Crystal Chemistry of Alkali–Aluminum–Iron Sulfates from the Burnt Mine Dumps of the Chelyabinsk Coal Basin, South Urals, Russia

Technogenic steklite, KAl(SO4)2, and unnamed mineral phase (K,Na)3Na3(Fe,Al)2(SO4)6 from burnt dumps of the Chelyabinsk Coal Basin have been investigated by single-crystal X-ray diffraction and electron microprobe analysis. Steklite is trigonal, space group P3, a = 4.7277(3), c = 7.9871(5) Å, V = 154.60(2) Å3. The crystal structure was refined to R1 = 0.026 (wR2 = 0.068). It is based upon the [Al(SO4)2]− layers formed by corner sharing of SO4 tetrahedra and AlO6 polyhedra. The anionic [Al(SO4)2]− layers are parallel to the (001) plane and linked via interlayer K+ ions. The regular octahedral coordination of Al is observed that distinguishes technogenic steklite from that found in Tolbachik fumaroles. The (K,Na)3Na3(Fe,Al)2(SO4)6 phase is trigonal, space group R3, a = 13.932(2), c = 17.992(2) Å, V = 3024.4(7) Å3, R1 = 0.073 (wR2 = 0.108). The crystal structure is based upon the anionic chains [(Fe,Al)(SO4)3]3− running parallel to the c axis and interconnected via K+ and Na+ ions. There are no known minerals or synthetic compounds isotypic to (K,Na)3Na3(Fe,Al)2(SO4)6, due to the presence of separate K and Na sites in its structure.


Introduction
This study is a continuation of the series of our publications devoted to the specific features of the crystal chemistry of technogenic minerals from burnt dumps of the Chelyabinsk Coal Basin [1][2][3][4][5].
Here, we report, in detail, the features of two alkali-bearing Al-Fe sulfates crystallized from gaseous phases in high-temperature technogenic environments.
In general, sulfates constitute one of the most important classes of minerals which contains numerous species diverse from both chemical and structural points of view [6]. The technogenic sulfates formed at burning dumps of coal mines are close in their genetic features to purely natural sulfates crystallizing in oxidizing-type volcanic fumaroles (for recent discoveries, see [7][8][9][10][11]).

Materials
The samples of steklite ( Figure 1) studied by us are of technogenic origin and were selected from the collection of Boris V. Chesnokov currently deposited at the Natural Science Museum of the Ilmen State Reserve (Miass, Russia). The sample originates from the coal dump of the mine No. 47 in the city of Kopeisk, Chelyabinsk area, Southern Urals, Russia [13], where it was found as transparent colorless hexagonal lamellar crystals with the average size of 0.3-0.5 mm.

Chemical Composition
The chemical composition of technogenic steklite was determined using a Hitachi S-3400N equipped with Oxford X-Max 20 energy dispersive X-ray spectrometer at the Resource Centre "Geomodel" of St. Petersburg State University. The spectra were processed automatically using the AzTec Energy software package using the TrueQ technique. The electron beam accelerating voltage was 20 kV and 1 nA current was used. The X-ray acquisition time was 60 s both in spot mode and small area scanning. Quantification of elemental compositions was conducted using standard samples of natural and synthetic compounds (NaAlSi 3

Chemical Composition
The chemical composition of technogenic steklite was determined using a Hitachi S-3400N equipped with Oxford X-Max 20 energy dispersive X-ray spectrometer at the Resource Centre "Geomodel" of St. Petersburg State University. The spectra were processed automatically using the AzTec Energy software package using the TrueQ technique. The electron beam accelerating voltage was 20 kV and 1 nA current was used. The X-ray acquisition time was 60 s both in spot mode and small area scanning. Quantification of elemental compositions was conducted using standard samples of natural and synthetic compounds (NaAlSi3O8 for Na and Al, KAlSi3O8  The chemical composition of the (K,Na)3Na3(Fe,Al)2(SO4)6 phase was determined on a Jeol JSM-6480LV scanning electron microscope equipped with an INCA-Wave 500 wavelength-dispersive spectrometer (Laboratory of Analytical Techniques of High Spatial Resolution, Dept. of Petrology, Moscow State University), with an acceleration voltage of 20 kV, a beam current of 20 nA, and a 3 μm beam diameter. The following standards were used: albite (Na), microcline (K), diopside (Ca, Mg), Mn (Mn), Al2O3 (Al), magnetite (Fe), and SrSO4 (S). The average composition (wt.%) is Na2O

Single-Crystal X-ray Diffraction
Single-crystal X-ray diffraction study of steklite and (K,Na)3Na3(Fe,Al)2(SO4)6 from Kopeisk was performed at the X-ray Diffraction Resource Centre of St. Petersburg State University. In the case of steklite, a Bruker Kappa APEX DUO diffractometer operated at 45 kV and 0.6 mA and equipped with a CCD area detector was used for data collection. The study was done by means of monochromatic MoKα radiation (λ = 0.71073 Å), frame widths of 0.5° in ω, and 10 s counting time for each frame. The intensity data were reduced and corrected for Lorentz, polarization, and background effects using Bruker software APEX2 [22]. A semiempirical absorption-correction based upon the intensities of equivalent reflections was applied [23]. The (K,Na)3Na3(Fe,Al)2(SO4)6 phase was studied on a STOE IPDS II diffractometer. More than half of the diffraction sphere were collected using MoKα radiation and scanning along ω with a step of 2° and 3 min exposition. The distance of the crystal detector was The empirical formula calculated based on 24 oxygen atoms is (K 1.95 Na 1.05 ) Σ3 (Na 3.05 Ca 0.02 ) Σ3.07 (Fe 3+ 1.08 Al 0.84 Mg 0.02 Mn 0.01 ) Σ1.95 S 6.02 O 24 .

Single-Crystal X-ray Diffraction
Single-crystal X-ray diffraction study of steklite and (K,Na) 3 Na 3 (Fe,Al) 2 (SO 4 ) 6 from Kopeisk was performed at the X-ray Diffraction Resource Centre of St. Petersburg State University. In the case of steklite, a Bruker Kappa APEX DUO diffractometer operated at 45 kV and 0.6 mA and equipped with a CCD area detector was used for data collection. The study was done by means of monochromatic MoKα radiation (λ = 0.71073 Å), frame widths of 0.5 • in ω, and 10 s counting time for each frame. The intensity data were reduced and corrected for Lorentz, polarization, and background effects using Bruker software APEX2 [22]. A semiempirical absorption-correction based upon the intensities of equivalent reflections was applied [23]. The (K,Na) 3 Na 3 (Fe,Al) 2 (SO 4 ) 6 phase was studied on a STOE IPDS II diffractometer. More than half of the diffraction sphere were collected using MoKα radiation and scanning along ω with a step of 2 • and 3 min exposition. The distance of the crystal detector was chosen equal to 100 mm. The numerical absorption correction based on measured crystal faces was applied using the X-RED [24] and X-SHAPE [25] programs.
The crystal structure of steklite was solved by direct methods and refined in the space group P3 to R 1 = 0.026 (wR 2 = 0.068) for 565 unique observed reflections with I ≥ 2σ(I) using ShelX program package [26] within the Olex2 shell [27]. The crystal structure of (K,Na) 3 Na 3 (Fe,Al) 2 (SO 4 ) 6 was solved and refined in the space group R3 to R 1 = 0.073 (wR 2 = 0.108) for 1200 unique observed reflections with I ≥ 2σ(I) using ShelX program package [26]. Crystal data, data collection information, and structure refinement details for both phases are given in Table 1; atom coordinates and displacement parameters are in Tables 2 and 3, and selected interatomic distances are in Table 4. Crystallographic data have been deposited at Cambridge Crystallographic Data Centre (2,042,224 for steklite; 2,042,223 for (K,Na) 3 Na 3 (Fe,Al) 2 (SO 4 ) 6 ) (also see Supplementary Materials). Table 1. Crystal data and structure refinement parameters for both studied sulfates.

Results
The crystal structure of synthetic KAl(SO4)2 was first reported in 1970 [29] as based upon the [Al(SO4)2] − layers formed by corner sharing of SO4 tetrahedra and AlO6 polyhedra (Figure 2). The anionic [Al(SO4)2] − layers are parallel to the (001) plane and linked via interlayer K + ions. The layer topology in steklite can be described using the 2D graph representation, where black and white vertices symbolize the Al and S polyhedra, respectively ( Figure 3). The observed topology is identical to that of the [Fe(SO4)2] − layers in yavapaiite, KFe(SO4)2 [30,31], and the [Mg(PO4)2] 4− layers in brianite, Na2CaMg(PO4)2 [32], as well as to the similar anionic layers in a number of synthetic compounds [33]. The average <S-O> bond length in sulfate tetrahedra in steklite is 1.471 Å , with the S-O distances for the O atoms bridging between sulfate tetrahedra with AlO6 polyhedra (1.478 Å ) essentially longer than that for the terminal O atoms (1.443 Å ) ( Table 4). The Al atom has an octahedral coordination with the <Al-O> bond length equal to 1.889 Å (see Discussion for more details). The interlayer K + cations are coordinated by 12 oxygen atoms with the <K-O> bond length equal to 3.061 Å (Figure 4b).      In the crystal structure of (K,Na) 3 Na 3 (Fe,Al) 2 (SO 4 ) 6 , there are three symmetrically independent sites with octahedral coordination. All of them are predominantly occupied by Fe 3+ with some amount of Al ( Table 2). The average interatomic cation-anion distances for these sites approximately correlate with the amount of Al in these positions. Two crystallographic positions of S are in tetrahedral coordination; the average <S-O> bond lengths are 1.468 and 1.470 Å for S1 and S2, respectively. There are two sites occupied by the Na + and K + cations. The 9-coordinated K site (Figure 4a) contains mainly K, with the rather high Na content (27%) reflected in the noticeable shortening of the K-O bond lengths compared to pure K. The Na site is fully occupied by sodium and has a strongly distorted octahedral coordination (Figure 4a).
The projection of the crystal structure of (K,Na) 3 Na 3 (Fe,Al) 2 (SO 4 ) 6 phase is shown in Figure 5. It is based upon the anionic chains [(Fe,Al)(SO 4 ) 3 ] 3− , running parallel to the c axis and interconnected via K + and Na + ions. Within the chains, each [(Fe,Al)O 6 ] octahedron is corner-linked to six adjacent (SO 4 ) tetrahedra, while only two O atoms of each tetrahedron are bridging; that is, they form the S-O-Fe bonds (Figure 6a). The S-O bond lengths for the bridging O atoms are longer (1.482-1.501 Å) than those for terminal oxygen atoms (1.439-1.450 Å), in agreement with the similar observation for steklite. The calculated crystal chemical formula of (K,Na) 3 Na 3 (Fe,Al) 2 (SO 4 ) 6 phase can be represented as (K 2.19 Na 0.81 ) Σ3 Na 3 (Fe 3+ 1.44 Al 0.56 ) Σ2 (SO 4 ) 6 , which is generally in good agreement with the empirical formula, taking into account variations in the composition of the studied phase.
The average <S-O> bond lengths for both sulfates studied herein are consistent with the average value of 1.473 Å, determined for sulfate minerals by Hawthorne et al. [6]. In both phases, the S-O bond lengths for bridging O atoms are longer than those for terminal oxygen atoms, which is typical for heteropolyhedral complexes with tetrahedral oxoanions and is widely manifested, for example, in uranyl salts [34]. via K and Na ions. Within the chains, each [(Fe,Al)O6] octahedron is corner-linked to six adjacent (SO4) tetrahedra, while only two O atoms of each tetrahedron are bridging; that is, they form the S-O-Fe bonds (Figure 6a). The S-O bond lengths for the bridging O atoms are longer (1.482-1.501 Å ) than those for terminal oxygen atoms (1.439-1.450 Å ), in agreement with the similar observation for steklite. The calculated crystal chemical formula of (K,Na)3Na3(Fe,Al)2(SO4)6 phase can be represented as (K2.19Na0.81)Σ3Na3(Fe 3+ 1.44Al0.56)Σ2(SO4)6, which is generally in good agreement with the empirical formula, taking into account variations in the composition of the studied phase.  The average <S-O> bond lengths for both sulfates studied herein are consistent with the average value of 1.473 Å , determined for sulfate minerals by Hawthorne et al. [6]. In both phases, the S-O bond lengths for bridging O atoms are longer than those for terminal oxygen atoms, which is typical for heteropolyhedral complexes with tetrahedral oxoanions and is widely manifested, for example,

Discussion
The most interesting aspect of the crystal structure of steklite studied here is related to the shape of the AlO 6 polyhedra. In general, the AlO 6 coordination polyhedron in KAl(SO 4 ) 2 may theoretically adopt either trigonal prismatic or octahedral geometry. For the KM(SO 4 ) 2 compounds (where M is a trivalent cation), the octahedral AlO 6 configuration was reported for the P3 space group, and the trigonal prismatic configuration was observed for the P312 space group [33]. West et al. [35] indicated that synthetic KAl(SO 4 ) 2 crystallizes in the P3 space group with the octahedral coordination of Al and disorder in the stacking of adjacent layers due to the rotation of polyhedra within the layers around the direction parallel to the c axis. However, Murashko et al. [14] described natural, fumarolic steklite from the Tolbachik volcano with coordination polyhedron of Al intermediate in a shape between a trigonal prism and octahedron with the rotation angle between the upper and lower triangular bases of about 10 • (it is equal to 0 and 30 • for a trigonal prism and an octahedron, respectively). In the case of fumarolic steklite, no disorder induced by the rotation of AlO 6 polyhedra was detected, and the crystal structure is well described by the P312 space group. In the case of technogenic steklite studied here, the regular octahedral coordination of Al was observed (space group P3) that distinguishes Kopeisk steklite from that found in Tolbachik fumaroles. The origin of different AlO 6 configurations in steklite from different occurrences requires additional comment. The difference in unit cell volumes between Tolbachik and Kopeisk samples of steklite is in fact negligible (154.76 vs. 154.60 Å 3 , respectively) and does not allow considering them as temperature-related. According to Murashko et al. [14], the temperature of formation of steklite in fumaroles cannot be determined reliably; the possible temperature range is 150-340 • C. For the Kopeisk sulfates, Chesnokov et al. [13] assigned the crystallization temperature of 650-750 • C, i.e., sufficiently higher than that in volcanic fumaroles. There is a general agreement that the symmetry and shape of local configurations in crystals tends to be higher with the increase of temperature [36]. This agrees well with the possible higher-temperature origin of the Kopeisk steklite compared to the Tolbachik sample, since the symmetry of an octahedron (m-3m; point group order = 48) is higher than that of a trigonal prism (−62m; point group order = 12). However, the problem is far from being clear and needs further experimental investigation.
The topology of the octahedral-tetrahedral chains in (K,Na) 3 Na 3 (Fe,Al) 2 (SO 4 ) 6 can be described using the 1D graph shown in Figure 6b, where black and white vertices symbolize octahedra and tetrahedra, respectively. Figure 6c shows an idealized graph in which each black vertex is 6-connected, and each white vertex is 2-connected. The topology shown in Figure 6c is quite common for the crystal structures of oxosalts of metals in octahedral coordination [33]. Among minerals, it was observed in the crystal structures of kaatialaite, Fe(H 2 AsO 4 ) 3 (H 2 O) 5 [37], and ferrinatrite, Na 3 [Fe 3+( SO 4 ) 3 ](H 2 O) 3 [38,39], as well as in fumarolic minerals such as aluminocoquimbite, AlFe(SO 4 ) 3 (H 2 O) 9 [40], and pyracmonite, (NH 4 ) 3 Fe(SO 4 ) 3 [41]. However, none of these minerals is isotypic to (K,Na) 3 Na 3 (Fe,Al) 2 (SO 4 ) 6 due to the presence of separate K and Na sites in its structure. Ferrinatrite, aluminocoquimbite, and pyracmonite were found in the fumaroles of the La Fossa Crater (Vulcano Island, Sicily, Italy), which indicates the possibility of the formation of (K,Na) 3 Na 3 (Fe,Al) 2 (SO 4 ) 6 under natural conditions during crystallization from high-temperature volcanic gases.
According to the current recommendations of the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the IMA, the newly formed minerals found in burnt coal dumps can be considered as valid mineral species, assuming that "the fire occurred as a result of natural events (self-ignition or lightning) and, beyond a shadow of doubt, was not of anthropogenic origin" [21]. Therefore, the new (K,Na) 3 Na 3 (Fe,Al) 2 (SO 4 ) 6 phase reported herein can be considered as a separate mineral species, despite its techogenic origin in burnt mine dumps of the Chelyabinsk coal basin.