Zn(NH 3 ) 2 Cl 2 , a Mineral-Like Anthropogenic Phase with Ammine Complexes from the Burned Dumps of the Chelyabinsk Coal Basin, South Urals, Russia: Crystal Structure, Spectroscopy and Thermal Evolution

: The mineral-like anthropogenic phase Zn(NH 3 ) 2 Cl 2 , with ammine (NH 30 ) complexes from the burned dumps of the Chelyabinsk coal basin (South Urals, Russia), has been investigated using single-crystal and high-temperature powder X-ray diﬀraction, and Raman and infrared (IR) spectroscopy. The anthropogenic Zn(NH 3 ) 2 Cl 2 is orthorhombic, Imma , a = 7.7399(6), b = 8.0551(5), c = 8.4767(8) Å, V = 528.49(7) Å 3 , R 1 = 0.0388 at −73 °C. Its crystal structure is based upon isolated ZnN 2 Cl 2 tetrahedra connected by hydrogen bonds (between NH 3 groups and Cl atoms) into a three-dimensional network. Upon heating, the Zn(NH 3 ) 2 Cl 2 phase is stable up to about 150 °C, which is in good agreement with the data on the temperature of its formation. The crystal structure of Zn(NH 3 )Cl 2 expands anisotropically with the strongest thermal expansion observed along the a axis. The thermal expansion of the structure is controlled by the changes in the hydrogen bonding sys-tem. The Raman and IR spectroscopic characteristics of this phase are close to those of the mineral ammineite, CuCl 2 (NH 3 ) 2 . The studied anthropogenic phase, formed in the unique conditions of burned coal dumps, is identical to the synthetic Zn(NH 3 ) 2 Cl 2 .


Introduction
Zn(NH3)2Cl2, an anthropogenic mineral-like phase, was found by Chesnokov and coauthors in the burned coal dumps of the Chelyabinsk coal basin (ChCB) in the South Urals, Russia in 1984 [1].The Chelyabinsk burned coal dumps are well-known for their anthropogenic mineral-like phases.More than 240 different compounds have been found in this region and about 50 of them were unique at the time of their first description.Eight phases found in the Chelyabinsk dumps were approved as new mineral species, with the ChCB as a type locality [2].
At least sixteen different ammonium-bearing phases from the ChCB are of special interest as they are formed through the contact of burning coal with organic ma er at elevated temperatures.Their formation involves crystallization from gases, the phases are formed during the so-called "pseudofumarole" stage or as a result of supergene process, and they have much in common with the fumarolic ammonium minerals and similar minerals formed by the reactions of mineral components with organic substance, for example, guano [3][4][5].At the same time, only one phase containing ammine (NH3 0 ) complexes, Zn(NH3)2Cl2, has been described from the ChCB burned dumps.This work is devoted to the crystal chemical features of this phase found at the ChCB.
The synthetic compound Zn(NH3)2Cl2 (zinc diammine chloride, or diamminedichlorozinc) is well known in the chemistry of complex compounds [11] and belongs to the class of diamminechlorides (chlorides containing two NH3 complexes) of various divalent metals with the general formula M 2+ (NH3)2Cl2, where M 2+ = Cu, Zn, Mg, Fe, Co, Ni, Cd, Hg, Ca and Pt.In nature, except for ammineite, CuCl2(NH3)2 [6], no such compounds have been described so far.Some synthetic metal chlorides with ammine complexes have applications in technologies and medicine: for example, the cis-[PtCl2(NH3)2] complex (Cisplatin) is an important anticancer drug [12] and the compound Mg(NH3)6Cl2 is considered as a solid matrix for hydrogen storage [13].
The purpose of this work is a detailed crystal chemical description of a mineral-like anthropogenic Zn(NH3)2Cl2 from the ChCB burned dumps, including the determination of its crystal structure, spectroscopic studies and the correlation of the obtained results with the data known for its synthetic analogue.One of the aims of the current study is to investigate the thermal evolution of anthropogenic Zn(NH3)2Cl2 by means of in situ singlecrystal and powder X-ray diffraction studies at different temperatures.Such an investigation is of interest since Zn(NH3)2Cl2 was found in discharged air-zinc ba eries and may also be a byproduct of hydrocracking of heavy oil fractions (see [14,15] and references therein).The thermal stability and thermodynamic characteristics of synthetic Zn(NH3)2Cl2 have been well studied (see [15] and references therein).However, data from in situ X-ray diffraction studies at different temperatures are absent in the literature and would complement the characterization of the thermal evolution of Zn(NH3)2Cl2, taking into account its formation under the conditions of the burning dumps of the ChCB.

Occurrence
The studied sample of the mineral-like anthropogenic Zn(NH3)2Cl2 phase was taken from the collection of Prof. Boris V. Chesnokov (1928Chesnokov ( -2005)), which is deposited in the Natural Science Museum of the Ilmen State Reserve (Miass, Russia) under catalogue number 099-10.It was described by Chesnokov and co-authors as "amminite" from a burning coal heap located near the town of Gornyak at the northern border of the city Kopeisk.This compound was found among the alteration products of a zinc plate placed in the near-surface part of burning coal-bearing material for nine days.It forms colorless or brownish crystals (Figure 1) closely associated with ZnO (anthropogenic analogue of zincite) and Zn5Cl2(OH)8 ("chlorozincite").According to Chesnokov and co-authors, the formation of Zn(NH3)2Cl2 occurred at temperatures lower than 200 °C as a result of the interaction between Zn or ZnO with hot gas enriched by chlorine and nitrogen compounds [1].

Chemical Composition
Several crystals of Zn(NH3)2Cl2 were mounted in epoxy blocks and polished without exposure to water.The samples were coated with a 10 nm conductive carbon layer for scanning electron microscopy (SEM) studies.Quantitative elemental analysis was carried out using a Hitachi S-3400N scanning electron microscope with an Oxford Instruments Energy Dispersive Spectrometer X-Max (20 kV and 1.7 nA, the working distance is 10 mm).The energy-dispersive spectra were processed automatically using the AzTec Energy software package using the TrueQ technique.The X-ray acquisition time was 30 s in spot mode.Quantification of elemental compositions was conducted using standard samples of natural and synthetic compounds: Zn for Zn, NaCl for Cl and BN for N. Since nitrogen was only determined semi-quantitatively, its amount was calculated based on structural data, as well as the amount of hydrogen.

Raman Spectroscopy
The Raman spectrum of Zn(NH3)2Cl2 was obtained using a Horiba Jobin-Yvon LabRam HR 800 system at room temperature in the range of 70-4000 cm −1 using a solid state laser (λ = 532 nm, power on the sample 8 mW) and 50× objective.The equipment was calibrated according to a silicon standard.The sample was oriented randomly.The data accumulation time took from 2 to 10 s.The obtained spectrum was visualized in OriginPro 2018 SR1 b9.5.1.195.

Infrared Spectroscopy
The infrared (IR) spectrum of Zn(NH3)2Cl2 was obtained using a Bruker Vertex 70 FTIR spectrometer with the KBr pellet method (200 mg of KBr and 2 mg of the sample) in the range of 4000-370 cm −1 .

Single-Crystal X-ray Diffraction
Single-crystal X-ray diffraction (SCXRD) studies of Zn(NH3)2Cl2 were performed using a Rigaku XtaLAB Synergy-S diffractometer (MoKα radiation, 50 kV and 1.0 mA, frame widths 0.5° in ω and 10 s counting time for each frame) with high-stability microfocus Xray source PhotonJet-S and a high-speed hybrid photon counting detector HyPix-6000HE.The single-crystal X-ray diffraction studies in air were kept in situ at the following temperatures: −173 °C, −123 °C, −73 °C, −23 °C, 27 °C, 77 °C and 127 °C (at 127 °C, the crystal lost crystallinity within a relatively short time; for this reason, SCXRD data for this temperature were not obtained) using the Oxford Cryosystems Cryostream.SCXRD data were collected at different temperatures without changing the orientation of the crystal.The main coefficients of the thermal expansion tensor were determined using the TTT program package [16].
The CrysAlisPro software was used for data processing [17].An absorption correction was introduced using the SCALE3 ABSPACK algorithm.The structures have been solved and refined using ShelX program package [18] within the Olex2 shell [19].Crystal data, data collection information and structure refinement details at −73 °C (at this temperature, the refinement parameters were the best, so this is the reason why we have provided all the main structural data for this temperature) are given in Table 1; atom coordinates and displacement parameters are shown in Tables 2 and 3 and selected interatomic distances and angles are presented in Tables 4 and S1, respectively.H-atoms were derived from an analysis of the Fourier difference electron-density maps and refined freely in an isotropic approximation.The parameters of the hydrogen bonds of Zn(NH3)2Cl2 at different temperatures are given in Table 5.The unit-cell parameters of Zn(NH3)2Cl2 at different temperatures are shown in Table 6.Vesta software was used to visualize the structural data [20].

High-Temperature Powder X-ray Diffraction
The thermal behavior of Zn(NH3)2Cl2 was studied in air using an in situ high-temperature powder X-ray diffraction (HTXRD) method, using a Rigaku Ultima IV diffractometer (CuKα radiation, 40 kV/30 mA, Bragg-Brentano geometry, high-speed DTEX Ultra 1D detector, Pt substrate) with a Rigaku SHT 1500 high-temperature a achment.The sample was studied in the temperature range 30-250 °C with 10 °C steps (Figure S1).The roomtemperature data were loaded into PDXL [21] to check the phase composition.The TOPAS software package [22] was used for the refinement of unit-cell parameters at all temperatures using the Pawley method.The background was modelled using a Chebyshev polynomial approximation of the 9 th 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 [16].

Structural Complexity
The structural complexity of Zn(NH3)2Cl2 was calculated using the ToposPro software package [23] for the model with localized hydrogen atoms (our data), as well as for the models without localized hydrogen atoms [24].The structural complexity calculation approach is based on the amount of Shannon information measured in bits per atom (IG, bits/atom) and per unit cell (IG,total, bits/cell), the method of numerical evaluation of structural complexity was developed by S.V. Krivovichev (see [25,26] and references therein).

Chemical Composition
The empirical formula of Zn(NH3)2Cl2 was calculated on the basis of 2 Cl atoms per formula unit; the N and H amounts were calculated using stoichiometry from the structure refinement (Table 7).Unfortunately, the Zn(NH3)2Cl2 phase is unstable and apparently partially oxidized, so the analyses were normalized to 100 wt.%.Despite this fact, the analyses show a good agreement with the Zn:Cl~1:2 stoichiometry.

Raman Spectroscopy
In general, the spectrum obtained for Zn(NH3)2Cl2 has many similarities with the spectrum described for ammineite, CuCl2(NH3)2 (see Table S2 for details), which also contains ammine complexes [27].The most intense modes of the spectrum correspond to the stretching vibrations of the NH3 groups and Zn-N and Zn-Cl stretching and bending vibrations (Figure 2).The bands at <200 cm −1 can be assigned to la ice modes.The intense band at 281 cm −1 is probably connected with the symmetric stretching vibrations (νs) of zinc-halogen (Zn-Cl), whereas the band at 417 cm −1 corresponds to the νs Zn-N vibration modes [27,28].The medium intense bands at 633 cm −1 and 682 cm −1 are connected with rocking vibration modes of NH3.The region from 3000 to 3500 cm −1 (3255 cm −1 (νs), 3170 (m), 3332 (m)) can be a ributed to the stretching N-H vibration modes [29].The low-intensity bands at 1333 cm −1 , 1457 cm −1 and 1594 cm −1 can relate to the symmetric (δs) and asymmetric (δas) bending modes of NH3 [28].The bands in the ranges 1750-3000 cm −1 and also 3500-4000 cm −1 are not clearly identified, but it can be assumed that they correspond to spectral artifacts.

Infrared Spectroscopy
The IR spectrum of Zn(NH3)2Cl2 (Figure 3) is close to the spectrum of "amminite", Zn(NH3)2Cl2, from Kopeisk earlier reported by Chukanov [30].Most intense bands of Zn(NH3)2Cl2 are assigned to the vibrations of the NH3-groups.Thus, according to Bojar et al. (see [6] and references therein), the bands in the range 3400-3000 cm −1 (3328, 3192, 3161 and their shoulders) correspond to antisymmetric and symmetric (NH3) stretching vibrations.We assume that the band at 3497 cm −1 can also be assigned to antisymmetric and symmetric (NH3) stretching modes.The degenerate bending vibration of δd-(NH3) has a wavenumber of 1617 cm −1 .The symmetric deformation of δs-(NH3) appears at 1247 cm −1 .The intense band at 1404 cm −1 can also be assigned to N-H vibrations (it may also probably be a trace of an ammonium salt impurity [6]).Libration vibrations of ρr-(NH3) form the triplet with the wavenumbers 684, 642 and 622 cm −1 at the IR spectrum of Zn(NH3)2Cl2.The weak band at 478 cm −1 can be assigned to the Zn-N antisymmetric deformations, and the bands at 417 and 391 cm −1 correspond to la ice modes.The bands at 2364 and 2339 cm −1 probably correspond to uncompensated atmospheric CO2 (as well as the "ripples" in the range 1600-1550 cm −1 corresponding to atmospheric water).The band at 1124 cm −1 probably belongs to an impurity (we can assume that the impurity is a Zn sulphate phase, e.g.goslarite, ZnSO4•7H2O, which is a common supergene zinc sulphate and the most intense band in its IR spectrum demonstrates maximum at 1125 cm −1 [30]).The IR spectrum of Zn(NH3)2Cl2 is similar to that of ammineite, CuCl2(NH3)2 [6,30].The main NH3-bands in similar positions were also recently described for Co(NH3)6Cl3 by Wang et al. [31].Noteworthy, the spectra of both "amminite", Zn(NH3)2Cl2, (reported by Chukanov) and ammineite, CuCl2(NH3)2, do not contain bands in the region 1200-1100 cm −1 [6,30].
The crystal structure of Zn(NH3)2Cl2 is based upon isolated ZnN2Cl2 tetrahedra (Figure 4) connected by hydrogen bonds (between NH3 groups and Cl atoms) into a threedimensional network (Figures 5 and 6).The positions of hydrogen atoms localized by our refinement are generally consistent with the model proposed by Ivšić et al. [14].The parameters of the hydrogen bonds are given in Table 5 and shown in Figures 5 and 6.The Zn-Cl bond length is 2.269 Å and the Zn-N bond length is 2.008 Å (for −73 °C).The SCXRD experiments at different temperatures (from −173 to 127 °C in 50 °C increments) indicated that, at 127 °C, the crystal lost its crystallinity.An analysis of the data shows that, with the increasing temperature, there is a consistent increase in the unit-cell parameters and volume (Table 6).

High-Temperature Powder X-ray Diffraction
The temperature dependencies of the unit-cell parameters are shown in Figure 7.The values obtained from the powder data are shown together with the data from the single crystal experiments at different temperatures.As can be seen from Figure 7, they are in a good agreement.According to the HTXRD experiments, the Zn(NH3)Cl2 phase is stable up to about 150 °C (Figure S1).At the same time, as per the SCXRD data, the phase begins to lose its crystallinity at 127 °C.It is worth noting that, according to the literature, the synthetic Zn(NH3)Cl2 phase is stable upon heating up to 190 °C [15].The difference can probably be explained by the different heating rates and exposure times used in the different methods.The crystal structure of Zn(NH3)Cl2 expands anisotropically with the strongest thermal expansion observed along the αa direction (Table 8, Figure 5); the αmax/αmin value is about 1.64.The parameters for the approximation equations describing the temperature dependences of the unit-cell parameters of Zn(NH3)Cl2 are given in Table 9.

Structural Complexity
The structural complexity values for Zn(NH3)2Cl2, including all atoms (IG, IG,total) and excluding localized H-atoms (IG(no H), IG,total(no H)), are provided in Table 10.The structural complexity per unit cell (IG,total) is about three times higher when including H atoms.

Discussion
The crystal chemical studies demonstrate that anthropogenic Zn(NH3)2Cl2 ("amminite") from the ChCB burned dump is completely identical to its synthetic analogue.Although phases from burned dumps under some circumstances could now be considered as valid mineral species [33], this zinc diammine chloride is definitely man-made (has anthropogenic or technogenic origin) due to the ad hoc introduction of the zinc plate.
Thus, the possibility of the formation of Zn(NH3)2Cl2 under completely natural conditions is still questionable.
In general, there are very few minerals containing ammine complexes (see Introduction).All these minerals contain copper and were found at the Pabellon de Pica guano deposit within the contact zone between bird guano and the surface of gabbro enriched with chalcopyrite, CuFeS2.The la er, being exposed to air, oxidizes and serves as a source of Cu 2+ for numerous supergene minerals [7][8][9].From these five minerals, two are chlorides, and only ammineite, CuCl2(NH3)2 (not to be confused with the anthropogenic "amminite" (Chesnokov et al.)), belongs to the class of diamminechlorides of the divalent metal cations with the general formula M 2+ (NH3)2Cl2 (see above), which also includes the studied Zn(NH3)2Cl2.It is of interest that the crystal structures of CuCl2(NH3)2 and Zn(NH3)2Cl2 are not isotypic.In ammineite, CuCl2(NH3)2, the coordination of Cu 2+ is distorted octahedral, due to the Jahn-Teller effect, and the structure is based upon zigzag chains of distorted octahedra [6].According to Bojar et al. [6], the formation of ammineite in nature was due to the reaction of Cu minerals with guano.
Thus, although the question remains open, under certain geochemical conditions, the formation of Zn(NH3)2Cl2 may occur in nature, for instance, in the environments that involve reactions of Zn-bearing minerals (for example, sphalerite-rich rocks) with complicated organic ma er, such as guano which contains organic compounds with ammine groups.Apparently, the source of ammonia for the formation of Zn(NH3)2Cl2 in the ChCB burned coal dumps was a special organic substance contained in coal-bearing dump material; however, we cannot say whether this is directly related to coal.This fact explains the difference in the number of compounds with ammonia (NH3 0 ) and ammonium (NH 4+ ) described from the burned coal dumps of the ChCB: one species vs at least sixteen species, respectively.The presence of such special (specific) organic substances within the burning coal dumps of the ChCB is indirectly confirmed by the presence of other crystalline organic phases found there in sublimates of the "pseudofumaroles", such as C10H12N8O8 and kladnoite, C6H4(CO)2NH [2].
Upon heating, the Zn(NH3)2Cl2 phase is stable up to about 150 °C.This is in good agreement with the data of Chesnokov and co-authors, who suggested temperatures of Zn(NH3)2Cl2 formation lower than 200 °C [1].The crystal structure of Zn(NH3)2Cl2 expands anisotropically (Table 8, Figure 5); at the same time, the bond lengths within the ZnN2Cl2 tetrahedra practically do not change (within the errors; see Table S1 for details), which indicates that the thermal expansion of the structure is controlled by the changes in the hydrogen bonding system (most obviously fixed by the increase in the distance D...A, Table 5).The observed anisotropy is probably connected to a specific arrangement of hydrogen bonds relative to the crystallographic axes.Thus, the maximum expansion is

Figure 1 .
Figure 1.The colorless and brownish crystals of Zn(NH3)2Cl2 from the burned dump of the ChCB.

Figure 4 .
Figure 4.The tetrahedra ZnN2Cl2 with bond lengths (Å) indicated for −73 °C.Legend: Zn is shown as gray ellipsoids (displacement ellipsoids, probability 50%), N as blue and Cl as green.Hydrogen atoms are shown as white balls.

Figure 5 .
Figure 5.The crystal structure of Zn(NH3)2Cl2 and the orientation of the section of the figure of thermal expansion coefficients (red) in the plane (relative to the shown projections): projection on the plane bc (A), on the plane ac (B) and on the plane ab (C).The unit cell is highlighted by the solid black line.Hydrogen bonds are shown with a do ed line.The legend is as in Figure 4.

Figure 6 .
Figure 6.Fragment of the crystal structure of Zn(NH3)2Cl2: projection on the plane ab (A) and on the plane ac (B).The do ed line shows the hydrogen bonds (the bond lengths Cl...H (Å) are given for temperature −73 °C).The legend is as in Figure 4.