The Crystal Chemistry of Boussingaultite, (NH₄)₂Mg(SO₄)₂·6H₂O, and Its Derivatives in a Wide Temperature Range

Abstract

The crystal structure, thermal behavior, and vibrational spectra of the anthropogenic analogue of boussingaultite, (NH₄)₂Mg(SO₄)₂·6H₂O, and its dehydrated counterpart efremovite, (NH₄)₂Mg₂(SO₄)₃, were studied in detail. The sample from the Chelyabinsk burning coal dumps has the composition of (NH₄)_1.92(Mg_1.02Mn_0.01Fe_0.01)_∑1.04(SO₄)₂·6H₂O and crystallizes in the space group P2₁/a, with a = 9.3183(4), b = 12.6070(4), c = 6.2054(3) Å, β = 107.115(5)°, V = 696.70(5) ų (at 20 °C), Z = 2. The thermal evolution steps are as follows: boussingaultite (NH₄)₂Mg(SO₄)₂·6H₂O (25–90 °C) → X-ray amorphous phase (100–150 °C) → efremovite (NH₄)₂Mg₂(SO₄)₃ (160–340 °C) → MgSO₄ Cmcm + Pbnm (340–580 °C) → MgSO₄ Pbnm (580–700 °C). Thermal expansion is anisotropic, with the coefficients (×10⁶ °C⁻¹) α₁₁ = 52(2), α₂₂ = 68(2), α₃₃ = –89(3), and α_v = 31(3) at T = –123 °C; and α₁₁ = 53(2), α₂₂ = 67(2), α₃₃ = 15(1), and α_v = 136(3) at T = 60 °C. The maximal thermal expansion is along the b-axis and is due to straightening of corrugated pseudolayers (within the ab plane) of Mg(H₂O)₆ octahedra and SO₄ tetrahedra with NH₄ groups in the interlayer space. Vibrational spectroscopy outlines the general trend of dehydration and deammonization as the difference in the temperature intervals of these transformation steps allows separation of O–H and N–H vibrations in the process of dehydration by infrared and Raman spectroscopy. The intermediate partially dehydrated modification of boussingaultite was detected by in situ Raman spectroscopy at 110 °C that may correspond to ammonium leonite.

1. Introduction

Boussingaultite, (NH₄)₂Mg(SO₄)₂·6H₂O, belongs to the picromerite group of minerals, with the general formula A₂M(XO₄)₂·6H₂O, where, in natural species, A = K or NH₄; M = Mg, Cu, Zn, Fe, and Ni; and X = S [1]. The synthetic inorganic compounds with this stoichiometry are known as Tutton’s salts, with broad chemical variability [2]. Only two ammonium and magnesium sulfates are known in nature as minerals: hydrated form, boussingaultite; and dehydrated form, efremovite, (NH₄)₂Mg(SO₄)₂. They are usually found in association with each other [3]. Boussingaultite has been known of since 1863 [4], but it can be considered a rare mineral species, in accordance with its known findings (in 14 countries). This is probably the major reason for the fact that most of the structural data in the literature have been reported for the synthetic phases (Tutton’s ammonium salts). The crystal structure of naturally occurring boussingaultite was revealed very recently by neutron diffraction [5]. Boussingaultite, efremovite, and the majority of other ammonium minerals are associated with specific geological environments, such as volcanism (fumaroles) [6,7,8,9] and coal fires (including pseudofumaroles) [10,11,12].

The astrobiological aspect of the crystal chemical study of ammonium minerals and phases is in developing methodologies for nitrogen (that is the fifth most common element in the Solar system) detection in extraterrestrial bodies, like planets and comets [13,14]. The Ni-bearing boussingaultite has been identified recently in carbonaceous chondrite meteorites [15]. The essential feature of nitrogen in ammonium form is its biological accessibility, in contrast to most of nitrogen, which is triple-bonded atmospheric and biologically inaccessible [16]. Thus, ammonium minerals are of high biochemical importance as (1) material prototypes for agricultural fertilizers (see the literature survey by [5]) and (2) the component of prebiotic life, and as evidence of biologically mediated process [16]. On Earth, most of the nitrogen is concentrated in solid material form [17] and transported in NH₃ form by magmatic liquids and fumarolic fluids [13]. The latter can produce rare associations of ammonium minerals, as well as the emission of coal-fire pseudofumaroles. Often, the phases formed as a result of coal fires are represented by more perfect crystals suitable for detailed crystal chemical studies, while, as shown by a structural analysis, burning coal dump phases are full structural analogues of fumarolic counterparts [18]. Recently, nitrogen was detected in Martian sediments by the Curiosity rover in the form of nitrate [19]. Taking into account the abundance of sulfates (including K-sulfate jarosite) on Martian surface [14] that could be of post-volcanic origin and approximating to terrestrial conditions, the fixation of nitrogen in the ammonium form seems promising. At the same time, the detection of nitrogen in natural phases by analytical methods is still problematic even for ammonium-dominant ones because nitrogen is a light element, and its peak in energy-dispersive spectra can be almost invisible [14].

Another analytical complication is due to the overlapping positions of N–H and O–H vibrations in Raman and infrared spectra [20,21,22]. Nitrogen detection becomes even more complex when taking into account that most of the ammonium occurs as an impurity in potassium minerals. Therefore, detecting ammonium in extraterrestrial minerals is a complex scientific task that can be solved by the detailed crystal chemical characterization of terrestrial well-crystallized phases, including those found on burnt coal dumps [23,24,25].

Mineralogically, the formation of boussingaultite is often confined to fumarolic and pseudofumarolic environments, which are characterized by different ranges of elevated temperatures at atmospheric pressure. The high-temperature experiments, such as those reported below, can be interpreted as a simulation of boussingaultite formation and transformation under natural conditions. Previously, the high-temperature behavior of the synthetic analogue of boussingaultite (ammonium magnesium sulfate hexahydrate) was studied by calorimetric and thermogravimetric methods [26], and the main transformation steps were revealed: (1) incongruent melting of (NH₄)₂Mg(SO₄)₂·6H₂O compound detected as two effects at 150–170 °C, (2) evaporation of water ~280 °C, (3) formation of efremovite ~290 °C and (4) phase transition of ammonium sulfate at ~370 °C. The study of Kosova and coauthors [26] revealed anomalies on differential scanning calorimetry (DSC), dielectric and permittivity curves interpreted as hypothesized reversible order–disorder phase transition that can be responsible for the appearance of ferroelectric properties of the material.

The aim of the present research is to characterize the crystal structure of boussingaultite and partly its derivatives under low (similar to Martian conditions) and high (similar to conditions of volcanic fumaroles and coal-fire pseudofumaroles) temperatures and to define N–H and O–H vibrations via the dehydration of boussingaultite.

2. Occurrence

The Chelyabinsk Coal Basin (CCB) is located east of Chelyabinsk City (from the northeast to the south of the city) and has an area of 1300 km². The CCB area is dominated mainly by brown coal of the Triassic–Jurassic age. Similar to other coal basins, the CCB deposits are subject to spontaneous combustion with the formation of so-called pseudofumaroles, around which some phases crystallize as sublimates or as part of hypergene processes. The status (mineral or technogenic phase) of such sublimates is discussed by the Commission on New Minerals Nomenclature and Classification (CNMNC). Eight minerals from CCB, bazhenovite, godovikovite, dmisteinbergite, svyatoslavite, rorisite, efremovite, srebrodolskite, and fluorellestadite, were approved by the CNMNC during the 1985–1990 period, with the later decision not to consider such phases as minerals. Despite the fact that the CNMNC recently revised its approach to the phases of burnt dumps and began to approve minerals from such environments, in our work, the studied samples are considered technogenic phases.

Burnt dumps of the Chelyabinsk Coal Basin are a unique object of technogenic phase formation. More than 240 different mineral-like compounds were described there, and about 50 of them were unique at the time of their first description. Of particular interest are ammonium-containing phases formed as a result of contact with organic matter at elevated temperatures. At least 16 ammonium-containing phases have been described within the burnt CCB dumps of which two phases have been confirmed as valid mineral species: godovikovite, (NH₄)Al(SO₄)₂ [27]; and efremovite, (NH₄)₂Mg₂(SO₄)₃ [28]. It is worth noting that the mineral status of godovikovite and efremovite has been confirmed by their findings in solely natural environments such as volcanic fumaroles.

The investigated therein technogenic analogue of boussingaultite (in the text, called boussingaultite for the sake of simplicity) from CCB has been taken from the personal collection of B.V. Chesnokov that is stored in the Natural Science Museum of the Ilmen State Reserve (Miass, Russia), sample No. 057–sch–12 (catalog number). It originates from the burnt dumps of mine No. 43bis, where boussingaultite was described in significant quantities, forming ammonium-type sulfate crusts [29,30]. Boussingaultite from burnt dumps of CCB is considered to be the final product of the hydration of efremovite, (NH₄)₂Mg₂(SO₄)₃, but the direct crystallization of boussingaultite from ammonium-containing solutions at temperatures up to 100 °C is a second possible scenario [31]. Another ammonium mineral mentioned in association with boussingaultite and efremovite is the ammonium analogue of leonite, K₂Mg(SO₄)₂·4H₂O.

The second sample of boussingaultite comes from the Kladno mine (Central Bohemian Region, Czech Republic)—the burning coal waste piles that were stored in the same collection as the sample from CCB.

3. Materials and Methods

3.1. Chemical Composition

Several crystals of boussingaultite were mounted in epoxy blocks and polished. Some crystals were deposited onto carbon tape, and all samples were coated with carbon. The elemental analyses were carried out using a Hitachi S–3400N scanning electron microscope equipped with an energy-dispersive spectrometer Oxford X–Max 20, at an accelerating voltage of 20 kV and a probe current of 1 nA, and with various electron beam diameters of minimum 5 μm due to possible dehydration under the electron beam. The standards are given in

Table 1. Chemical composition of boussingaultite (in wt. %).

Constituent	Mean	Range	Atoms per Formula Unit for S = 2	Probe Standard
(NH4)2Omeas (1)	12.90	11.97–15.39	1.77	BN (N)
[(NH4)2Ocalc] (2)	14.55	–	2.00	–
MgO	11.47	11.29–12.64	1.02	MgO (Mg)
MnO	0.26	0.19–0.37	0.01	Mn (Mn)
FeO	0.30	0.27–0.34	0.01	FeS2 (Fe)
SO3	44.85	44.64–45.95	2.00	FeS2 (S)
H2O (3)	30.27	–	6.00	–
Total	100.05 [101.70]

⁽¹⁾ Nitrogen (ammonium) content can be taken as approximate due to the semi-quantitative element content with Z < 8, using energy-dispersive spectroscopy; ⁽²⁾ calculated from the crystal-structure data; ⁽³⁾ calculated by stoichiometry.

. The energy-dispersive spectra were processed automatically using the AzTec Energy software package.

3.2. Low-Temperature Single-Crystal X-Ray Diffraction Analysis (LT SCXRD)

The crystal structure of boussingaultite at different temperatures was studied by performing a single-crystal X-ray diffraction analysis in the range from −173 up to −73 °C (temperature step was 25 °C). Additional data were obtained at 52 °C to record the presence or absence of order–disorder phase transitions assumed by Kosova and coauthors [26]. The analysis was carried out using the single-crystal four-circle Agilent Technologies (Oxford Diffraction) “Supernova” diffractometer (MoKα radiation, 40 kV and 1.5 mA, frame widths of 1° in ω, and 45 s counting time for each frame) equipped with a low-temperature Oxford Cryostream system. Single-crystal X-ray diffraction analysis data were processed in the CrysAlisPro software package [32], an empirical absorption correction was calculated based on spherical harmonics using the SCALES ABSPACK algorithm. The crystal structures were solved and refined using the ShelX program package [33] within the Olex2 software shell [34]. Crystal data, data-collection information, and structure-refinement details are given in

Table 2. Crystal data and structure refinement of boussingaultite collected at T = −173 °C (or 100 K) and 52 °C (or 325 K).

Crystal System	Monoclinic
Space group	P21/a
Temperature, °C	–173	52
a, Å	9.1992(5)	9.2964(12)
b, Å	12.4049(7)	12.5825(9)
c, Å	6.2590(3)	6.1942(7)
β, °	107.026(6)	107.106(13)
Volume, Å3	682.94(7)	692.50(14)
Z	2
ρcalc, g/cm3	1.754	1.883
μ, mm−1	0.508	0.520
F(000)	380.0	412.0
Radiation	MoKα (λ = 0.71073)
2Θ range for data collection, °	from 6.57 to 65.228	from 7.608 to 65.046
Index ranges	−13 ≤ h ≤ 8, −16 ≤ k ≤ 18, −19 ≤ l ≤ 9	−13 ≤ h ≤ 11, −18 ≤ k ≤ 17, −8 ≤ l ≤ 8
Reflections collected	4772	4880
Independent reflections [Rint, Rsigma]	2227 [Rint = 0.0362, Rsigma = 0.0548]	2162 [Rint = 0.0700, Rsigma = 0.1102]
Data/restraints/parameters	2227/0/128	2162/0/128
Goodness-of-fit on F2	1.082	1.053
Final R indexes [I ≥ 2σ(I)]	R1 = 0.0392, wR2 = 0.0824	R1 = 0.0626, wR2 = 0.1232
Final R indexes [all data]	R1 = 0.0556, wR2 = 0.0920	R1 = 0.1378, wR2 = 0.1794
Largest diff. peak/hole/e Å−3	0.55/−0.61	0.48/−0.69

(for temperature of −173 and 52 °C); atom coordinates and displacement parameters are in Supplementary Tables S1 and S2, and selected interatomic distances are in Supplementary Table S3. The positions of H atoms were derived from the analysis of Fourier difference electron–density maps and refined freely in an isotropic approximation. The parameters of the hydrogen bonds of boussingaultite (for temperature of −173 °C) are given in Supplementary Table S4. Vesta software was used to visualize the structural data [35].

Additionally, the crystal structure of boussingaultite from Kladno was refined by X-ray diffraction analysis at 20 °C, which was 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, sharp-focus X-ray source PhotonJet–S and a high-speed, direct-action detector HyPix–6000HE. The data were processed using the same software packages as for low-temperature single-crystal X-ray diffraction analysis. All crystal structure data of boussingaultite from Kladno are given in Supplementary Tables S5–S9.

3.3. High-Temperature In Situ Powder X-Ray Diffraction Analysis (HTXRD)

Anisotropy of thermal expansion and high-temperature transformations of boussingaultite were investigated by high-temperature in situ powder X-ray diffraction (HTXRD) analysis, which was carried out using a Rigaku Ultima IV diffractometer (CuKα radiation, 40 kV/30 mA). The sample was studied in the temperature range from 30 up to 700 °C; and the temperature step was 10 °C in the range from 30 to 240 °C and 20 °C in the range from 240 to 700 °C. Unit-cell parameters of boussingaultite were refined using the Rietveld method implemented in the Topas software package [36]. The crystal-structure data obtained at 20 °C were used for unit-cell refinement of HTXRD data. The main thermal-expansion coefficients were calculated using the TTT program package [37].

3.4. Infrared Spectroscopy

The infrared spectra were registered using a Ge–ATR probe of a Micran–3 infrared microscope with a liquid nitrogen cooled MCT detector connected to a Fourier Transform Infrared (FTIR) spectrometer FT–801, made by Simex. The spectra were measured using 32 scans, with a resolution of 1 cm⁻¹, and processed by the Simex software Zair v.3.5. The dehydration was studied on material subsequently heated in a muffle furnace in air for about 10 min at each temperature in the interval between 20 and 400 °C. Attenuated total internal reflection (ATR) spectroscopy in the infrared region was carried out for samples of boussingaultite annealed at temperatures of 150, 200, 250, 300, and 400 °C, as well as for the untreated (initial) sample.

3.5. Raman Spectroscopy

Raman spectra were obtained using the Horiba LabRam HR800 Evolution spectrometer equipped with the confocal Olympus BX–FM microscope, Ar laser (radiation wavelength of 488 nm), and the Linkam TSM 600 heating/cooling stage. A diffraction grating of 1800 gr/mm and an electrically cooled CCD detector were used for recording. The spectrometer calibration was guided along the Rayleigh line and the emission lines of a neon lamp. The spectral resolution was about 1 cm⁻¹. The spatial lateral resolution was about 1 μm. The temperature-dependent spectra were collected during the heating of the sample in the temperature range from −190 to 500 °C, with a step of 1–10 °C (depends on temperature range); the exposure time of the sample to stabilize the temperature was 20 s. The heating rate was 20 °C/min. The accuracy of maintaining the temperature was 0.1 °C. The temperature-measurement range is limited due to the increasing photoluminescent background from the sample. The peak fitting procedure was carried out using LabSpec6 software, ArDI web-application [38], and Origin 2020 by approximating spectra with Voigt contours. The background was subtracted by a third-order polynomial, each spectrum in the equation has its own characteristic coefficients.

4. Results

4.1. Chemical Composition

In the initial boussingaultite sample, the following elements were detected: N, O, Mg, S, and minor Mn and Fe. The data were used for the calculation of empirical chemical formula of boussingaultite on the basis of S = 2 atoms per formula unit (apfu). The measurement gave a somewhat underestimated value for N (Table 1). Therefore, it was corrected in the chemical formula based on charge-balancing requirements. The amount of H₂O was calculated based on the crystal-structure data. The obtained formula is (NH₄)_1.92(Mg_1.02Mn_0.01Fe_0.01)_∑1.04(SO₄)₂·6H₂O, and it is close to the ideal one, (NH₄)₂Mg(SO₄)₂·6H₂O.

4.2. Crystal Structure

The structure refinement based on single-crystal data was performed for six temperatures (at −173, −148, −123, −98, −73, and 52 °C). All refinement results are available as CIF–files from the CCDC database (CSD # 2372190, 2372191, 2372192, 2372193, 2372194, and 2372198, respectively). The crystal structure of boussingaultite was refined in the space group P2₁/a; the unit-cell parameters (at –173 °C) are a = 9.1992(5), b = 12.4049(7), c = 6.2590(3) Å, β = 107.026(6) °, V = 682.94(7) ų to R₁ = 0.0392 (Table 2). The calculated powder X-ray diffraction patterns are given in Supplementary Figure S1. The structure refinements performed at other temperatures are available as supplementary data (see data-availability paragraph). In the text, we provide data for T = −173 °C since this is the best refinement with minimized atomic displacement parameters, whereas data at T = 52 °C correspond to the highest temperature and show absence of structure transformation.

The structure of boussingaultite is built from regular Mg(H₂O)₆ octahedra, with the average <Mg–O> bond length of 2.068 Å; SO₄ tetrahedra (<S–O> = 1.479 Å); and NH₄ tetrahedra (<N–H> = 0.89 Å) (

Table 3. Geometric parameters of boussingaultite structure at T = −173 (or 100 K) and 52 °C (or 325 K).

Parameter	−173 °C	52 °C
<Mg–O>, Å	2.068	2.063
<S–O>, Å	1.479	1.468
<N–H>, Å	0.887	0.852
VMg, Å3	11.78	11.70
Vsulf, Å3	1.66	1.62
Vamm, Å3	0.36	0.31
Distortion index for Mg(H2O)6	0.0057	0.0066
Distortion index for SO4	0.0059	0.0055
Distortion index for NH4	0.0357	0.0640

and Supplementary Tables S3 and S8) interconnected by a system of hydrogen bonds (Supplementary Tables S4 and S9). The structure can be considered as consisting of pseudolayers (within the ab plane) of Mg(H₂O)₆ octahedra and SO₄ tetrahedra, while the NH₄ groups are located in the interlayer space. In general, the crystal chemical characteristics of boussingaultite studied in this work are similar to those revealed for boussingaultite from Pecs–Vasas, Mecsek Mountains, South Hungary, by neutron diffraction [5], except for the occupancy of monovalent cation that is fully populated by N in our sample and has ~10% of K, in addition to N, in the sample studied by Gatta and coauthors [5]. The hydrogen positions and hydrogen bonding scheme are similar for both cases despite the different techniques used for the localization of H atoms.

No principal structure changes occur in the temperature range from −173 to 52 °C; the main deformations are expansion and contraction in different directions (see below). Through our analysis of geometric parameters of boussingaultite based on atomic displacement parameters and residual electron density, we did not find any indications of disordering of sulfate tetrahedra at 52 °C (or any other polyhedra) that have been suggested previously [26]. The decomposition of boussingaultite was registered at 77 °C by SCXRD. The crystal structure of boussingaultite consists of isolated fragments linked by hydrogen bonds; thus, the thermal behavior can be determined by both the expansion of structural fragments and the distances between them, as shown by the data in Table 3; the second factor is the main one.

4.3. Phase Evolution upon Temperature

The boussingaultite maxima are observed in the HTXRD pattern up to 90 °C (Figure 1 and Supplementary Figure S2). Further heating leads to the formation of an X-ray amorphous phase in the temperature range of 100–150 °C. The efremovite reflections are registered in the range of 160–340 °C, two modifications of MgSO₄ (Cmcm and Pbnm) are identified between 340 and 580 °C, and the only Pbnm modification of MgSO₄ is observed above 580 °C (Figure 1). Thus, upon boussingaultite heating, the dehydration and deammonization processes were registered at 100–150 °C and ~340 °C, respectively.

4.4. Thermal Expansion

The unit-cell parameters of boussingaultite were refined for seven temperatures in the high-temperature region from 25 to 90 °C, in addition to the low-temperature SCXRD region, with five values in the temperature range from −173 to −73 °C. As shown by Figure 2 for the unit-cell parameters a and b and the unit-cell volume, V, the dependencies of lattice parameters versus temperatures show linear approximation for both low- and high-temperature regions. The c lattice parameter undergoes contraction in the low-temperature region and expansion in the high-temperature region (approximated by two linear dependencies).

Thermal expansion of boussingaultite is anisotropic (Figure 3;

Table 4. Thermal-expansion coefficients, ×10⁶ °C⁻¹ for boussingaultite.

T, °C	α11	α22	α33	αV
–123	52(2)	68(2)	–89(3)	31(3)
60	53(2)	67(2)	15(1)	136(3)

) in low- and high-temperature regions. A nearly isotropic section is obtained for the ab plane within the octahedral–tetrahedral pseudolayer (with α₁₁ and α₂₂). Maximal thermal expansion is observed parallel to the b-axis due to the straightening of corrugated angles in pseudolayer (Figure 3). Minimal thermal expansion (α₃₃) is observed perpendicular to the pseudolayer plane that is 15(1) × 10⁻⁶ °C⁻¹ at high temperature and −89(3) × 10⁻⁶ °C⁻¹ at low temperature (Table 4). Thus, the structure mainly expands within the plane of pseudolayers, with much less expansion or even contraction in the low-temperature region in perpendicular direction.

4.5. Infrared Spectroscopy

The infrared spectra of boussingaultite (initial sample) contains bands overlapping O–H and N–H stretching bands in the range of 3464–2839 cm⁻¹ and H–O–H, H–N–H bending in the range of 1660–1423 cm⁻¹ (Figure 4) [20,21,22,23,24,25,39,40]; the bands associated with SO₄ tetrahedra are found in the range of 1167–1012 cm⁻¹ and 700–657 cm⁻¹, and H–O–H libration mode is registered between 880 and 892 cm⁻¹; the detailed band assignment is given in

Table 5. IR absorption bands of boussingaultite and its high-temperature derivatives, efremovite and magnesium sulfate.

IR Band Maxima (cm−1) at Temperatures (°C)						Band Assignment
Boussingaultite		Efremovite			MgSO4
RT	150	200	250	300	400
3464	3464 ↓	-	-	-	-	O–H stretching
3350	3350 ↓	-	-	-	-	O–H stretching
3272	3271	3267	3266	3265 ↓	-	N–H stretching
3233	3233 ↓	-	-	-	-	O–H stretching
3189	3189	3189	3188	3187 ↓	-	N–H stretching
3155	3155 ↓	-	-	-	-	O–H stretching
3124	3125	3125	3127	3130 ↓	-	N–H stretching
3077	3075	3075	3074	3071 ↓	-	N–H stretching
3040	3040 ↓	-	-	-	-	O–H stretching
3012	3012 ↓	-	-	-	-	O–H stretching
2980	2980	2980	2980	2980 ↓	-	N–H stretching
2920	2922	2920	2920	2920 ↓	-	N–H stretching
2839	2840	2842	2844	2846 ↓	-	N–H stretching
1660	1660 ↓	-	-	-	-	H–O–H bending
1468	1468 ↓	-	-	-	-	H–N–H bending *
1423	1420	1418 ↓	1418 ↓	1414 ↓	-	H–N–H bending
-	-	-	-	1167 w	1167	v3 (SO4)
1140	1140	1140	1140	1140	1140	v3 (SO4)
1060	1083 1125 w	1125	1125	1125	-	v3 (SO4)
981	981 1040 w	1040	1040	1040	1062	v1 (SO4)
-	-	-	-	-	1012	v1 (SO4)
-	880	880	-	-	-	H–O–H libration
-	-	892	-	-	-	H–O–H libration
-	-	657	657	657	680 700	v4 (SO4)

↓ intensity reduced. Note: w—weak. * The band is splinted into two components (1468 and 1463 cm⁻¹), one of the components disappears upon heating up to 150 °C. This may be due to either a change in the geometry of NH₄ cation or due to the loss of perturbed H₂O.

. The sample annealed at 150 °C corresponds to partly dehydrated boussingaultite and possibly amorphous phase characterized by band broadening. The samples annealed at 200, 250, and 300 °C correspond to efremovite (the sample treated at 200 °C is partly hydrated). The sample annealed at 400 °C contains MgSO₄ [41] (Table 5). In general, the phase-evolution sequence determined by infrared spectroscopy and diffraction methodologies coincides, but the transition temperatures are shifted to higher values due to the difference in the heating kinetics.

The transformation of boussingaultite to efremovite results in the decrease in intensities associated with O–H vibrations and disappearance of such bands at temperatures higher than 200 °C. At the same time, deammonization is complete at 400 °C; thus, for boussingaultite annealed at 250 and 300 °C, the O–H related bands are no longer observed, while the N–H bands remain. This allows for the separation of the N–H bonds from the O–H ones. The dehydration process is accompanied by the amorphization of boussingaultite, clearly seen by the broadening of the infrared absorption bands in the sample heated at 150 °C (Figure 4). The bands at 3464, 3350, 3233, 3155, 3040, and 3012 cm⁻¹ are associated with the O–H stretching modes. The bands corresponding to N–H stretching modes are observed at 3272, 3189, 3124, 3077, 2980, 2920, and 2839 cm⁻¹. The H–O–H bending mode is found at 1660 cm⁻¹ that is rather typical for that vibration. The band at ~1450–1460 cm⁻¹ is split into two components peaked at 1463 and 1468 cm⁻¹. The 1468 cm⁻¹ component disappears at temperatures higher than 150 °C that may be due to either a change in the geometry of NH₄ cation or the loss of perturbed H₂O. The component at 1463 cm⁻¹ remains above 150 °C and attributed to the H–N–H bending vibration. The bands in the region 1200–600 cm⁻¹ correspond to the different vibration modes of SO₄ tetrahedra [20,21,22,23,24,25,39,40]. In the samples heated at 150 and 200 °C, the bands in the interval of 880–890 cm⁻¹ could be related to H–O–H libration modes (Table 5).

4.6. Raman Spectroscopy

The 45 of the 114 Raman-active vibration modes were recorded in the spectra of boussingaultite (Figure 5), all predicted via theoretical group analysis: Γ = 57A_g + 59A_u + 57B_g + 58B_u (Bilbao Crystallographic Server [42]), where A_g and B_g are Raman-active modes, while A_u and B_u are IR-active modes. The detection of weak modes can be hampered by spectrally broad photoluminescence emission that is possibly due to the presence of Fe³⁺. The Raman spectra of the boussingaultite crystal are characterized by narrow bands, which reflect a high degree of structural perfection. The band assignment, in general, agrees with those reported previously [20]; however, the 3500–2800 cm⁻¹ region was analyzed in more detail with the separation of O–H and N–H vibrations. The band assignment is listed in Table 6.

Table 6. Raman bands of boussingaultite and its high-temperature derivatives, efremovite, magnesium sulfate, and possibly ammonium leonite.

Boussingaultite				Ammonium Leonite		Amor- Phous	Efremovite	MgSO4	Phase
Raman Band Maxima (cm−1) at Temperatures (°C)
−190	−110	10	80	95	110	120	155	200	Band Assignment
3432	3444	3436	3438	3439	3439	–	–	–	ν(H2O)
3390	3388	3389	3391	3396	3405	–	–	–
3364	3363	3364	3364	3363	3372	–	–	–
3339	3338	3339	3339	n.i.*	n.i.	–	–	–
3309	3308	3309	3309	3319	n.i.	–	–	–
3279	3278	3273	3276	3279	3268	–	–	–
3239	3239	3236	3237	3241	n.i.	–	–	–	ν(NH4)
3202	3199	3200	3199	3204	n.i.	–	–	–
3152	3150	3151	3152	3155	3166	–	–	–	ν(H2O)
3130	3128	3127	3127	3131	n.i.	–	–	–	ν(NH4)
3109	3110	3111	3110	3118	n.i.	–	–	–	ν(H2O)
3091	3091	3089	3090	3085	3076	–	–	–
3063	3068	3067	3066	n.i.	n.i.	–	–	–	ν(NH4)
3014	3024	3022	3021	n.i.	n.i.	–	–	–
2925	n.i.	n.i.	n.i.	n.i.	n.i.	–	–	–
2872	n.i.	n.i.	n.i.	n.i.	n.i.	–	–	–
2844	n.i.	n.i.	n.i.	n.i.	n.i.	–	–	–
1766	1766	1769	1770	1765	n.i.	–	–	–	ν4(NH4) + δ(HOH)
1714	1712	1713	1706	1698	1701	–	–	–
1678	1676	1679	1679	n.i.	n.i.	–	–	–
1423	1429	1431	1433	n.i.	n.i.	–	n.i.	–	ν4(NH4)
1144	1139	1139	1136	1200	n.i.	–	n.i.	n.i.	ν3(SO4)
1091	1091	1093	1094	1102	1100	–	n.i.	n.i.
1070	1069	1071	1070	n.i.	n.i.	–	n.i.	n.i.
1059	1062	1062	1060	n.i.	n.i.	–	n.i.	n.i.
982	982	983	982	981	996	1010	1045	1053	ν1(SO4)
803	797	800	798	n.i.	–	–	–	–	ν(H2O)
722	719	717	716	n.i.	–	–	–	–
630	627	627	626	639	n.i.	642	n.i.	n.i.	ν4(SO4)
617	617	617	618	612	610	617	n.i.	n.i.
610	610	610	610	n.i.	n.i.	–	n.i.	n.i.
592	583	583	583	n.i.	n.i.	–	n.i.	n.i.
552	557	558	556	n.i.	n.i.	–	–	–	ν(H2O)
465	463	461	459	464	463	479	n.i.	n.i.	ν2(SO4)
451	451	451	450	446	446	448	n.i.	n.i.
391	387	n.i.	n.i.	n.i.	n.i.	–	–	–	ν(H2O)
368	367	365	361	n.i.	n.i.	–	–	–
322	320	n.i.	n.i.	n.i.	n.i.	–	–	–
301	306	306	306	n.i.	n.i.	–	–	–
256	255	254	253	n.i.	n.i.	–	n.i.	n.i.	Lattice mode
227	226	224	221	n.i.	n.i.	–	n.i.	n.i.
201	195	191	189	n.i.	n.i.	–	n.i.	n.i.
191	191	185	183	n.i.	n.i.	–	n.i.	n.i.
172	174	172	173	n.i.	n.i.	–	n.i.	n.i.
164	163	159	158	n.i.	n.i.	–	n.i.	n.i.
137	133	128	127	n.i.	n.i.	–	n.i.	n.i.
129	129	125	122	n.i.	n.i.	–	n.i.	n.i.

n.i.—the exact wavenumber is not identified due to low signal-to-noise ratio; however, the band is visible in Figure 6 and Figure 7; a dash means the absence of a corresponding vibration in the spectrum. Interpretation of vibrations according to [20,24,39,43].

Table 6. Raman bands of boussingaultite and its high-temperature derivatives, efremovite, magnesium sulfate, and possibly ammonium leonite.

Boussingaultite				Ammonium Leonite		Amor- Phous	Efremovite	MgSO4	Phase
Raman Band Maxima (cm−1) at Temperatures (°C)
−190	−110	10	80	95	110	120	155	200	Band Assignment
3432	3444	3436	3438	3439	3439	–	–	–	ν(H2O)
3390	3388	3389	3391	3396	3405	–	–	–
3364	3363	3364	3364	3363	3372	–	–	–
3339	3338	3339	3339	n.i.*	n.i.	–	–	–
3309	3308	3309	3309	3319	n.i.	–	–	–
3279	3278	3273	3276	3279	3268	–	–	–
3239	3239	3236	3237	3241	n.i.	–	–	–	ν(NH4)
3202	3199	3200	3199	3204	n.i.	–	–	–
3152	3150	3151	3152	3155	3166	–	–	–	ν(H2O)
3130	3128	3127	3127	3131	n.i.	–	–	–	ν(NH4)
3109	3110	3111	3110	3118	n.i.	–	–	–	ν(H2O)
3091	3091	3089	3090	3085	3076	–	–	–
3063	3068	3067	3066	n.i.	n.i.	–	–	–	ν(NH4)
3014	3024	3022	3021	n.i.	n.i.	–	–	–
2925	n.i.	n.i.	n.i.	n.i.	n.i.	–	–	–
2872	n.i.	n.i.	n.i.	n.i.	n.i.	–	–	–
2844	n.i.	n.i.	n.i.	n.i.	n.i.	–	–	–
1766	1766	1769	1770	1765	n.i.	–	–	–	ν4(NH4) + δ(HOH)
1714	1712	1713	1706	1698	1701	–	–	–
1678	1676	1679	1679	n.i.	n.i.	–	–	–
1423	1429	1431	1433	n.i.	n.i.	–	n.i.	–	ν4(NH4)
1144	1139	1139	1136	1200	n.i.	–	n.i.	n.i.	ν3(SO4)
1091	1091	1093	1094	1102	1100	–	n.i.	n.i.
1070	1069	1071	1070	n.i.	n.i.	–	n.i.	n.i.
1059	1062	1062	1060	n.i.	n.i.	–	n.i.	n.i.
982	982	983	982	981	996	1010	1045	1053	ν1(SO4)
803	797	800	798	n.i.	–	–	–	–	ν(H2O)
722	719	717	716	n.i.	–	–	–	–
630	627	627	626	639	n.i.	642	n.i.	n.i.	ν4(SO4)
617	617	617	618	612	610	617	n.i.	n.i.
610	610	610	610	n.i.	n.i.	–	n.i.	n.i.
592	583	583	583	n.i.	n.i.	–	n.i.	n.i.
552	557	558	556	n.i.	n.i.	–	–	–	ν(H2O)
465	463	461	459	464	463	479	n.i.	n.i.	ν2(SO4)
451	451	451	450	446	446	448	n.i.	n.i.
391	387	n.i.	n.i.	n.i.	n.i.	–	–	–	ν(H2O)
368	367	365	361	n.i.	n.i.	–	–	–
322	320	n.i.	n.i.	n.i.	n.i.	–	–	–
301	306	306	306	n.i.	n.i.	–	–	–
256	255	254	253	n.i.	n.i.	–	n.i.	n.i.	Lattice mode
227	226	224	221	n.i.	n.i.	–	n.i.	n.i.
201	195	191	189	n.i.	n.i.	–	n.i.	n.i.
191	191	185	183	n.i.	n.i.	–	n.i.	n.i.
172	174	172	173	n.i.	n.i.	–	n.i.	n.i.
164	163	159	158	n.i.	n.i.	–	n.i.	n.i.
137	133	128	127	n.i.	n.i.	–	n.i.	n.i.
129	129	125	122	n.i.	n.i.	–	n.i.	n.i.

n.i.—the exact wavenumber is not identified due to low signal-to-noise ratio; however, the band is visible in Figure 6 and Figure 7; a dash means the absence of a corresponding vibration in the spectrum. Interpretation of vibrations according to [20,24,39,43].

Figure 5. Raman spectra of boussingaultite at different temperatures by in situ measurements (a) and detailed 3600–2800 cm⁻¹ region at different temperatures (b).

Figure 5. Raman spectra of boussingaultite at different temperatures by in situ measurements (a) and detailed 3600–2800 cm⁻¹ region at different temperatures (b).

Figure 6. Raman spectra of boussingaultite and its high-temperature derivatives: partly dehydrated modification interpreted as ammonium leonite at 95, 110 °C, amorphous phase at 120 °C, and efremovite at 155 °C) in 2000–100 cm⁻¹ (a), 1100–900 cm⁻¹ (b), and 3600–3000 cm⁻¹ (d) regions. Frame (c) shows the dependence of the ν₁(SO₄) mode position on the temperature of the boussingaultite (black dots) and ammonioleonite (red dots) in the temperature range from −190 to 90 °C.

Figure 6. Raman spectra of boussingaultite and its high-temperature derivatives: partly dehydrated modification interpreted as ammonium leonite at 95, 110 °C, amorphous phase at 120 °C, and efremovite at 155 °C) in 2000–100 cm⁻¹ (a), 1100–900 cm⁻¹ (b), and 3600–3000 cm⁻¹ (d) regions. Frame (c) shows the dependence of the ν₁(SO₄) mode position on the temperature of the boussingaultite (black dots) and ammonioleonite (red dots) in the temperature range from −190 to 90 °C.

Figure 7. The dependence of corrugated angle of pseudolayer from temperature in boussingaultite structure.

Figure 7. The dependence of corrugated angle of pseudolayer from temperature in boussingaultite structure.

The Raman bands in the 3500–2800 cm⁻¹ region correspond to an overlap of N–H and O–H vibrations [20,21,22,23,24,25,39,40] that have been assigned in detail in accord with infrared spectroscopy (Figure 5b; Supplementary Figure S3 and the procedure described as note to it). The O–H stretching is revealed by the following band components (at T = 10 °C): 3436, 3389, 3364, 3339, 3309, 3273, 3151, 3111, and 3089 cm⁻¹. The N–H vibrations are observed at 3236, 3200, 3127, 3067, and 3022 cm⁻¹.

The bands at 1769, 1713, and 1679 cm⁻¹ in the Raman spectrum are assigned to the combination of the (ν₂) NH₄ and H–O–H deformation vibration of H₂O molecules. The (ν₄) NH₄ stretching vibration is demonstrated by the 1431 cm⁻¹ band. The antisymmetric stretching vibration, ν₃, of the SO₄ group is outlined by the 1139, 1093, 1071, and 1062 cm⁻¹ bands. The strongest band in the Raman spectrum at 983 cm⁻¹ is attributed to the ν₁ symmetric stretching vibration of the SO₄ group, and the band at 981 cm⁻¹ at the infrared spectrum is also intensive, suggesting the symmetry reduction of sulfate group that may be considered as distorted tetrahedra. Medium-intensity bands at wavenumbers 625 and 615 cm⁻¹ (Raman) and 626 and 616 cm⁻¹ (infrared) are attributed to the four bending vibrations of the SO₄ group. A medium-intensity band at 454 cm⁻¹ in the Raman spectrum is attributed to the ν₂ bending vibration of the SO₄ group. The broad bands at wavenumbers 787, 724, 676, and 564 cm⁻¹ can be attributed to the torsion vibrations of H₂O molecules. The Raman bands at 360 and 310 cm⁻¹ can be attributed to the vibrations of H₂O molecules, while the 222 cm⁻¹ band and lower wavenumbers are attributed to the lattice modes. In general, the band assignment is in agreement with that of [20,24]; however, the detailed assignment of the 3600–2500 cm⁻¹ region is carried out for the first time.

Temperature-dependent Raman Spectroscopy. Three heating cycles from −190 to 500 °C were performed using three different boussingaultite crystals. Two of the three cycles are characterized by the following sequence of transformations (Figure 5):

(NH₄)₂Mg(SO₄)₂·6H₂O (−190–85 °C) →

(NH₄)₂Mg(SO₄)₂·6H₂O + amorphous phase (85 °C) →

amorphous phase (86–150 °C) →

(NH₄)₂Mg(SO₄)₂ (from ~160 °C).

It should be noted that MgSO₄ is also likely observed, the band of which is clearly visible at ~1050 cm⁻¹ (assigned to ν₁(SO₄) in accord with [43]). But the observation of the band at higher temperatures is problematic due to the strong photoluminescent background.

The third temperature cycle of measurements showed some differences in the Raman spectra (Figure 6). The intensive Raman band at 981 cm⁻¹ is observed for boussingaultite and assigned to sulfate anion vibrations (Table 6). In the third heating cycle at temperature 90 °C, an additional band occurs at 996 cm⁻¹, and only this band is presented at the temperatures 95 and 110 °C, while the 981 cm⁻¹ band disappeared at these temperatures (Figure 6b,c). However, the position of the boussingaultite mode ν₁(SO₄) is practically independent of temperature and varies from ~981 to ~983 cm⁻¹ in the temperature range from −190 to 90 °C (Figure 6c), which also shows the absence of SO₄-tetrahedra ordering in this temperature range, in accordance with X-ray diffraction single-crystal analyses data. Another interesting observation is that, in the third cycle, the “intermediate” (partly dehydrated) phase was detected that was not revealed by other methods and two other temperature-dependent Raman cycles. Partial dehydration is proven by the presence of H₂O vibrations in the region of 3600–3000 cm⁻¹ at 95 °C. The phase was identified only by Raman spectroscopy in heating cycles with steps of 1–2 °C; thus, this phase has a very narrow temperature-stability field. This “intermediate” phase was identified as the ammonium analogue of leonite, i.e., phase with the composition (NH₄)₂Mg(SO₄)₂·4H₂O. It is suggested that thermal decomposition of boussingaultite may pass through an intermediate stage, accompanied by the removal of a certain number of H₂O molecules, like in gypsum [44]; however, it has not been detected by other methods.

5. Discussion

Boussingaultite is stable up to 90 °C. In the temperature range of 100–150 °C, it transforms into the X-ray amorphous phase due to the dehydration process. Efremovite crystallizes from the X-ray amorphous phase at ~160 °C; the compound is stable up to 340 °C, and, after that, two MgSO₄ modifications (Cmcm and Pbnm) form due to the deammonization process in the temperature range from 340 to 580 °C, and the single Pbnm modification of MgSO₄ is stable above 580 °C. A partly dehydrated form of boussingaultite, possibly “ammonioleonite”, was detected by Raman spectroscopy only in a narrow temperature range and slow heating rate. It is noteworthy that the technogenic “ammonioleonite”, (NH₄)₂Mg(SO₄)₂·4H₂O (i.e., the chemical analogue of boussingaultite with 4 H₂O pfu instead of 6), has been reported from CCB in association with boussingaultite and efremovite [31]. Based on our experimental data and conditions of technogenic phase formation, we may suggest that “ammonioleonite” may indeed form easily via the hydration of efremovite because it was almost undetectable in heating experiments of boussingaultite. The phase transformation of boussingaultite → ammonioleonite → efremovite → MgSO₄ is followed by the following crystal chemical changes (accompanying the processes of dehydration and deammonization until their completion): isolated structure → layers from Mg(SO₄)₂(H₂O)₄ clusters interconnected by the NH₄⁺ → tetrahedral–octahedral framework with NH₄⁺ in the framework voids → dense structure of edge-sharing MgO₆-octahedra chains interconnected via SO₄ tetrahedra. The formation of a dehydrated chemical analogue of a phase may pass through intermediate forms. As an example, novograblenovite from CCB, (NH₄)MgCl₃·6H₂O, which, at a temperature of about 90 °C, transforms to the crystalline partly dehydrated phase, (NH₄)MgCl₃·2H₂O that, in turn, is stable up to 150 °C [18]. Meanwhile, contrasting behavior is observed for another ammonium sulfate, tschermigite, (NH₄)Al(SO₄)₂·12H₂O, which melts (at 60–70 °C) with the formation of amorphous phase and further crystallization (at 210 °C) of its anhydrous counterpart godovikovite, (NH₄)Al(SO₄)₂, with its later transformation (at 380–390 °C) to millosevichite, Al₂(SO₄)₃ [40].

Previously, the phase transition of boussingaultite has been suggested based on differential scanning calorimetry (DSC), dielectric permittivity, and heat-capacity curves that depended on crystal size [26]. The crystal chemical proposal was based on the assumption that the phase transition is associated with the disorder of sulfate tetrahedra. Neither the phase transition nor disordering of sulfate tetrahedra is observed in our study. However, we, for the first time, obtained signatures of existence of a crystalline partly dehydrated difficult-to-detect phase identified as “ammonioleonite”.

Thermal expansion of boussingaultite is strongly anisotropic and changes dramatically upon heating, which is reflected by the variation in character of thermal expansion of c and β unit-cell parameters (Figure 2). Thus, the crystal structure of boussingaultite experiences compression along the c-axis in the temperature range from −173 to −73 °C, while the a, b, and V parameters are increasing. However, in the temperature range from 25 to 90 °C, a, b, c, and V increase, while the monoclinic angle, β, slightly decreases. At the same time, expansion along the c-axis is weak (minimal).

It is worth noting that the c-axis direction is perpendicular to the plane of the pseudolayers, allowing us to assume that the thermal-expansion anisotropy of boussingaultite is controlled by its structural motif. Moreover, the direction of maximal thermal expansion (the b-axis) is parallel to the direction of the corrugation of pseudolayers, leading us to analyze the change in corrugation angle with temperature (Figure 7). Thus, the corrugation angle was calculated for the LT SCXRD data, using Olex2 software [34], as 180 − n, where n = normal to the angle between two symmetrical S–Mg–S planes. As a result, it was revealed that the corrugated layer straightened (with increasing temperature) from 92.391(5) to 98.240(9)°, causing the maximal thermal expansion along the b-axis.

The vibrational spectroscopy data allowed for the distinction of N–H bands to be carried out for the first time (given in detail in Table 5 and Table 6 and visualized in Figure 5 and Figure 8). This can be helpful for the identification of ammonium minerals. Also, according to vibrational spectroscopy, the heating leads to the oxidation of Fe²⁺ impurity to Fe³⁺. It is registered by the increasing luminescence signal in the Raman spectra of the heated samples. The mechanism of Fe oxidation is accompanied by the deprotonation of H₂O molecules (Figure 8). Figure 8 also demonstrates dehydration and deammonization steps of boussingaultite based on the N–H and O–H bands’ intensity dependences on temperature: dehydration occurs in the temperature range from 100 to 200 °C, while deammonization occurs from 200 to 400 °C, which is, generally, in accordance with HTXRD data.

6. Conclusions

The high-temperature behavior of boussingaultite has been studied on the sample of technogenic origin from the burnt-coal dumps. Upon heating, boussingaultite transforms to efremovite, which, at a higher temperature, transforms to MgSO₄, according to X-ray diffraction and vibrational spectroscopy data. The shift of ν₁(SO₄) mode from 981 to 996 cm⁻¹ was observed at the temperature range of 95–110 °C in one of three Raman heating cycles, where the temperature step was 1–2 °C. This has been interpreted as the formation of a crystalline partly dehydrated modification that may correspond to “ammonioleonite”. The thermal expansion of boussingaultite is different for low (from −173 to −73 °C) and high (from 25 to 90 °C) regions. In both regions, the thermal behavior of boussingaultite is anisotropic, with the maximal thermal expansion parallel to the b-axis. The structure analysis shows that the strongest expansion is due to the straightening of corrugated pseudolayers with increasing temperature, while the minimal expansion (or even contraction in the low-temperature interval) is perpendicular to pseudolayers. Thus, anisotropy of thermal expansion of boussingaultite is mostly controlled by its layered structural character. We did not detect either the phase transition or disordering of sulfate tetrahedra suggested for synthetic analogue of boussingaultite previously. The study of boussingaultite and its dehydrated counterpart efremovite by infrared and Raman spectroscopy allowed for the separation of N–H and O–H vibrations that could be helpful for the diagnostic of ammonium minerals, including those studied by rovers.