Ca₃SiO₄Cl₂—An Anthropogenic Phase from Burnt Mine Dumps of the Chelyabinsk Coal Basin: Crystal Structure Refinement, Spectroscopic Study and Thermal Evolution

Abstract

The mineral-like phase Ca₃SiO₄Cl₂, an anthropogenic anhydrous calcium chlorine-silicate from the Chelyabinsk coal basin has been investigated using single-crystal and high-temperature powder X-ray diffraction and Raman spectroscopy. The empirical formula of this phase was calculated as Ca_2.96[(Si_0.98P_0.03)_Σ1.01O₄]Cl₂, in good agreement with its ideal formula. Ca₃SiO₄Cl₂ is monoclinic, space group P2₁/c, Z = 4, a = 9.8367(6) Å, b = 6.7159(4) Å, c = 10.8738(7) Å, β = 105.735(6)°, V = 691.43(8) ų. The crystal structure is based upon the pseudo-layers formed by Ca–O and Si–O bonds separated by Cl atoms. The pseudo-layers are parallel to the (100) plane. The crystal structure of Ca₃SiO₄Cl₂ was refined (R₁ = 0.037) and stable up to 660 °C; it expands anisotropically with the direction of the strongest thermal expansion close to parallel to the [−101] direction, which can be explained by the combination of thermal expansion and shear deformations that involves the ‘gliding’ of the Ca silicate layers relative to each other. The Raman spectrum of the compound contains the following bands (cm^–1): 950 (ν₃), 848 (ν₁), 600 (ν₄), 466 (ν₂), 372 (ν₂). The bands near 100–200 cm⁻¹ can be described as lattice modes. The compound had also been found under natural conditions in association with chlorellestadite.

1. Introduction

The current work is devoted to the detailed crystal chemistry and spectroscopic study of Ca₃SiO₄Cl₂ [1], an anthropogenic mineral-like phase which originates from the burnt coal dumps of the Chelyabinsk coal basin. In addition to the most common rock-forming silicates such as amphiboles and pyroxenes, the Chelyabinsk burnt coal dumps contain a set of rare mineral species observed there for the first time [2]. Among the nine minerals found in this locality and approved by the International Mineralogical Association (IMA), there are three silicates: the monoclinic (pseudo-orthorhombic) and trigonal (or hexagonal) polymorphs of Ca(Al₂Si₂O₈) (svyatoslavite and dmisteinbergite) [3,4] and the apatite-like sulfate-silicate fluorellestadite, Ca₅(SiO₄)_1.5(SO₄)_1.5F, with the ideal Si:S ratio equal to 1:1 [5]. Generally, silicates are predominate among other classes of mineral-like phases in the burned rocks of the Chelyabinsk coal basin [2]. Additionally, the phases enriched with chlorine and fluorine are quite common in burnt coal dumps. Some of them are Cl analogues of well-known minerals such as mayenite or cuspidine [6,7], and others are evident primilary in burnt coal dumps (

Table 1. Cl-bearing silicates of the Chelyabinsk coal basin ¹.

Chemical Composition	Name
Ca7[SiO4]2Cl6	‘Rhytmite’ or ‘Rhythmite’
Ca3Al2[SiO4]2Cl4	‘Igumnovite’
Ca3[SiO4]Cl2	‘Albovite’
Ca3Al2[SiO4]3-xCl4x (x = 0.3–0.4)	‘Chlorohibschite’
Ca13Al14(SiO4)4O24Cl4	‘Silicochlormayenite’
Ca18Fe3+15AlSi4O47Cl6	‘Demidovskite’
Ca10[Si2O7]3Cl2	Rusinovite (‘Chesofiite’)
Ca8[Si2O7]2Cl2O	‘Afanasievaite’
Ca4[SiO4]2(Cl,F)2	‘Chlorcuspidine’
Ca12Al14O32[□4Cl2]	Chlormayenite

¹ The minerals and mineral-like phases with Cl content less than 4 (in mass percentages) are not shown [8]. The names which are enclosed in quotes are not approved by the CNMNC and were given by Chesnokov and co-authors and are used here for information purposes only (as bywords). The formulas are given according to the IMA list of minerals and Chesnokov and co-authors [2].

).

The compound Ca₃SiO₄Cl₂, an anhydrous calcium chloride-silicate, is known as a metastable phase in Portland cement clinkers. The crystal structure of Ca₃SiO₄Cl₂ was solved and refined by Treushnikov et al. for the first time [9]. Several studies have demonstrated that Ca₃SiO₄Cl₂ can be used for immobilization of halide-containing waste from the chemical reprocessing of plutonium [10,11]. In addition, the green phosphor Ca₃SiO₄Cl₂:Eu²⁺ was investigated as a perspective material for UV and blue light-stimulated LEDs [12].

Ca₃SiO₄Cl₂ was also found by B.V. Chesnokov and co-authors in the interlayer of the annealed petrified tree (dump of mine No 42, Chelyabinsk coal basin) and named ‘albovite’ (not approved by the CNMNC) [1,2]. It is always found in association with spurrite, Ca₅(SiO₄)₂(CO₃), and occurs as small isometric inclusions. Chesnokov et al. [1,2] pointed out that, under exposure to the air, the grains of this phase may be covered by drops of calcium chloride and a white plaque.

Ca₃SiO₄Cl₂ was also detected via electron microscopy in association with chlorellestadite, Ca₅(SiO₄)_1.5(SO₄)_1.5Cl; larnite, β-Ca₂SiO₄; and spurrite, Ca₅(SiO₄)₂(CO₃), in an altered xenolith of the Shadil-Khokh Volcano, South Ossetia. The crystal structure determination of the phase founded in a natural environment was impossible due to its instability under atmospheric conditions and tiny crystal size [13]. The fact that this compound was found in a natural environment is important for its potential establishment as a separate mineral species.

Herein we continue the series of our studies of crystal chemistry of anthropogenic mineral-like compounds from the Chelyabinsk coal basin [3,5,14,15] and provide detailed results on Ca₃SiO₄Cl₂.

2. Materials and Methods

2.1. Occurrence

The sample of Ca₃SiO₄Cl₂ (No. 059-18) studied in this work was taken from the personal collection of B.V. Chesnokov (dump of the mine No. 42) and provided by the Natural Science Museum of the Ilmen State Reserve, Miass, Russia. The crystals of this phase have a columned appearance with a crystal size up to 100 μm. The grains are white, sometimes colorless. The streak is white, and the luster is glassy; on the cleavage planes, the luster changes to pearlish, with cleavage in one direction. The crystals are brittle, with a hardness of 4 on the Mohs scale (Figure 1).

2.2. Chemical Composition

The study of the chemical composition of Ca₃SiO₄Cl₂ was carried out using a Hitachi S-3400N scanning electron microscope with an Oxford Instruments Energy Dispersive Spectrometer X-Max (20 kV and 1.0 nA). The AzTec Energy software was used for automatic spectra processing. The X-ray acquisition time was 30 s in spot mode. The following standards were used: CaSO₄ for the determination of Ca; MgO for Mg; Mn for Mn; Al₂O₃ for Al; FeS₂ for Fe; SiO₂, albite for Si; and NaCl for Cl.

2.3. Raman Spectroscopy

The Raman spectra of Ca₃SiO₄Cl₂ were recorded using the Horiba Jobin-Yvon LabRam HR800 spectrometer (solid state laser, 532 nm). The equipment was calibrated according to a silicon standard (520.7 cm⁻¹). The studied sample was placed on a slide glass without a given orientation. Raman spectrum was recorded at room temperature in the range of 70–4000 cm⁻¹, the power on the sample reached 8 mW. The data accumulation time took from 2 to 10 s. The obtained spectra were studied in the program CrystalSleuth [16], built on the RRUFF database. The drawings of the spectra were constructed in the Veusz program (version 3.6.2) [17].

2.4. High-Temperature Powder X-ray Diffraction

The thermal behavior of Ca₃SiO₄Cl₂ was studied in air via the 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 energy dispersion detector, Ni filter, Pt substrate). The sample was studied in the temperature range 30–900 °C with 30 °C steps (Figure S1). A thin powder sample was deposited on a Pt sample holder (20 × 12 × 2 mm³) from an ethanol suspension. The data concerning room temperature were loaded into PDXL [18] to refine the phase composition, then the TOPAS software package [19] was used for refining the unit cell parameters for all temperatures via the Rietveld method (Table S1). The coordinates of atoms, site scattering, and isotropic displacement parameters were fixed. The background was modeled using the Chebyshev polynomial approximation of the 12th order. The peak profile was described using the fundamental parameters approach. The thermal expansion coefficients were calculated and visualized in the TTT program [20].

2.5. Single-Crystal X-ray Diffraction

Single-crystal X-ray diffraction studies of Ca₃SiO₄Cl₂ were carried out using a Rigaku XtaLAB Synergy-S diffractometer (MoKα radiation) with a high-stability sharp-focus X-ray source PhotonJet-S and a high-speed direct-action detector HyPix-6000HE. The studied single crystal was kept at room temperature (frame widths 1.0° in ω and 150 s counting time for each frame). The CrysAlisPro software was used for the data processing [21]. An absorption correction was introduced using the SCALE3 ABSPACK algorithm. The obtained data were loaded into the Olex2 program software (version 1.3) [22], and the crystal structure was solved and refined using the ShelX program package (2014) [23]. Crystal data and structure refinement parameters are shown in

Table 2. Crystallographic data and structure refinement parameters for Ca₃SiO₄Cl₂.

Crystal System, Space Group, Z	Monoclinic, P21/c, 4
a, b, c, Å	9.8367(6), 6.7159(4), 10.8738(7)
β, °	105.735(6)
V, Å3	691.43(8)
ρcalc, g cm−3	2.721
μ/mm-1	3.278
F(000)	560.0
Radiation; λ, Å	MoKα; 0.71073
2Θ range for data collection, °	7.21 to 55.964
Index ranges h, k, l	−12 ≤ h ≤ 12, −8 ≤ k ≤ 8, −14 ≤ l ≤ 14
Reflections collected,	4603
Independent reflection	1585
Rint	0.0457
Rsigma	0.0528
Data/restrains/parameters	1585/0/91
R1/wR2(I ≥ 2σ (I))	0.0366/0.1363
R1/wR2(all data)	0.0548/0.2148
S	1.136
Largest diff. peak/hole/e Å−3	1.53/−1.34

; the atomic coordinates and equivalent isotropic parameters and selected interatomic distances are given in

Table 3. Atomic coordinates and isotropic displacement parameters (Ų) for Ca₃SiO₄Cl₂.

Atom	x/a	y/b	z/c	Ueq, Å2
Ca1	0.19638(11)	0.25598(13)	0.54605(9)	0.0105(4)
Ca2	0.07921(10)	0.04320(13)	0.22909(8)	0.0107(3)
Ca3	0.42903(12)	0.77403(19)	0.61100(10)	0.0193(4)
Si1	0.12068(14)	0.74981(16)	0.53023(11)	0.0067(4)
O1	0.2311(3)	0.5819(5)	0.6064(3)	0.0119(7)
O2	0.2223(3)	0.9275(5)	0.5042(3)	0.0104(7)
O3	0.0166(3)	0.687(5)	0.3921(3)	0.0107(7)
O4	0.0174(4)	0.8053(5)	0.619(3)	0.0116(7)
Cl1	0.71965(14)	0.6781(2)	0.68259(11)	0.0198(4)
Cl2	0.49226(15)	0.7081(2)	0.87529(13)	0.0268(4)

and

Table 4. Selected bond lengths (Å) in the crystal structure of Ca₃SiO₄Cl₂.

Ca1-O2	2.281(3)	Ca3-O2	2.295(3)
-O1	2.284(3)	-O1	2.325(4)
-O3	2.399(3)	Ca3-Cl2	2.799(1)
-O4	2.401(3)	-Cl2	2.806(1)
Ca1-Cl1	2.860(1)	-Cl1	2.826(1)
-Cl1	2.888(1)	-Cl2	3.010(2)
-Cl2	2.966(1)	<Ca3-O>	2.31
<Ca1-O>	2.341	<Ca3-Cl>	2.853
<Ca1-Cl>	2.904
		Si1-O4	1.623(3)
Ca2-O4	2.347(3)	-O3	1.627(3)
-O3	2.348(3)	-O1	1.628(3)
-O4	2.378(3)	-O2	1.630(3)
-O3	2.416(3)	<Si1-O>	1.627(3)
-O2	2.553(3)
-O1	2.636(3)
-Cl1	3.105(2)
<Ca2-O>	2.446

, respectively.

All studies were carried out in the research centers of St. Petersburg State University (Geo Environmental Research Center ‘Geomodel’ and Research Centre for X-ray diffraction studies).

3. Results

3.1. Chemical Composition

The chemical formula was calculated on the basis of O = 4. The ranges of the values of the main chemical components are quite narrow (wt.%): Ca 59.09 (58.64–59.84), SiO₂ 20.96 (20.68–21.44), P₂O₅ 0.85 (0.67–1.15), Cl₂ 5.72, total 100.49. The empirical formula, Ca_2.96[(Si_0.98P_0.03)_Σ1.01O₄]Cl₂, is in good agreement with the ideal formula obtained by B.V. Chesnokov et al. [1].

3.2. Raman Spectroscopy

The Raman spectrum of Ca₃SiO₄Cl₂ is shown in Figure 2. In general, it correlates well with the spectra of orthosilicates [24,25]. The main band at 848 cm⁻¹ corresponds to the symmetric stretching vibrations (ν₁) of SiO₄ tetrahedra. The bands in the range of 900–1000 cm⁻¹ belong to the asymmetric stretching vibrations (ν₃) of silicate tetrahedra [26]. A low-intensity band at 600 cm⁻¹ can be attributed to in-plane bending vibrations (ν₄) of tetrahedra [27]. The ν₂ vibrations are located in the region of 500–350 cm⁻¹ [26]. Additional bands are associated with the Ca–O–Ca and O–Ca–O vibration modes. Wide bands up to 350 cm⁻¹ correspond to lattice modes. The bands that appeared in the 350–200 cm⁻¹ (237 cm⁻¹) region are related to the vibrations of the O–Ca–O bonds [26] and rotations of the SiO₄ groups [27].

3.3. Crystal Structure

The artificial analogue of this mineral-like phase Ca₃SiO₄Cl₂ was found as a metastable phase in cement clinkers and structurally characterized by Treushnikov and co-authors in 1970 [9]. The crystal structure contains three symmetrically independent Ca sites and one Si site (Figure 3). The Ca1 site is coordinated by seven anions, four oxygen atoms with an average bond length <Ca1-O> of 2.341 Å, and three chlorine atoms (<Ca1-Cl> = 2.904 Å). The geometry of the Ca1 coordination can be described as a distorted pentagonal bipyramid. The Ca2 site forms a CaO₆ distorted trigonal prism, with one of the rectangular faces capped by the additional Cl1 atom. The Ca3 position is coordinated by two oxygen and four chlorine atoms to form strongly distorted Ca3O₂Cl₄ octahedra. SiO₄ tetrahedra are isolated from each other and have typical geometric characteristics (<Si-O> = 1.627 Å).

The projection of the crystal structure of Ca₃SiO₄Cl₂ along the b axis is shown in Figure 4a. Figure 4b shows the same projection but with Cl atoms omitted. It clearly demonstrates that Ca–O and Si–O bonds form two-dimensional layers parallel to the (100) plane. The topology of the Ca silicate layer is depicted in Figure 5. The dense backbone of the layer shown in Figure 5b consists of CaO₆ trigonal prisms sharing edges with each other and SiO₄ tetrahedra. The Ca1 and Ca3 atoms are attached to the backbone through four (Ca1) and two (Ca3) Ca–O bonds. Thus, the crystal structure of Ca₃SiO₄Cl₂ has a pseudo-two-dimensional character, with the Ca–O and Si–O bonds between hard cations and hard anions forming a two-dimensional topology separated by Ca–Cl bonds between hard low-polarizable Ca²⁺ cations and soft high-polarizable Cl^- anions. Such a characteristic of chemical bond distribution results in the remarkable thermal expansion behavior of Ca₃SiO₄Cl₂ that will be discussed below.

3.4. High-Temperature Behaviour

No phase transitions have been detected during the high-temperature X-ray diffraction study. The temperature dependencies of the unit cell parameters are shown in Figure 6. The crystal structure of Ca₃SiO₄Cl₂ expands anisotropically, with the strongest thermal expansion observed along the α₁₁ (μ_c1 = 13.5°) (Table 5). The compound is stable up to 660 °C and then starts to decompose with the formation of Ca₂[SiO₄] and CaO. The following equations can be used for the approximation of the temperature dependences of the unit cell parameters:

a = 9.8161 + 0.2 × 10⁻³T

b = 6.7048 + 0.1 × 10⁻³T

c = 10.855 + 0.1 × 10⁻³T

β = 105.73 + 0.03T

V = 687.60 + 0.01T

Table 5. Thermal expansion coefficients (°C^–1) × 10⁶ and angles for the orientation of the tensor of thermal expansion for Ca₃SiO₄Cl₂.

T °C	100	200	300	400	500	600
α11	22.7(3)	22.7(3)	22.6(3)	22.6(3)	22.5(3)	22.5(3)
α22	18.7(2)	18.7(2)	18.6(2)	18.6(2)	18.6(2)	18.5(2)
α33	12.4(1)	12.3(1)	12.3(1)	12.3(1)	12.3(1)	12.2(1)
μa1	13.4	13.5	13.5	13.5	13.6	13.6
μb1	0	0	0	0	0	0
μc1	29.2	29.3	29.4	29.4	29.5	29.6
αa	22.1(3)	22.1(3)	22.1(3)	22.0(3)	21.9(3)	21.9(3)
αc	14.8(3)	14.8(3)	14.8(3)	14.8(3)	14.8(3)	14.7(3)
αV	53.8(7)	53.7(7)	53.6(7)	53.5(7)	53.3(7)	53.2(7)

Table 5. Thermal expansion coefficients (°C^–1) × 10⁶ and angles for the orientation of the tensor of thermal expansion for Ca₃SiO₄Cl₂.

T °C	100	200	300	400	500	600
α11	22.7(3)	22.7(3)	22.6(3)	22.6(3)	22.5(3)	22.5(3)
α22	18.7(2)	18.7(2)	18.6(2)	18.6(2)	18.6(2)	18.5(2)
α33	12.4(1)	12.3(1)	12.3(1)	12.3(1)	12.3(1)	12.2(1)
μa1	13.4	13.5	13.5	13.5	13.6	13.6
μb1	0	0	0	0	0	0
μc1	29.2	29.3	29.4	29.4	29.5	29.6
αa	22.1(3)	22.1(3)	22.1(3)	22.0(3)	21.9(3)	21.9(3)
αc	14.8(3)	14.8(3)	14.8(3)	14.8(3)	14.8(3)	14.7(3)
αV	53.8(7)	53.7(7)	53.6(7)	53.5(7)	53.3(7)	53.2(7)

Figure 6. Temperature dependencies of the unit cell parameters of Ca₃SiO₄Cl₂.

Figure 6. Temperature dependencies of the unit cell parameters of Ca₃SiO₄Cl₂.

4. Discussion

The crystal structure refinement, chemical analysis, and spectroscopic studies of the mineral-like Ca₃SiO₄Cl₂ phase from the burnt mine dumps of the Chelyabinsk coal basin indicate that the phase is identical to the synthetic compound Ca₃[SiO₄]Cl₂, which is formed when CaCl₂ is added to a cement clinker consisting of calcium silicates.

The thermal expansion anisotropy of this phase deserves a special comment. The direction of maximal thermal expansion is approximately parallel to the (a–c) vector, i.e., to the [−101] direction. Such an expansion character can be explained by the shear deformations frequently observed for monoclinic structures [28,29]. The presence of the pseudo-two-dimensional calcium silicate layers in the crystal structure of Ca₃SiO₄Cl₂ described above agrees well with the hypothesis of the shear deformations. The schemes shown in Figure 7 may help to visualize the mechanisms of the thermal expansion revealed for Ca₃SiO₄Cl₂. For the truly layered structure (Figure 7a), the expansion should be maximal along the direction perpendicular to the plane of the layers with the β angle decreasing with the rise in temperature. If the expansion is accompanied by the shear deformation (i.e., by the shift or gliding of the layers relative to each other), the β angle increases with the increasing temperature. This is exactly the kind of thermal behavior that is observed for Ca₃SiO₄Cl₂ (Figure 6). Thus, the thermal behavior of the crystal structure of the phase can be considered as a combination of shear and expansion, resulting in the direction of maximal expansion being not perpendicular, but inclined relative to the plane of the Ca silicate layer. The ‘gliding’ of the Ca silicate layers relative to each other is provided by the interlayer of soft and highly polarizable Cl⁻ ions.

The coal stored in the burnt dumps is prone to ignition and spontaneous combustion. In high-temperature anhydrous environments, the host rocks are transformed into a number of specific paragenetic associations. In coal dumps, the temperatures of the phase formation are unusually high (up to 1200 °C) and result in the formation of uncommon mineral phases, including kinetically stabilized metastable polymorphs. As an essential element involved and present in burned coal dumps, chlorine occurs as one of the phase-forming constituents in many mineral-like phases. The maximum concentration of chlorine was detected in such phases as salammoniac (NH₄Cl) and aquasidite (CaCl₂). The compound Ca₃[SiO₄]Cl₂ is the most chlorine-rich silicate among the mineral-like phases containing chlorine (in wt. %) that occurs along with Ca₃Al₂[SiO₄]₂Cl₄ and Ca₇[SiO₄]₂Cl₆ in so-called ‘black blocks’(products of calcination of the heterogeneous mass components under reducing conditions) of the burnt dumps of the Chelyabinsk coal basin [2]. The process of the Ca₃[SiO₄]Cl₂ formation is directly related to the interaction of quartz and a carbonate matrix (quartz inclusions in petrified wood) in the presence of chlorine-bearing gas streams [8].

The chlorine-bearing silicates Ca₃Al₂[SiO₄]₂Cl₄, Ca₇[SiO₄]₂Cl₂, Ca₈[Si₂O₇]₂Cl₂O, and Ca₁₀[Si₂O₇]₃Cl₂ were described by Chesnokov and co-authors in the burned mine dumps of the Chelyabinsk coal basin [2]. Later, the Ca₁₀[Si₂O₇]₃Cl₂ phase was found in natural conditions in two different localities and approved as a mineral species under the name rusinovite [30]. The mineral was found (in one of the two localities) in a high-temperature, carbonate–silicate xenolith also containing the phase Ca₃SiO₄Cl₂ reported herein [13].

Taking into account the occurrence of Ca₃SiO₄Cl₂ in geological environments (high-temperature contact metamorphism), we can expect that it can be described as a separate mineral species in the near future.