct frequency ranges may be assigned as follows: Based on the d-ν correlation for hydrogen bonds (Libowitzky, 1999), the Raman-shift values observed in the O-H stretching region are in very good agreement with the refined O−H···O bond lengths ranging between 2.63(2) and 3.12(1) Å (corresponding Raman bands obtained between 2500 and 3700 cm -1 ) (Fig. 7).

Structural relations to the associated minerals and inorganic compounds
SrNi 2 (AsO 4 ) 2 ·2H 2 O (I), is isostructural with minerals of the tsumcorite group, which adopt the monoclinic symmetry, space group C2/m (Z = 2). The general formula of the tsumcorite group is A(M 2+,3+ ) 2 (XO 4 ) 2 (H 2 O,OH) 2 (A is a large cation, M 2+,3+ is usually an octahedrally coordinated transition metal cation and X is a tetrahedrally coordinated small cation). The   (Mihajlović and Effenberger, 2004). minerals of this group have been described with Pb, Ca, Bi and Na on the A site, Zn, Fe 3+ , Cu 2+ , Mn 3+ and Co on the M site, and As 5 + , P 5+ , V 5+ and S 6+ on the X site (Tillmanns and Gebert, 1973;Pring et al., 1989;Kharisun et al., 1997;Krause et al., 1998Krause et al., , 2001aKrause et al., , 2001bKrause et al., , 2002Effenberger et al., 2000;Brugger et al., 2000Brugger et al., , 2001Yang et al., 2012;Elliott and Pring, 2015;Pekov et al., 2016). They display either monoclinic or triclinic symmetry, depending on the chemical composition as well as ordering of the hydrogen bonds and cations on the M sites. The basic tsumcorite-type structure mentioned previously is monoclinic (space group C2/m, Z = 2) and represents the parental structure of the group referred to as tsumcorite-type structure. It was determined by Tillmanns and Gebert (1973). The majority of the tsumcorite-group minerals (Table 9) crystallise in space group C2/m, but some members display lower symmetry. The next modification is also monoclinic referred to as the Sr-Co-type structure. It is characterised by protonated anion tetrahedra (Mihajlović and Effenberger, 2004) and a doubled cell volume (Z = 4). In addition, the space-group symmetry is lowered to the P2 1 /a. The triclinic members of the tsumcorite group (Table 9) crystallise in space group P 1 (Z = 1) and can be divided into the helmutwinklerite and rappoldite subgroup . In the helmutwinklerite subgroup, referred to as a helmutwinklerite-type structure, the symmetry reduction is due to the ordering of the hydrogen bonds. The characteristic of this subgroup is two H 2 O per formula unit and the hydrogen atoms are rearranged in order to avoid the symmetry-restricted hydrogen bonds. These changes in hydrogen bonding induce structural transformations that cause the transition from the monoclinic tsumcorite-type structure to the triclinic helmutwinklerite-type structure (Krause et al., , 2001bEffenberger et al., 2000;Elliott and Pring, 2015). In the second triclinic subgroup, referred to as the gartrellite type, symmetry reduction is due to the cation ordering on the two M positions in the triclinic unit cell.
Formula, mineral name a (Å) b (Å) c (Å) α /β /γ (°) Volume (Å 3 ) Reference OH or H 2 O depends on the amount of M 3+ and M 2+ cations and the valence and quantity of the large cation A. The novel-type crystal structure of III is characterised by the unique octahedral-tetrahedral-quadruple, [Fe(AsO 4 )] n infinite chains, which are composed of one central double and two peripheral single chains extended along the a axis. Similar verticessharing double-sided chains of octahedra and tetrahedra has been found in galliskiite, Ca 4 Al 2 (PO 4 ) 2 F 8 ·5H 2 O (Kampf et al., 2010) and some phosphates (Mills et al., 2010a(Mills et al., , 2010b and references therein). Dissimilar quadruple chains are found in the synthetic ferric sulfate trihydrate, Fe 2 (SO 4 ) 3 ·3H 2 O (Xu and Parise, 2011). Besides the exceptional occurrence of quadruple chains of FeO 6 and AsO 4 polyhedra, the characteristics of this structure are the novel chemical composition and the extremely high positional disorder of Sr2 atoms.

Conclusion
The results of phase formation studies in the system Sr-M 2+ -As-O-H increase the knowledge on SrM-oxoarsenates(V) and their crystal structures. All three structures have different chains as the primary building units. Compound I is a new strontium nickel analogue of tsumcorite PbZn 2 (AsO 4 ) 2 ·2H 2 O (monoclinic, s.g. C2/m) having a structure which is built up of infinite linear edge-sharing NiO 4 (OH 2 ) 2 octahedral chains with AsO 4 tetrahedra and SrO 8 polyhedra linking them by sharing corners. Compound II exists as an unusual intermediate form between carminite, PbFe 3+ 2 (AsO 4-) 2 (OH) 2 and arsenbrackebuschite, Pb 2 Fe 3+ (AsO 4 ) 2 (OH) type structures. The characteristic feature of compound II is that it hosts two type of chains: (i) octahedral vertex-and edge-sharing FeO 4 (OH) 2 chains like in carminite and (ii) chains similar to arsenbrackebuschite which involves SrO 11 coordination polyhedra, known as Edshammar polyhedra, that can be described as a combination of a trigonal bipyramid and trigonal prism centred by the same Sr atom surrounded by 11 oxygen atoms.
The MΦ 6 octahedra (Φ = O, OH or OH 2 ) in I like in tsumcorite and arsenbrackebuschite is all edge sharing, in contrast to the alternating edge and vertices sharing in carminite and II. In carminite and II structures two MΦ 6 octahedra share edges to form a M 2 Φ 10 dimer and the M 2 Φ 10 dimers share corners to form an M 2 Φ 8 chain that is decorated by XO 4 tetrahedra. These parallel chains are linked to form a framework by sharing octahedron and tetrahedron vertices. Contrary to this, the arsenbrackebuschite and tsumcorite MΦ 6 octahedra share one set of trans edges with adjacent octahedra to form a MΦ 4 chain. Adjacent octahedra are bridged by XO 4 tetrahedra to form a decorated chain. The parallel chains in tsumcorite are further linked in the M 2 (XO 4 ) 2 Φ 2 sheets similar to those in natrochalcite (nominally NaCu 2 (SO 4 ) 2 (OH)·2H 2 O). No such linkage occurs in arsenbrackebuschite where the MΦ 6 octahedra occupy only half of the octahedral sites, because every second octahedral chain is omitted and replaced by chains of 11-coordinated Pb. Similar substitutions were found in II i.e. nearly 20% of the octahedra chains are replaced by SrO 11 polyhedra chains. The 8-and 11-coordinated Sr atoms in II adopt an almost identical polyhedral environment, such as in the synthetic compound Sr 2 V(VO 4 ) 2 (OH) with a brackebuschite-type structure .
Compound III exhibits a new type of structure having interesting novel quadruple chains which are composed of FeO 6 octahedra alternating with AsO 4 tetrahedra sharing corners. In the structure of III the quadruple chains are connected with AsO 4 tetrahedra in a 3D open framework with the channels occupied by Sr atoms.
The chains in I have the same topology as the chains in tsumcorite and the chains in II have the topology as the chains in carminite and brackebuschite. Quadruple chains of a topology different to that of III were found in the synthetic compound Fe 2 (SO 4 ) 3 ·3H 2 O (Xu and Parise, 2011).
The further interesting feature of these structures is variable coordination geometry of Sr 2+ cations combined with the substitutional (in II) and positional (in II and III) disorder. The strontium coordination geometries include near-prismatic SrO 8 coordination polyhedra in I and II, the chains of Edshammar SrO 11 coordination polyhedra in II and mutually isolated Sr 2 O 16 dimers in III.