Crystal Chemistry and High-Temperature Behaviour of Ammonium Phases NH4MgCl3·6H2O and (NH4)2Fe3+Cl5·H2O from the Burned Dumps of the Chelyabinsk Coal Basin

The technogenic mineral phases NH4MgCl3·6H2O and (NH4)2Fe3+Cl5·H2O from the burned dumps of the Chelyabinsk coal basin have been investigated by single-crystal X-ray diffraction, scanning electron microscopy and high-temperature powder X-ray diffraction. The NH4MgCl3·6H2O phase is monoclinic, space group C2/c, unit cell parameters a = 9.3091(9), b = 9.5353(7), c = 13.2941(12) Å, β = 90.089(8)° and V = 1180.05(18) Å3. The crystal structure of NH4MgCl3·6H2O was refined to R1 = 0.078 (wR2 = 0.185) on the basis of 1678 unique reflections. The (NH4)2Fe3+Cl5·H2O phase is orthorhombic, space group Pnma, unit cell parameters a = 13.725(2), b = 9.9365(16), c = 7.0370(11) Å and V = 959.7(3) Å3. The crystal structure of (NH4)2Fe3+Cl5·H2O was refined to R1 = 0.023 (wR2 = 0.066) on the basis of 2256 unique reflections. NH4MgCl3·6H2O is stable up to 90 °C and then transforms to the less hydrated phase isotypic to β-Rb(MnCl3)(H2O)2 (i.e., NH4MgCl3·2H2O), the latter phase being stable up to 150 °C. (NH4)2Fe3+Cl5·H2O is stable up to 120 °C and then transforms to an X-ray amorphous phase. Hydrogen bonds provide an important linkage between the main structural units and play the key role in determining structural stability and physical properties of the studied phases. The mineral phases NH4MgCl3·6H2O and (NH4)2Fe3+Cl5·H2O are isostructural with natural minerals novograblenovite and kremersite, respectively.


Introduction
The present publication is devoted to the detailed crystal chemical study of NH 4 MgCl 3 ·6H 2 O and (NH 4 ) 2 Fe 3+ Cl 5 ·H 2 O, two ammonium phases from the burned dumps of the Chelyabinsk coal basin (Chelyabinsk Oblast, Russia). Nowadays the phases from that mine are considered as anthropogenic, despite the fact that eight phases from the same locality have previously been accepted as mineral species: bazhenovite [1], godovikovite [2], dmisteinbergite [3], svyatoslavite [4], rorisite [5], efremovite [6], srebrodolskite [7] and fluorellestadite [8]. It is worthy to note that many phases first described from the burned dumps of the Chelyabinsk coal basin by Chesnokov and co-authors [9] were later found in Table 1. Recently discovered ammonium minerals.

Mineral Locality, Scenarios of Formation Reference
Acmonidesite (NH 4 ,K,Pb 2+ ,Na) 9 [32] Cuatrocapaite-(NH 4 ) (NH 4 ) 3 (NaM )(As 2 O 3 ) 6 Cl 6 ·16H 2 O Torrecillas Mine, Salar Grande, Iquique Province, Tarapacá Region, Chile. [33] Davidbrownite-(NH 4 ) (NH 4 ) 5 (V 4+ O) 2  Oxidation zone of a sphalerite-rich orebody. [40] Novograblenovite (NH 4  Russia. Exhalation due to volcanic gas exposure. [10] Russoite (NH 4 )ClAs 2 O 3 (H 2 O) 0. 5 Solfatara di Pozzuoli, Pozzuoli, Napoli, Italy. Fumarolic phase. [41] Minerals 2019, 9, 486 3 of 16 These minerals originate either as fumarolic or mofettic (i.e., "volcanic" minerals) [10,26,30,41] or as secondary phases formed due to the contact with organic matter including burning of coal seams and dumps as well as in guano deposits ( Table 1). The burned dumps of the Chelyabinsk coal basin appeared to be an important locality of ammonium compounds: at least 16 minerals and technogenic mineral-like phases were described there ( Table 2). The genesis of mineral-like phases at burning coal dumps is closely related to the fumarolic formation, because both processes occur at elevated temperatures and require the principal role of gases as mineral-forming media. In principle, ammonium compounds are typical for burned coal dumps and they can form mostly during the "pseudofumarolic" stage and as a result of supergene processes [42]. The appearance of ammonium phases on burned coal dumps can be a useful indicator of the presence of underground fires [42]. At the same time, it should be noted that the origin of ferrierite-NH 4 [37], which is wide spread over a large area, is not connected with underground fires. The high-temperature study of such exhalative minerals is of interest for establishing relations between species formed at the same geochemical environment, but under different temperature regimes. Such studies have been recently performed for fumarolic minerals: saranchinaite, Na 2 Cu(SO 4 ) 2 , and euchlorine, KNaCu 3 O(SO 4 ) 3 , from the Tolbachik volcano (Kamchatka, Russia) [43,44] and the efflorescence mineral tschermigite, (NH 4 )Al(SO 4 ) 2 ·12H 2 O, from geothermal fields of Southern Kamchatka (Russia) [45]. The aim of the present study is to provide the first crystal chemical description of NH 4 MgCl 3 ·6H 2 O and (NH 4 ) 2 Fe 3+ Cl 5 ·H 2 O phases from the Chelyabinsk coal basin, including their first structure refinements and determinations of their stability at increasing temperatures coupled with the calculation of thermal expansion coefficients and their crystal chemical analysis.

Occurrence
The samples of NH 4  "Redikortsevite", NH 4 MgCl 3 ·6H 2 O, crystallizes as colourless or light-yellow crystals with a flat prismatic shape. The crystals of "redikortsevite" are associated with "kopeiskite", (NH 4 ) 2 Fe 3+ Cl 5 ·H 2 O, that forms prismatic and pseudo-octahedral crystals with a bright orange colour [9]. Both phases are very hygroscopic, easily dissolve in water and should be stored under favourable environmental conditions only, which hinders their investigation under usual atmospheric conditions.

Chemical Composition
Four crystals of "redikortsevite", NH 4 MgCl 3 ·6H 2 O, and "kopeiskite", (NH 4 ) 2 Fe 3+ Cl 5 ·H 2 O, were mounted in epoxy blocks and polished. The polishing process was started on dry sandpaper and was completed on paper medium with 1 µm diamond suspension oil. The samples avoided water at all stages of polishing. The samples were coated with a 10 nm conductive carbon layer for the scanning electron microscopy (SEM) studies. Quantitative elemental analyses were carried out using a scanning electron microscope Hitachi S3400N equipped with the Oxford X-Max 20 energy-dispersive spectrometer at the Resource Centre "Geomodel" of St. Petersburg State University. The working conditions were 20 kV accelerating voltage and 1.5 nA beam current. The spectra were obtained at the spot mode for 30 s each and revealed that NH 4   1 Nitrogen (ammonium) content can be taken as approximate due to problems with measuring elements with Z < 8 using energy-dispersive spectroscopy; 2 calculated from the crystal-structure data; 3 calculated on the basis of Mg = 1; 4 calculated on the basis of Fe + Cl = 6.

Single-Crystal XRD
Single-crystal X-ray diffraction studies of NH 4 MgCl 3 ·6H 2 O and (NH 4 ) 2 Fe 3+ Cl 5 ·H 2 O were performed at the Resource Centre "X-ray Diffraction Methods" of St. Petersburg State University using a Bruker Kappa APEX DUO diffractometer operated at 45 kV and 0.6 mA and equipped with a CCD area detector. The study was done by means of a monochromatic MoKα X-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 the Bruker software APEX2 [46]. A semiempirical absorption-correction based upon the intensities of equivalent reflections was applied [47]. The unit-cell parameters were refined by least square techniques. The structures were solved and refined using ShelX program package [48] within the Olex2 shell [49]. Crystal data, data collection information and structure refinement details are given in Table 4; atom coordinates and displacement parameters are in Tables 5 and 6, and selected interatomic distances and angles are in Table 7. All H atoms were derived from the analysis of Fourier difference electron-density maps and refined in an isotropic approximation. The H atoms forming ammonium groups have been fixed using DFIX instruction in order to maintain a reasonable geometry of the NH 4 groups with Uiso(H) set to 1.5 Ueq(N) and N-H 0.86 Å. The crystals of NH 4 MgCl 3 ·6H 2 O were found to display a non-merohedral twinning. De-twinning treatment was achieved by the application of the CrysAlisPro software [50].

High-Temperature Powder X-Ray Diffraction
In situ high-temperature powder X-ray diffraction (HTXRD) experiments of NH 4 MgCl 3 ·6H 2 O and (NH 4 ) 2 Fe 3+ Cl 5 ·H 2 O phases up to 540 and 260 • C ( Figure 1) were done in air using a Rigaku Ultima IV powder X-ray diffractometer (CoKα 1+2 radiation, 40 kV/30 mA, Bragg-Brentano geometry, PSD D-Tex Ultra) with Rigaku HT 1500 high-temperature attachment. Thin powder samples were deposited on Pt sample holders (20 × 12 × 2 mm 3 ) from heptane suspensions. The temperature steps and the average heating rates were 10 • C and 2/3 • /min, respectively; the collecting time at each temperature step was about 20 min. Silicon was used as an external standard. to 140 °C. No reflections were observed in the powder patterns in the temperature range 160-370 °C. Above 370 °C, broad reflections of periclase, MgO, appeared. The unit-cell parameters of NH4MgCl3·6H2O and its high-temperature phase (HT phase) (Tables S1 and S2) were refined by the Rietveld method. The refinements were based on the reflections in the 2θ region 10-75°. The refinement of the unit-cell parameters was done in the temperature ranges 23-80 °C and 100-140 °C for NH4MgCl3·6H2O and HT phase, respectively (powder pattern recorded at 90 °C was excluded from the refinement due to the appearance of reflections of the HT phase). (NH4)2Fe 3+ Cl5·H2O is stable up to 120 °C and then transforms to an X-ray amorphous phase. It is interesting that the shape of reflections and their intensity is slightly different at each of the patterns recorded at different temperatures, most likely, due to some changes in the structure of the phase. Therefore, the unit-cell parameters refined by the Rietveld method in the 2θ region 10-75° and in temperature range 24-80 °C (Table S3) should be taken as approximate only.
In all cases, the Rietveld refinements were carried out using Topas 4.2 [52]; the crystallographic data of starting models used for refinement are given in Table 8. Rietveld refinements were done with the fixed atom coordinates, site scattering and isotropic-displacement parameters. The background was modelled using a Chebyshev polynomial approximation of the 18th order. The It was found that NH 4 MgCl 3 ·6H 2 O transforms to another phase at T~90 • C, which is stable up to 140 • C. No reflections were observed in the powder patterns in the temperature range 160-370 • C. Above 370 • C, broad reflections of periclase, MgO, appeared. The unit-cell parameters of NH 4 MgCl 3 ·6H 2 O and its high-temperature phase (HT phase) (Tables S1 and S2) were refined by the Rietveld method. The refinements were based on the reflections in the 2θ region 10-75 • . The refinement of the unit-cell parameters was done in the temperature ranges 23-80 • C and 100-140 • C for NH 4 MgCl 3 ·6H 2 O and HT phase, respectively (powder pattern recorded at 90 • C was excluded from the refinement due to the appearance of reflections of the HT phase).
(NH 4 ) 2 Fe 3+ Cl 5 ·H 2 O is stable up to 120 • C and then transforms to an X-ray amorphous phase. It is interesting that the shape of reflections and their intensity is slightly different at each of the patterns recorded at different temperatures, most likely, due to some changes in the structure of the phase. Therefore, the unit-cell parameters refined by the Rietveld method in the 2θ region 10-75 • and in temperature range 24-80 • C (Table S3) should be taken as approximate only.
In all cases, the Rietveld refinements were carried out using Topas 4.2 [52]; the crystallographic data of starting models used for refinement are given in Table 8. Rietveld refinements were done with the fixed atom coordinates, site scattering and isotropic-displacement parameters. The background was modelled using a Chebyshev polynomial approximation of the 18th order. The peak profile was described using the fundamental parameters approach. The main coefficients of the thermal-expansion tensor were determined using the TTT program package [53,54] and the TEV program [55].

Chemical Composition
The chemical compositions were averaged based on 15/10 analyses for NH 4 MgCl 3 ·6H 2 O and (NH 4 ) 2 Fe 3+ Cl 5 ·H 2 O, respectively. Due to the low quality of crystal surfaces and the high rate of their dehydration in vacuum, the data were normalized to 100 wt. %. The ratios between the specie-defining cations and anions were determined as NH 4 /Mg/Cl = 0.96/1/3.08 and NH 4 /Fe/Cl = 1.70/0.94/5.06 (Table 3), which is close to those in the ideal chemical formulas of the studied phases.
The NH4MgCl3·6H2O phase is characterized by rather strong anisotropy of thermal expansion with the αa/αb/αc ratio equal to 3.2/1/2.2 ( Table 11). The strongest anisotropy is observed within the xy plane ( Figure 6). In contrast, thermal expansion of (NH4)2Fe 3+ Cl5·H2O is rather uniform with the αa/αb/αc ratio equal to 1/1.1/1.2 (Table 11). The data obtained for the HT phase are less accurate with larger ESDs and only five points available for approximation (Table S5). In general, the behaviour of the HT phase at elevated temperatures can be described as a strong expansion with the thermal expansion coefficients changing abruptly, due to the rather abrupt and nonlinear changes of the unit-cell parameters ( Figure S1).       (25) 134 (9) 1 At T = 120 • C; α-coefficient of thermal expansion (α 11 , α 22 , α 33 -eigenvalues (main values); α a , α b , α c -values along crystallographic axes); <α 11 a-angle between α 11 and a.
The NH 4 MgCl 3 ·6H 2 O phase is characterized by rather strong anisotropy of thermal expansion with the α a /α b /α c ratio equal to 3.2/1/2.2 ( Table 11). The strongest anisotropy is observed within the xy plane ( Figure 6). In contrast, thermal expansion of (NH 4 ) 2 Fe 3+ Cl 5 ·H 2 O is rather uniform with the α a /α b /α c ratio equal to 1/1.1/1.2 (Table 11). The data obtained for the HT phase are less accurate with larger ESDs and only five points available for approximation (Table S5). In general, the behaviour of the HT phase at elevated temperatures can be described as a strong expansion with the thermal expansion coefficients changing abruptly, due to the rather abrupt and nonlinear changes of the unit-cell parameters ( Figure S1). with the distance between two donor oxygen atoms O1 … O1 equal to 6.418 Å) than the Cl1-H2A distance (2.45 (5) Å with the O2 … O2 distance of 6.270 Å) ( Figure 6). Therefore, the anisotropy of thermal expansion within the xy plane reflects the anisotropy of the strength of the hydrogen bonding network involving Cl atoms. Along the c axis, the Mg(H2O)6 octahedra are linked via hydrogen bonds through Cl2 atoms ( Figure 6) with the Cl2-H distances ranging from 2.39 to 2.45 Å. However, the Cl2-H bonds are oriented differently (i.e., the network is not unidirectional) and thus cannot be directly compared with the Cl1-H distances. It is worthy to note that the dehydration-induced NH4MgCl3·6H2O  NH4MgCl3·2H2O transformation is associated with restructuring of the coordination sphere of the NH4 + cations. The latter are coordinated mainly by H2O molecules in the high hydrated phase, whereas, in the dihydrate, the coordination is mostly by the Cl − anions. For NH4MgCl3·6H2O, the hydrogen bonding network plays a crucial role in the structural stability and the character of the physical properties such as thermal expansion. For (NH4)2Fe 3+ Cl5·H2O, no preferred orientation of the Cl-H bonds is observed, resulting in the absence of thermal expansion anisotropy.

Conclusions
The study of the technogenic NH4MgCl3·6H2O and (NH4)2Fe 3+ Cl5·H2O phases from the burned dumps of the Chelyabinsk coal basin shows that they are complete analogues of novograblenovite and kremersite, respectively. In contrast to the two minerals, the technogenic phases do not contain K and are almost pure ammonium compounds. As was shown earlier [10], and confirmed in the present study, structural integrity of the studied phases is determined by the hydrogen bonding

Discussion
The crystal-structure study of the technogenic phase "kopeiskite" confirmed that (NH 4 ) 2 Fe 3+ Cl 5 ·H 2 O is a complete analogue of kremersite. The technogenic phase "redikortsevite", NH 4 MgCl 3 ·6H 2 O, is not a dimorph, but an direct analogue of novograblenovite, (NH 4 ,K)MgCl 3 ·6H 2 O. The only difference is the presence of K in the mineral, while the technogenic phase contains ammonium only. It is interesting that the pure potassium analogue of novograblenovite, carnallite, KMgCl 3 ·6H 2 O, belongs to a different structure type [10]. The previous studies of XMgCl 3 ·6H 2 O compounds (X = Li, K, NH 4 , Rb, Cs) [10,25,64] demonstrated that the crystal structures of the Li and K phases differ from those of the NH 4 , Rb and Cs phases. In contrast, for (X) 2 Fe 3+ Cl 5 ·H 2 O compounds (X = K, NH 4 , Rb, Cs), the crystal structure of the Cs phase differs from those of the NH 4 , K and Rb phases. Therefore, the ionic radii have a significant impact upon the structure type in the compounds under consideration (Table 12). 1 Effective ionic radii for CN = 6 by Shannon [69] and for NH 4 by Sidey [70].
The structural nature of thermal expansion anisotropy observed for NH 4 MgCl 3 ·6H 2 O phase deserves special attention. The three-dimensional integrity of its structure is controlled by the hydrogen bonding network. Assuming that weaker bonds are more prone to stretching, one should analyse the hydrogen bonding system responsible for the linkage of Mg(H 2 O) 6 octahedra via Cl atoms ( Figure 6). The analysis of interatomic distances indicates that the Cl1-H bonds are located either along a (Cl1-H1B bond) or b (Cl1-H2A bond) axes. The Cl1-H1B distance is longer (2.50 (5) Å with the distance between two donor oxygen atoms O1 . . . O1 equal to 6.418 Å) than the Cl1-H2A distance (2.45 (5) Å with the O2 . . . O2 distance of 6.270 Å) ( Figure 6). Therefore, the anisotropy of thermal expansion within the xy plane reflects the anisotropy of the strength of the hydrogen bonding network involving Cl atoms. Along the c axis, the Mg(H 2 O) 6 octahedra are linked via hydrogen bonds through Cl2 atoms ( Figure 6) with the Cl2-H distances ranging from 2.39 to 2.45 Å. However, the Cl2-H bonds are oriented differently (i.e., the network is not unidirectional) and thus cannot be directly compared with the Cl1-H distances. It is worthy to note that the dehydration-induced NH 4 MgCl 3 ·6H 2 O → NH 4 MgCl 3 ·2H 2 O transformation is associated with restructuring of the coordination sphere of the NH 4 + cations. The latter are coordinated mainly by H 2 O molecules in the high hydrated phase, whereas, in the dihydrate, the coordination is mostly by the Cl − anions. For NH 4 MgCl 3 ·6H 2 O, the hydrogen bonding network plays a crucial role in the structural stability and the character of the physical properties such as thermal expansion. For (NH 4 ) 2 Fe 3+ Cl 5 ·H 2 O, no preferred orientation of the Cl-H bonds is observed, resulting in the absence of thermal expansion anisotropy.

Conclusions
The study of the technogenic NH 4 MgCl 3 ·6H 2 O and (NH 4 ) 2 Fe 3+ Cl 5 ·H 2 O phases from the burned dumps of the Chelyabinsk coal basin shows that they are complete analogues of novograblenovite and kremersite, respectively. In contrast to the two minerals, the technogenic phases do not contain K and are almost pure ammonium compounds. As was shown earlier [10], and confirmed in the present study, structural integrity of the studied phases is determined by the hydrogen bonding networks that serve as the most important intermediates providing linkage between main structural units. The distribution of the hydrogen bonds also explains the high-temperature behaviour of the crystal structures of the compounds under consideration and therefore determines their physical properties.
The analogy between the two studied phases and their natural counterparts, novograblenovite and kremersite, points to the geochemical analogy between technogenic (burned coal dumps) and natural (volcanic fumaroles) environments. Thus, one may expect the discoveries of more NH 4 -bearing mineral species analogous to the technogenic phases previously described by Chesnokov et al. [9] ( Table 2).