Ca 3 SiO 4 Cl 2 —An Anthropogenic Phase from Burnt Mine Dumps of the Chelyabinsk Coal Basin: Crystal Structure Refinement, Spectroscopic Study and Thermal Evolution

: The mineral-like phase Ca 3 SiO 4 Cl 2 , 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 0.98 P 0.03 ) Σ1.01 O 4 ]Cl 2 , in good agreement with its ideal formula. Ca 3 SiO 4 Cl 2 is mono-clinic, space group P 2 1 / с , 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) Å 3 . 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 Ca 3 SiO 4 Cl 2 was refined (R 1 = 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 ‘glid-ing’ of the Ca silicate layers relative to each other. The Raman spectrum of the compound contains the following bands (cm –1 ): 950 (ν 3 ), 848 (ν 1 ), 600 (ν 4 ), 466 (ν 2 ), 372 (ν 2 ). 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.


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

Chemical Composition Name
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:Eu 2+ 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.

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).

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.

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].

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 mm 3 ) 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].

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 Xray 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; the atomic coordinates and equivalent isotropic parameters and selected interatomic distances are given in Tables 3 and 4, 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).

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 lowintensity 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].

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> = 1.627Å).
The projection of the crystal structure of Ca3SiO4Cl2 along the b axis is shown in Fig- ure 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 pseudotwo-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 lowpolarizable Ca 2+ 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.

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 (μс1 = 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:

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 phaseforming 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

Figure 1 .
Figure 1.The appearance of the columnar crystals of Ca3SiO4Cl2 from the burnt dumps of the Chelyabinsk coal basin (No. 059-18).

Figure 3 .
Figure 3. Coordination of Ca atoms in the crystal structure of Ca3SiO4Cl2.Legend: Ca = blue; Cl = green; O = red; Si = yellow.

Figure 4 .
Figure 4. Projection of the crystal structure of Ca3SiO4Cl2 along the b axis (a), the same projection but with omitted Cl atoms (b), and the orientation of the section of the figure of thermal expansion coefficients in the (010) plane (relative to the projections shown in (a,b)) (c).Legend as in Figure 3.

Figure 5 .
Figure 5. Topology of the Ca silicate layer in the crystal structure of Ca3SiO4Cl2 (a) and dense backbone of the layer formed by Ca2 atoms and SiO4 tetrahedra (b).Legend as in Figure 3.

Figure 7 .
Figure 7.The schemes showing the behavior of monclinic layered structures under truly thermal expansion (the layers are oriented horizontally) (a) and when thermal expansion is accompanied by shear deformations, i.e., by shifts of the layer relative to each other (b).The β and β′ angles correspond to the values at low and high temperatures, respectively.Under shear deformation, the β angle increases under increasing temperature, whereas the opposite is observed for pure thermal expansion.The single and striated lines correspond to the contours of the unit cell at low and high temperatures, respectively.

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

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

Table 5 .
Thermal expansion coefficients (°C -1 ) × 10 6 and angles for the orientation of the tensor of thermal expansion for Ca3SiO4Cl2.
Figure 6.Temperature dependencies of the unit cell parameters of Ca3SiO4Cl2.