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

Abstract

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.

1. Introduction

The present publication is devoted to the detailed crystal chemical study of NH₄MgCl₃·6H₂O and (NH₄)₂Fe³⁺Cl₅·H₂O, 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 natural environments (e.g., novograblenovite [10], pyracmonite [11], kumtyubeite [12], harmunite [13], ghiaraite [14], steklite [15], khesinite [16], rusinovite [17], etc.).

The mineral phase with the chemical composition NH₄MgCl₃·6H₂O was first described from the Chelyabinsk coal basin as an ammonium analogue of carnallite, KMgCl₃·6H₂O [18], and named as “redikortsevite” [19], but this naming as well as the mineral species have not been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (CNMNC IMA). Later, this phase was found in Germany [20] and Poland [21]. Very recently, the same phase was found in a natural environment forming on the surface of basalts of the 2012–2013 Tolbachik effusive eruption (Plosky Tolbachik volcano, Kamchatka area, Far-Eastern Region, Russia) and approved by the CNMNC IMA as a mineral species named novograblenovite (NH₄,K)MgCl₃·6H₂O [10]. Another phase studied here is the technogenic analogue of the rare mineral kremersite, (NH₄,K)₂[Fe³⁺Cl₅(H₂O)], that was originally described from Mt. Vesuvius (Somma-Vesuvius Complex, Naples, Campania, Italy) [22]. The pure ammonium phase, (NH₄)₂Fe³⁺Cl₅·H₂O, from the burned dumps of Kopeisk (Chelyabinsk coal basin) was proposed by Chesnokov [9] as a separate mineral species, the ammonium analogue of erythrosiderite, K₂Fe³⁺Cl₅·H₂O [23], and named “kopeiskite”. However, this proposal was not approved by the CNMNC MMA as a species identical to kremersite. The synthetic analogue of kremersite has been suggested recently as a new multiferroic material with a strong magnetoelectric coupling [24,25].

In general, about 80 NH₄-bearing minerals are known to date, with many of them being discovered recently (Table 1).

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-NH₄ [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, Na₂Cu(SO₄)₂, and euchlorine, KNaCu₃O(SO₄)₃, from the Tolbachik volcano (Kamchatka, Russia) [43,44] and the efflorescence mineral tschermigite, (NH₄)Al(SO₄)₂·12H₂O, from geothermal fields of Southern Kamchatka (Russia) [45]. The aim of the present study is to provide the first crystal chemical description of NH₄MgCl₃·6H₂O and (NH₄)₂Fe³⁺Cl₅·H₂O 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.

2. Materials and Methods 2.1. Occurrence

The samples of NH₄MgCl₃·6H₂O and (NH₄)₂Fe³⁺Cl₅·H₂O phases investigated in this study were taken from the personal collection of B.V. Chesnokov currently deposited in the Natural Science Museum of the Ilmen State Reserve (Miass, Russia). Initially these samples were discovered from the coal dumps of mine No. 50 near Kopeisk city, Chelyabinsk area, Southern Urals, Russia [9]. Both phases were found in the chloride-rich crusts formed in the upper parts of the heaps of mines. “Redikortsevite”, NH₄MgCl₃·6H₂O, crystallizes as colourless or light-yellow crystals with a flat prismatic shape. The crystals of “redikortsevite” are associated with “kopeiskite”, (NH₄)₂Fe³⁺Cl₅·H₂O, 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.

2.2. Chemical Composition

Four crystals of “redikortsevite”, NH₄MgCl₃·6H₂O, and “kopeiskite”, (NH₄)₂Fe³⁺Cl₅·H₂O, 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 NH₄MgCl₃·6H₂O contains abundant Mg, Cl, O and N, whereas (NH₄)₂Fe³⁺Cl₅·H₂O contains abundant Fe, Cl, O and N (Table 3). No significant zoning of crystals was observed.

2.3. Single-Crystal XRD

Single-crystal X-ray diffraction studies of NH₄MgCl₃·6H₂O and (NH₄)₂Fe³⁺Cl₅·H₂O 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 Table 5 and Table 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 NH₄ groups with Uiso(H) set to 1.5 Ueq(N) and N–H 0.86 Å. The crystals of NH₄MgCl₃·6H₂O were found to display a non-merohedral twinning. De-twinning treatment was achieved by the application of the CrysAlisPro software [50].

2.4. High-Temperature Powder X-Ray Diffraction

In situ high-temperature powder X-ray diffraction (HTXRD) experiments of NH₄MgCl₃·6H₂O and (NH₄)₂Fe³⁺Cl₅·H₂O phases up to 540 and 260 °C (Figure 1) were done in air using a Rigaku Ultima IV powder X-ray diffractometer (CoKα₁₊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 mm³) 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.

It was found that NH₄MgCl₃·6H₂O 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 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)₂Fe³⁺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 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].

3. Results 3.1. Chemical Composition

The chemical compositions were averaged based on 15/10 analyses for NH₄MgCl₃·6H₂O and (NH₄)₂Fe³⁺Cl₅·H₂O, 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 NH₄/Mg/Cl = 0.96/1/3.08 and NH₄/Fe/Cl = 1.70/0.94/5.06 (Table 3), which is close to those in the ideal chemical formulas of the studied phases.

3.2. Crystal Structures

The crystal structure of synthetic NH₄MgCl₃·6H₂O was described by Solans et al. [57] and Marsh [58] without localization of hydrogen atoms. Recently, the crystal structure of novograblenovite, (NH₄,K)MgCl₃·6H₂O, was described in detail, including localization of hydrogen atoms, description of structural peculiarities and structural relationships with carnallite, KMgCl₃·6H₂O [10].

The crystal structure of NH₄MgCl₃·6H₂O is based upon regular Mg(H₂O)₆ octahedra that are connected to Cl⁻ anions, and indirectly to NH₄⁺ cations, through hydrogen bonds into a three-dimensional framework (Figure 2, Table 7 and Table 9). The Mg site is octahedrally coordinated by six H₂O molecules with the mean <Mg–O> bond length of 2.045 Å (Table 7). Each NH₄⁺ ion is surrounded by six CI⁻ ions in an octahedral arrangement with the average <Cl–N> distance of 3.330 Å (Table 7). In general, the crystal structure of NH₄MgCl₃·6H₂O can be represented as a perovskite-like network of (NH₄)Cl₆ octahedra with Mg(H₂O)₆ octahedra localized in framework cavities. Table 10 provides the results of bond-valence analysis for NH₄MgCl₃·6H₂O phase with bond-valence parameters taken from [59,60,61,62].

The crystal structure of kremersite, (NH₄,K)₂[Fe³⁺Cl₅(H₂O)], was described earlier based on the data obtained from its pure synthetic analogue, (NH₄)₂Fe³⁺Cl₅·H₂O [63]. It is based upon isolated FeCl₅(H₂O) octahedra that are connected to NH₄⁺ cations through hydrogen bonds into a three-dimensional framework (Figure 3, Table 9). The Fe site is octahedrally coordinated by five Cl atoms and one H₂O molecule with the average <Fe–Cl,O> distance of 2.325 Å (Table 7). A bond-valence calculation for (NH₄)₂Fe³⁺Cl₅·H₂O phase with bond-valence parameters taken from [59,60,61,62] is given in Table S4.

3.3. HTXRD

The NH₄MgCl₃·6H₂O phase dehydrates to NH₄MgCl₃·2H₂O (further denoted as HT phase), which is isotypic to β-Rb(MnCl₃)(H₂O)₂ [56]. The unit-cell parameters at different temperatures are provided in Tables S1–S3 for NH₄MgCl₃·6H₂O, NH₄MgCl₃·2H₂O, and (NH₄)₂Fe³⁺Cl₅·H₂O, respectively, and are visualized in Figure 4, Figure 5 and Figure S1. The main thermal expansion coefficients for NH₄MgCl₃·6H₂O and (NH₄)₂Fe³⁺Cl₅·H₂O were determined using linear approximations (Tables S5 and S6) of temperature dependencies of the unit-cell parameters (Table 11).

The NH₄MgCl₃·6H₂O phase is characterized by rather strong anisotropy of thermal expansion with the α_a/α_b/α_cratio 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)₂Fe³⁺Cl₅·H₂O is rather uniform with the α_a/α_b/α_cratio 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).

4. Discussion

The crystal-structure study of the technogenic phase “kopeiskite” confirmed that (NH₄)₂Fe³⁺Cl₅·H₂O is a complete analogue of kremersite. The technogenic phase “redikortsevite”, NH₄MgCl₃·6H₂O, is not a dimorph, but an direct analogue of novograblenovite, (NH₄,K)MgCl₃·6H₂O. 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, KMgCl₃·6H₂O, belongs to a different structure type [10]. The previous studies of XMgCl₃·6H₂O compounds (X = Li, K, NH₄, Rb, Cs) [10,25,64] demonstrated that the crystal structures of the Li and K phases differ from those of the NH₄, Rb and Cs phases. In contrast, for (X)₂Fe³⁺Cl₅·H₂O compounds (X = K, NH₄, Rb, Cs), the crystal structure of the Cs phase differs from those of the NH₄, K and Rb phases. Therefore, the ionic radii have a significant impact upon the structure type in the compounds under consideration (Table 12).

The structural nature of thermal expansion anisotropy observed for NH₄MgCl₃·6H₂O 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(H₂O)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(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)₂Fe³⁺Cl₅·H₂O, no preferred orientation of the Cl–H bonds is observed, resulting in the absence of thermal expansion anisotropy.

5. Conclusions

The study of the technogenic NH₄MgCl₃·6H₂O and (NH₄)₂Fe³⁺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 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 NH₄-bearing mineral species analogous to the technogenic phases previously described by Chesnokov et al. [9] (Table 2).

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Mineral	Locality, Scenarios of Formation	Reference
Acmonidesite (NH4,K,Pb2+,Na)9Fe42+(SO4)5Cl8	La Fossa Crater, Vulcano, Aeolian Islands, Sicily, Italy. Fumarolic phase, (T ~ 250 °C).	[26]
Ammoniolasalite [(NH4)2Mg2(H2O)20]·[V10O28]	Burro Mine, San Miguel County, Utah, USA. Secondary phases.	[27]
Ammoniomathesiusite (NH4)5(UO2)4(SO4)4(VO5)·4H2O		[28]
Burroite Ca2(NH4)2(V10O28)·15H2O		[29]
Ammoniovoltaite (NH4)2Fe2+5Fe3+3Al(SO4)12(H2O)18	Severo-Kambalny geothermal field, southern Kamchatka, Russia. Efflorescence around gas-steam hydrothermal vents.	[30]
Ammoniozippeite (NH4)2[(UO2)2(SO4)O2]·H2O	Blue Lizard Mine, San Juan County, Utah, and the Burro Mine, San Miguel County, Colorado, USA. Low-temperature, secondary phase within organic-rich beds.	[31]
Redcanyonite (NH4)2Mn[(UO2)4O4(SO4)2](H2O)4		[32]
Cuatrocapaite-(NH4) (NH4)3(NaM☐)(As2O3)6Cl6·16H2O	Torrecillas Mine, Salar Grande, Iquique Province, Tarapacá Region, Chile.	[33]
Davidbrownite-(NH4) (NH4)5(V4+O)2(C2O4)[PO2.75(OH)1.25]4·3H2O	Rowley Mine, Painted Rock Mts, Arizona, USA. Low-temperature, apparently post-mining suite of phases that include various vanadates, phosphates, oxalates and chlorides, some containing NH4+.	[34]
Phoxite (NH4)2Mg2(C2O4)(PO3OH)2(H2O)4		[35]
Rowleyite [Na(NH4,K)9Cl4][V5+,4+2(P,As)O8]6·n[H2O,Na, NH4,K,Cl]		[36]
Ferrierite-NH4 (NH4,Mg0.5)5(Al5Si31O72)·22H2O	Northern Bohemia, Czech Republic.	[37]
Greenlizardite (NH4)Na(UO2)2(SO4)2(OH)2·4H2O	Green Lizard Mine, Red Canyon, San Juan County, Utah, USA. Secondary alteration phase.	[38]
Meitnerite (NH4)(UO2)(SO4)(OH)·2H2O		[39]
Katerinopoulosite (NH4)2Zn(SO4)2·6H2O	Esperanza Mine, Lavrion District, Attikí Prefecture, Greece. Oxidation zone of a sphalerite-rich orebody.	[40]
Novograblenovite (NH4,K)MgCl3·6H2O	2012–2013 Tolbachik effusive eruption basalts, Plosky Tolbachik volcano, Kamchatka Oblast, Far-Eastern Region, Russia. Exhalation due to volcanic gas exposure.	[10]
Russoite (NH4)ClAs2O3(H2O)0.5	Solfatara di Pozzuoli, Pozzuoli, Napoli, Italy. Fumarolic phase.	[41]

Ideal Chemical Formula	Mineral Analogue If Known	Chesnakov’s Name
NH4Cl	salammoniac
NH4MgCl3·6H2O	novograblenovite	redikortsevite
(NH4)2Fe3+Cl5·H2O	kremersite	kopeiskite
(NH4)2Mg2(SO4)3	efremovite 1	efremovite
(NH4)Al(SO4)2	godovikovite 1	godovikovite
NH4Fe3+3(SO4)2(OH)6	ammoniojarosite
(NH4)2Mg(SO4)2·6H2O	boussingaultite
(NH4)2Ca(SO4)2·H2O	koktaite
(NH4)Fe3+(SO4)2	sabieite	terriconite
(NH4)2SO4	mascagnite
(NH4)3Fe3+(SO4)3	pyracmonite	(NH4)3Fe3+(SO4)3
(NH4)2Fe2+(SO4)2·6H2O	mohrite
NH4Al(SO4)2·12H2O	tschermigite
NH4Fe3+(SO4)2·12H2O	lonecreekite
NH4Al(SO4)2·4H2O	-	NH4Al(SO4)2·4H2O
(NH4)2Mg(SO4)2·4H2O	-	ammonioleonite

1 Chelyabinsk coal basin is the type of locality of the mineral.

NH4MgCl3·6H2O			(NH4)2Fe3+Cl5·(H2O)			Standard
Constituent	Wt. %	Atoms Per Formula Unit 3	Constituent	Wt. %	Atoms Per Formula Unit 4
(NH4)2O 1	9.68	0.96	(NH4)2O 1	15.96	1.70	BN (N)
MgO	15.61	1.00	Fe2O3	27.19	0.94	MgO (Mg) FeS2 (Fe)
Cl	42.34	3.08	Cl	64.64	5.06	NaCl (Cl)
H2O 2	41.93	6.00	H2O (2)	6.50	1.00
Cl2=O	–9.55		Cl2=O	−14.29
Total	100.00		Total	100.00

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.

Compound	NH4MgCl3·6H2O	(NH4)2FeCl5·H2O
Crystal system	monoclinic	orthorhombic
Space group	C2/c	Pnma
a, Å	9.3091(9)	13.725(2)
b, Å	9.5353(7)	9.9365(16)
c, Å	13.2941(12)	7.0370(11)
β, °	90.089(8)	-
V, Å3	1180.05(18)	959.7(3)
Z	4	4
ρcalc, g/cm3	1.445	1.988
μ, mm−1	0.822	2.900
F(000)	536.0	572.0
Radiation	MoKα (λ = 0.71073)	MoKα (λ = 0.71073)
2θ range, deg.	8.55–59.99	5.936–71.776
Index ranges	−13 ≤ h ≤ 12, −13 ≤ k ≤ 13, −18 ≤ l ≤ 18	−18 ≤ h ≤ 21, −13 ≤ k ≤ 16, −11 ≤ l ≤ 11
Reflections collected	5504	11813
Independent reflections	1678 (Rint = 0.066, Rsigma = 0.037)	2256 (Rint = 0.0230, Rsigma = 0.0173)
Data/restraints/parameters	1678/4/84	2256/7/65
Goodness of Fit	1.243	1.091
Final R indexes (I ≥ 2σ(I))	R1 = 0.0783, wR2 = 0.1847	R1 = 0.0229, wR2 = 0.0660
Final R indexes (all data)	R1 = 0.0883, wR2 = 0.1907	R1 = 0.0292, wR2 = 0.0690
Largest diff. peak/hole / e Å−3	0.53/−0.32	0.34/−0.80

Atom	x	y	z	Ueq
NH4MgCl3·6H2O
Cl1	0.5	0.5	0.5	0.0414(3)
Cl2	0.7542(1)	0.73838(9)	0.74664(6)	0.0401(3)
Mg	0	0.5	0.5	0.0225(3)
N	0.5	0.5012(6)	0.75	0.061(1)
HA	0.496(4)	0.454(2)	0.8028(7)	0.053
HB	0.429(4)	0.560(3)	0.749(3)	0.053
O1	0.1795(3)	0.6010(3)	0.4481(2)	0.0420(6)
O2	0.4099(3)	0.1877(3)	0.5379(2)	0.0398(6)
O3	0.9089(3)	0.5131(3)	0.3603(2)	0.0403(6)
H1A	0.198(5)	0.652(5)	0.404(4)	0.06(1)
H1B	0.250(5)	0.58105(4)	0.463(3)	0.04(1)
H2A	0.440(6)	0.248(5)	0.515(4)	0.05(1)
H2B	0.360(5)	0.193(5)	0.586(4)	0.05(1)
H3A	0.890(6)	0.447(6)	0.329(4)	0.06(2)
H3B	0.885(5)	0.579(5)	0.332(3)	0.04(1)
(NH4)2FeCl5·H2O
N	0.14167(9)	0.0006(1)	0.6594(2)	0.0360(2)
HA	0.191(1)	0.050(2)	0.671(2)	0.054
HB	0.114(1)	0.05(2)	0.771(2)	0.054
HC	0.119(1)	0.053(2)	0.585(2)	0.054
HD	0.148(1)	−0.075(2)	0.614(2)	0.054
Fe	0.11623(2)	0.25	0.18959(2)	0.02252(6)
Cl1	0.10486(2)	0.01065(3)	0.17604(4)	0.03113(8)
Cl2	0.00609(3)	0.25	0.45303(6)	0.03264(9)
Cl3	0.24790(3)	0.25	0.39805(5)	0.02664(8)
Cl4	0.22324(3)	0.25	−0.07176(6)	0.0340(1)
O1	−0.0033(1)	0.25	0.0011(2)	0.0388(3)
H1	−0.028(2)	0.184(2)	−0.033(3)	0.072(7)

Atom	U11	U22	U33	U23	U13	U12
NH4MgCl3·6H2O
Cl1	0.0346(6)	0.0335(6)	0.0561(8)	0.0098(4)	−0.0010(5)	−0.0015(4)
Cl2	0.0503(5)	0.0378(5)	0.0323(5)	0.0002(3)	−0.0001(3)	0.0102(3)
Mg	0.0214(7)	0.0227(6)	0.0234(7)	0.0006(4)	−0.0011(5)	−0.0007(4)
N	0.058(3)	0.063(4)	0.062(4)	0	−0.003(3)	0
O1	0.023(1)	0.056(2)	0.048(2)	0.020(1)	−0.002(1)	−0.008(1)
O2	0.049(2)	0.025(1)	0.046(2)	0.002(1)	0.018(1)	0.006(1)
O3	0.057(2)	0.033(1)	0.031(1)	0.002(1)	−0.020(1)	0.002(1)
(NH4)2FeCl5·H2O
N	0.0369(5)	0.0319(6)	0.0392(6)	0.0037(4)	−0.0035(3)	−0.0013(4)
Fe	0.0229(1)	0.0197(1)	0.0250(1)	0	−0.00326(7)	0
Cl1	0.0353(1)	0.0186(1)	0.0395(2)	−0.00229(9)	−0.0073(1)	−0.00072(9)
Cl2	0.0282(2)	0.0307(2)	0.0390(2)	0	0.0085(1)	0
Cl3	0.0244(1)	0.0293(2)	0.0263(2)	0	−0.0054(1)	0
Cl4	0.0398(2)	0.0372(2)	0.0250(2)	0	0.0054(1)	0
O1	0.0370(6)	0.0240(6)	0.0553(9)	0	−0.0237(6)	0

NH4MgCl3·6H2O
Mg–O		Cl–N		∠ H–N–H in NH4
Mg–O1	2.052(2) ×2	Cl1–N	3.3235(4) ×2	HA–N–HB	110(2)
Mg–O2	2.040(2) ×2	Cl2–N	3.273(5) ×2	HA–N–HA 1	115(3)
Mg–O3	2.044(2) ×2	Cl2 2–N	3.394(5) ×2	HB–N–HB 1	100(6)
<Mg–O>	2.045	<Cl–N>	3.330	HA–N–HB 1	111(2)
Di4	0.00153	Di4	0.01269
				∠ H–O–H in H2O
				H1A–O1–H1B	106(4)
				H2A–O2–H2B	120(5)
				H3A–O3–H3B	110(5)
(NH4)2FeCl5·H2O
Fe–Cl,O		∠ H–O–H in H2O		∠ H–N–H in NH4
Fe–Cl1	2.3853(5) ×2	H1–O1–H1 3	115(3)	HA–N–HB	104(1)
Fe–Cl2	2.3920(5)			HA–N–HC	90(2)
Fe–Cl3	2.3276(5)			HA–N–HD	118(2)
Fe–Cl4	2.3537(5)			HB–N–HC	113(2)
Fe–O1	2.109(1)			HB–N–HD	116(2)
<Fe–Cl,O>	2.325			HC–N–HD	112(2)
Di4	0.03097

1 1 − x, y, 1.5 − z; 2 x + 0.5, y + 0.5; 3 x, 0.5 − y, z; 4 Di is distortion index,Di=1n∑i=1n|li−lav|lav [51].

	NH4MgCl3·6H2O	β-Rb(MnCl3)·2H2O 1 (Isotypic to HT Modification of NH4MgCl3·6H2O)	(NH4)2FeCl5·H2O
Symmetry	Monoclinic	Triclinic	Orthorhombic
Space group	C2/c	P−1	Pnma
a (Å)	9.3091(9)	6.65	13.725(2)
b (Å)	9.5353(7)	7.01	9.9365(16)
c (Å)	13.2941(12)	9.03	7.0370(11)
α (°)	90	92.3	90
β (°)	90.089(8)	109.4	90
γ (°)	90	112.9	90
V (Å3)	1180.05(18)	358.74	959.7(3)
Reference	this work	[56]	this work

1 In the process of refinement Rb was replaced by N, and Mn was replaced by Mg, leading to the chemical formula NH4MgCl3·2H2O.

D–H	d(D–H) (Å)	d(H..A) (Å)	<DHA (°)	d(D..A) (Å)	A
NH4MgCl3·6H2O
O1–H1A	0.783	2.395	166.88	3.164	Cl2 1
O1–H1B	0.701	2.514	171.80	3.209	Cl1
O2–H2A	0.710	2.472	156.34	3.135	Cl1
O2–H2B	0.794	2.390	167.42	3.169	Cl2 2
O3–H3A	0.773	2.435	161.92	3.178	Cl2 3
O3–H3B	0.762	2.411	164.46	3.152	Cl2 4
N–HA	0.834	2.659	137.70	3.224	Cl1 5
N–HB	0.863	2.414	174.62	3.274	Cl2 5
(NH4)2FeCl5·H2O
O1–H1	0.775	2.424	173.29	3.195	Cl1 6
N–HA	0.842	2.862	132.12	3.483	Cl1 7
N–HA	0.842	2.873	123.80	3.414	Cl3
N–HA	0.842	2.726	128.12	3.313	Cl4 8
N–HB	0.872	2.855	156.62	3.672	Cl1 8
N–HC	0.806	2.907	125.71	3.440	Cl1
N–HC	0.806	2.662	158.11	3.423	Cl2
N–HC	0.806	2.947	119.25	3.414	Cl3
N–HD	0.818	2.784	123.53	3.307	Cl2 9
N–HD	0.818	2.802	145.15	3.505	Cl4 7

1x − 1/2, −y + 3/2, z − 1/2; 2 x − 1/2, y − 1/2, z; 3 x, −y + 1, z − 1/2; 4 –x + 3/2, −y + 3/2, −z + 1; 5 –x + 1, y, −z + 3/2; 6 −x, −y, −z; 7 –x + 1/2, −y, z + 1/2; 8 x, y, z + 1; 9 −x, −y, −z + 1.

Atom	NH4	Mg	H1A	H1B	H2A	H2B	H3A	H3B	Total
O1		0.38↓ ×2	0.80	0.87					2.05
O2		0.39↓ ×2			0.86	0.79			2.04
O3		0.39↓ ×2					0.81	0.82	2.02
Cl1	0.16↓ → ×2			0.11 → × 2	0.12 → × 2				0.78
Cl2	0.18↓ ×2 0.13↓ ×2		0.13			0.14	0.13	0.13	0.84
Total	0.96	2.32	0.93	0.98	0.98	0.93	0.94	0.95

	NH4MgCl3·6H2O	HT Phase 1	(NH4)2Fe3+Cl5·H2O
α11	36.3	−30.7	40.8
α22	11.5	161.1	45.9
α33	25.2	275.2	47.0
<α11a	12.8	70.7	-
<α22b	-	24.3	-
<α33c	12.7	35.8	-
αa	36(1)	182(9)	41(6)
αb	11.5(6)	159(22)	46(6)
αc	26(1)	172(7)	47(4)
αα	-	73(9)	-
αβ	−3(1)	97(12)	-
αγ	-	−10(8)	-
αV	73(2)	406(25)	134(9)

1 At T = 120 °C; α—coefficient of thermal expansion (α11, α22, α33—eigenvalues (main values); αa, αb, αc—values along crystallographic axes); <α11a—angle between α11 and a.

Formula	Symmetry	Cation X+	Ionic Radii 1 of X+, Å	Reference
XMgCl3·6H2O
CsMgCl3·6H2O	Monoclinic C2/c	Cs	1.81	[64]
RbMgCl3·6H2O	Monoclinic C2/c	Rb	1.52	[58]
NH4MgCl3·6H2O	Monoclinic C2/c	NH4	1.48	[57], this work
KMgCl3·6H2O	Orthorhombic Pnna	K	1.38	[65]
LiMgCl3·7H2O	Trigonal R3	Li	0.76	[66]
(X)2FeCl5·H2O
Cs2FeCl5·H2O	Orthorhombic Cmcm	Cs	1.81	[67]
Rb2FeCl5·H2O	Orthorhombic Pnma	Rb	1.52	[67]
(NH4)2FeCl5·H2O	Orthorhombic Pnma	NH4	1.48	[63], this work
K2FeCl5·H2O	Orthorhombic Pnma	K	1.38	[68]

1 Effective ionic radii for CN = 6 by Shannon [69] and for NH4 by Sidey [70].

Supplementary Materials

The following are available online at https://www.mdpi.com/2075-163X/9/8/486/s1, Figure S1: The dependencies of unit-cell parameters of HT modification of NH4MgCl3·2H2O, Table S1: The refined unit-cell parameters for NH4MgCl3·6H2O phase at different temperatures, Table S2: The refined unit-cell parameters for HT phase of NH4MgCl3·2H2O, at different temperatures, Table S3: The refined unit-cell parameters for (NH4)2Fe3+Cl5·H2O phase at different temperatures, Table S4: Bond-valence analysis (v.u. = valence units) for (NH4)2Fe3+Cl5·H2O phase, Table S5: Approximation equations for NH4MgCl3·6H2O phase and its HT modification, Table S6: Approximation equations for (NH4)2Fe3+Cl5·H2O phase, NH4MgCl3·6H2O.CIF: crystallographic Information file (CIF) for the crystal structure of NH4MgCl3·6H2O, (NH4)2FeCl5·H2O.CIF: crystallographic Information file (CIF) for the crystal structure of(NH4)2FeCl5·H2O.

Author Contributions

Conceptualization, A.A.Z., E.S.Z. and S.V.K.; methodology, A.A.Z. and E.S.Z.; investigation, A.A.Z., E.S.Z., M.G.K., M.A.R. and V.V.S.; writing–original draft preparation, A.A.Z. and E.S.Z.; writing–review and editing, S.V.K.; visualization, A.A.Z. and E.S.Z.

Funding

This research was funded by the Russian Foundation for Basic Research (grant No. 19-05-00628). E.S.Z. is grateful to the President Federation Grant for Young Candidates of Sciences (MK- 3246.2019.5).

Acknowledgments

The X-ray diffraction studies were performed in the X-ray Diffraction Resource Centre of St. Petersburg State University. The chemical analytical studies were done in the “Geomodel” Resource Centre of St. Petersburg State University.

Conflicts of Interest

The authors declare no conflicts of interest.

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