Extra-Framework Content in Sodalite-Group Minerals: Complexity and New Aspects of Its Study Using Infrared and Raman Spectroscopy

Nine samples of carbonate-free sodalite-group minerals, including those with abnormally high contents of polysulfide groups, fluoride anion and carbon dioxide molecules as well as synthetic fluoraluminate sodalite-type compound Na8(Si7Al5O24)(AlF6)3–·5H2O, have been studied by means of electron microprobe analyses, infrared and Raman spectroscopy; the CO2 content was determined using the selective sorption of gaseous ignition products. This article describes a semi-quantitative method for estimating the content of carbon dioxide molecules in these minerals, based on IR spectroscopy data. The data obtained demonstrate the existence of a sulfide sodalite-group mineral with the idealized formula Na7(Si6Al6O24)(S3−)·H2O, which differs significantly from the formula Na6Ca2(Si6Al6O24)S2–2 accepted for lazurite. According to single-crystal X-ray structural analysis, in the F-rich sodalite-group mineral from the Eifel paleovolcanic region, Germany with the idealized formula Na7(Si6Al6O24)F−·nH2O fluorine occurs as an isolated F− anion, unlike synthetic F-rich sodalite-type compounds.


Introduction
Natural aluminosilicates belonging to the sodalite group are feldspathoids whose crystal structures are based on the tetrahedral framework (Al x Si 1−x O 2 ) xwith large cavities (so-called β-cages) hosting large cations (Na + , K + , Ca 2+ ), additional anions (Cl − , F − , SO 4 2-, S 2-, S 3 − , etc.) and neutral molecules (H 2 O, CO 2 ) [1][2][3][4][5][6][7][8][9][10][11]. Sodalite-type compounds with different additional anions (CO 3 − , HCOO − , AlF 6 3-, OH − ) have been synthesized [12][13][14][15][16]. Vibrational (infrared and Raman) spectroscopy has proven to be an effective tool with which various anionic and neutral extra-framework groups in minerals of the sodalite group can be identified. However, a number of unresolved problems remain related to the quantitative determination of the relative contents of various groups containing C, S, and F, the limits of carbon dioxide contents, the       Table 1 lists the analytical methods which were used for the characterization of different samples ("+" means that the method was applied). Samples 3, 6, 7, and 9 with unusual compositional characteristics were investigated in the most detail. Samples 1, 2, 4, 5, and 8 played an auxiliary role and were used to obtain calibration dependence for the determination of the content of CO2 molecules from the IR spectra. Some data for Samples 1 and 10 were taken from literature sources, indicated in Table 1.   Table 1 lists the analytical methods which were used for the characterization of different samples ("+" means that the method was applied). Samples 3, 6, 7, and 9 with unusual compositional characteristics were investigated in the most detail. Samples 1, 2, 4, 5, and 8 played an auxiliary role and were used to obtain calibration dependence for the determination of the content of CO2 molecules from the IR spectra. Some data for Samples 1 and 10 were taken from literature sources, indicated in Table 1. Table 1 lists the analytical methods which were used for the characterization of different samples ("+" means that the method was applied). Samples 3, 6, 7, and 9 with unusual compositional characteristics were investigated in the most detail. Samples 1, 2, 4, 5, and 8 played an auxiliary role and were used to obtain calibration dependence for the determination of the content of CO 2 molecules from the IR spectra. Some data for Samples 1 and 10 were taken from literature sources, indicated in Table 1.
Electron microprobe analyses (three spot analyses for each sample) were carried out using a Tescan VEGA-II XMU electronic microscope (EDS mode, 20 kV, 400 pA). Data reduction was carried out by means of a modified INCA Energy 450 software package. The size of the electron beam was 157-180 nm. The beam was rastered on an area 20 µm × 20 µm in order to minimize sample damage. The time of data acquisition was 50 s. The sample-to-detector distance was 25 mm. The standards used are: albite for Na and Si, sanidine for K, wollastonite for Ca, Al 2 O 3 for Al, BaSO 4 for S, CaF 2 for F, and NaCl for Cl.
The content of fluorine in the F-rich Sample 9 was determined as the mean of 11 spot analyses using a Jeol JSM-6480LV scanning electron microscope equipped with an INCA-Wave 500 wavelength-dispersive spectrometer (Laboratory of Analytical Techniques of High Spatial Resolution, Department of Petrology, Moscow State University), with an acceleration voltage of 20 kV, a beam current of 20 nA, and a 3 µm beam diameter. MgF 2 was used as a reference.
For all samples except Sample 7, CO 2 was determined by the selective sorption of CO 2 on an askarite sorbent (an asbestiform matter saturated by NaOH) from gaseous products obtained by heating of the mineral at 1080 • C in oxygen at 1 atm. For Sample 7, CO 2 and H 2 O were determined by gas chromatography of products of ignition in oxygen at 1200 • C with a Vario Micro cube analyzer (Elementar GmbH, Frankfurt, Germany).
In order to obtain infrared (IR) absorption spectra, powdered samples were mixed with dried KBr, pelletized, and analyzed using an ALPHA FTIR spectrometer (Bruker Optics) in the range 360-4000 cm −1 with a resolution of 4 cm −1 and 16 scans. An IR spectrum of an analogous pellet prepared from pure KBr was used as a reference. The optical densities of the bands at 2341-2346 cm −1 were measured relative to the optical density at 2500 cm −1 . The optical densities of the band used as an internal standard was measured at the absorption maximum in the range 656-668 cm −1 ; the common tangents to the two deepest minima in the range 500-830 cm −1 were used as baselines.
Raman spectra of randomly oriented samples were obtained using an EnSpectr R532 spectrometer based on an OLYMPUS CX 41 microscope coupled with a diode laser (λ = 532 nm) at room temperature. The spectra were recorded in the range from 100 to 4000 cm −1 with a diffraction grating (1800 g mm −1 ) and spectral resolution of about 6 cm −1 . The output power of the laser beam was about 5 mW. The diameter of the focal spot on the sample was less than 5 µm. The backscattered Raman signal was collected with a 40× objective; signal acquisition time for a single scan of the spectral range was 1 s, and the signal was averaged over 50 scans. Crystalline silicon was used as a standard.
The single-crystal X-ray diffraction experiment was carried out for Sample 9 using Xcalibur S CCD diffractometer. The measured intensities were corrected for Lorentz, background, polarization and absorption effects. Data reduction was performed using CrysAlisPro Version 1.171.39.46 [24]. The studied sample is cubic, space group I-43m, a = 9.05887(10) Å, V = 743.40(2) Å 3 . The crystal structure of Sample 9 was obtained by direct methods and refined using the SHELX software package [25]. The structure was refined for the two-component twin with refined twin ratio 0.52:0.48. Experimental details are given in Table 2.

Chemical Composition
Chemical data for the studied sodalite-group minerals are given in Table 3. The CO 2 content in Sample 9 was determined by means of IR spectroscopy (see below). It is to be noted that the CO 2 contents in Samples 2-9 correspond to the CO 2 molecules in the sodalite cavities because IR spectra of these samples do not contain characteristic bands of the CO 3 2anion. Samples 6, 7 and 9 are characterized by abnormally high contents of carbon, sulfur and fluorine, respectively.

Infrared Spectroscopy
The IR spectra of the studied Samples 1-10 are given in Figures 5-8. All of them contain bands of O-H stretching and H-O-H bending vibrations (in the ranges 3400-3700 and 1640-1660 cm −1 , respectively) which are due to the presence of H 2 O molecules (zeolitic water). Taking into account that in the sodalite-type framework, each tetrahedral atom is coordinated by two O atoms, the presence of OH groups connected to the framework should be excluded. Additional bands in the ranges 3200-3400 and 1687-1709 cm −1 in the IR spectra of Samples 1, 2, 9 and 10 may correspond to H 3 O + or H 5 O 2 + cations [26]. This assumption is confirmed by the presence of a band in the range 1339-1358 cm −1 in the IR spectra of these four samples whereas IR spectra of other samples do not contain this band. The IR band in the range 1320-1390 cm −1 is a characteristic feature of isolated H + cation which may be formed as a result of dissociation of acid groups: Characteristic IR bands of S-bearing groups in the sodalite-group minerals are observed in the ranges 1124-1142 cm −1 (asymmetric stretching vibrations of the SO 4 2group, the F 2 (ν 3 ) mode), 614-637 cm −1 (bending vibrations of the SO 4 2group, the F 2 (ν 4 ) mode) and 575-585 cm −1 (antisymmetric stretching vibrations of the S 3 − radical anion, the ν 3 mode-A weak band) [26][27][28][29][30].
Taking        A narrow band at 2341-2346 cm -1 present in the IR spectra of Samples 2-9 is due to the antisymmetric stretching vibrations of CO2 molecules [11,32]. This band is accompanied by a very weak isotope satellite at 2275-2279cm -1 corresponding to 13 C 16 O2. Samples 3, 6 and 9 are the most CO2-rich. The three bands at about 670, 710 and 740 cm −1 are proof that the sodalite-type structure exists. In some spectra, including those of Samples 3, 4 and 7, the arrangements of these three characteristic bands are disturbed. Most probably, this phenomenon is due to structure modulation, which is a characteristic feature of sodalite-type minerals from metasomatic lazurite deposits [23].
The strongest band of the CO 3 2group in sodalite cavities is observed in the range 1436-1450 cm −1 [30,31]. In the IR spectra of natural samples investigated in this work, this band is absent, but a band at 1438 cm −1 is observed in the IR spectrum of the synthetic sodalite-type compound, with the presumed crystal-chemical formula Na 7.38 (Si 6.74 Al 5.26 O 24 )(AlF 6 ) 0.70 ·4.88H 2 O (Sample 10 [14]). The IR spectrum of Sample 10 was discussed elsewhere [15]. In accordance with structural data and by analogy with IR spectra of natural fluoraluminates [30], the band at 597 cm −1 corresponds to stretching vibrations of the AlF 6 3anion. This band is absent in the IR spectra of all other samples investigated in this work, including natural F-rich sodalite-type mineral (Sample 9). A narrow band at 2341-2346 cm −1 present in the IR spectra of Samples 2-9 is due to the antisymmetric stretching vibrations of CO 2 molecules [11,32]. This band is accompanied by a very weak isotope satellite at 2275-2279cm −1 corresponding to 13 C 16 O 2 . Samples 3, 6 and 9 are the most CO 2 -rich.
The optical density of the band at 2341-2346 cm −1 (D) shows positive correlation with the content of CO 2 determined by selective sorption (Figure 9). Consequently, IR spectroscopy can be used for the estimation of the content of CO 2 molecules in sodalite-group minerals.
We failed to assign the weak band at 2043-2044 cm −1 observed in the IR spectra of Samples 6 and 7. Hypothetically, this band may be associated with a minor admixture of an extra-framework component with the S-H covalent bond.
The optical density of the band at 2341-2346 cm -1 (D) shows positive correlation with the content of CO2 determined by selective sorption (Figure 9). Consequently, IR spectroscopy can be used for the estimation of the content of CO2 molecules in sodalite-group minerals.
We failed to assign the weak band at 2043-2044 cm -1 observed in the IR spectra of Samples 6 and 7. Hypothetically, this band may be associated with a minor admixture of an extra-framework component with the S-H covalent bond. Figure 9. The correlation between the content of CO2 determined by the selective sorption and relative optical density of the band at 2341-2346 cm -1 (D0 is the optical density at the maximum of the band at 656-668 cm -1 used as an internal standard). The circles correspond to Samples 1-8 and the square corresponds to Sample 9 with the CO2 content determined using this correlation.

Raman Spectroscopy
Among the nine minerals of the sodalite group studied in this work, there are several samples characterized by anomalous features of the chemical composition (Table 3). These are Samples 3, 6 and 9 with high contents of carbon dioxide molecules (≥ 0.36 CO2 groups pfu), Sample 7 with a high content of sulfur (2.74 atoms pfu), and Sample 9 characterized by a very high content of fluorine (1.81 wt.%, 0.83 atoms pfu), which has never been noted in natural sodalites before. Among minerals, a higher content of CO2 molecules (2.2 wt.%) was detected only in cordierite [33]. Figure  10; Figure 11 show the Raman spectra of these samples as well as the Raman spectrum of the synthetic F-rich compound with the idealized formula Na8(Si7Al5O24)(AlF6)·5H2O (Sample 10).
The F-rich Sample 9 shows a strong luminescence under a laser beam (the broad feature in the range 1900-3200 cm -1 : Figure 11). The cause of this phenomenon is unknown. Another specific feature of the Raman spectrum of Sample 9 is the presence of the weak bands at 605 and 1074 cm -1 which are absent in the Raman spectra of other samples and may correspond to translational and librational modes of HF [34].
The assignment of Raman bands is given in Table 4. For the assignment of the bands at 327-331 and 667-680 cm -1 to the vibrations of the S4 -radical anion, see the Discussion section.  Figure 9. The correlation between the content of CO 2 determined by the selective sorption and relative optical density of the band at 2341-2346 cm −1 (D 0 is the optical density at the maximum of the band at 656-668 cm −1 used as an internal standard). The circles correspond to Samples 1-8 and the square corresponds to Sample 9 with the CO 2 content determined using this correlation.

Raman Spectroscopy
Among the nine minerals of the sodalite group studied in this work, there are several samples characterized by anomalous features of the chemical composition (Table 3). These are Samples 3, 6 and 9 with high contents of carbon dioxide molecules (≥ 0.36 CO 2 groups pfu), Sample 7 with a high content of sulfur (2.74 atoms pfu), and Sample 9 characterized by a very high content of fluorine (1.81 wt.%, 0.83 atoms pfu), which has never been noted in natural sodalites before. Among minerals, a higher content of CO 2 molecules (2.2 wt.%) was detected only in cordierite [33]. Figures 10 and 11 show the Raman spectra of these samples as well as the Raman spectrum of the synthetic F-rich compound with the idealized formula Na 8 (Si 7 Al 5 O 24 )(AlF 6 )·5H 2 O (Sample 10).

X-ray Diffraction Data and Crystal Structure
The studied sample of the F-rich sodalite-group mineral (Sample 9) retains the sodalite-type framework built by corner-linked TO4 tetrahedra (T = Si, Al). Extra-framework cations (Na + and subordinate K + and Ca 2+ statistically replacing each other), anions (F -, Cl -, SO4 2-) and neutral molecules (H2O, CO2) occur in β-cages. The refined F content of 1.54 apfu is significantly higher than the value of 0.83 F apfu in the empirical formula. On the other hand, no separate sites were found for H2O and CO2 molecules. For this reason, we assume that these components may replace F anions. Additional water molecules may occur with a very low occupancy around O atoms of SO4 tetrahedron (OS). The Na site is split onto three subsites with partial occupancies. The site occupancy factors for all Na subsites were refined using Na scattering curve and Uanis restrained to be equal; then, K (K = K + Ca) admixture was added according to chemical data and the refined number of electrons.
Atom coordinates and displacement parameters for Sample 9 are given in Table 5 and interatomic distances in Table 6. The crystal structure of Sample 9 is shown in Figure 12.  (10) F0.128 12 Figure 11. Raman spectra of the F-rich sodalite-group mineral (Sample 9) and the synthetic fluoraluminate sodalite-type compound (Sample 10).
The F-rich Sample 9 shows a strong luminescence under a laser beam (the broad feature in the range 1900-3200 cm −1 : Figure 11). The cause of this phenomenon is unknown. Another specific feature of the Raman spectrum of Sample 9 is the presence of the weak bands at 605 and 1074 cm −1 which are absent in the Raman spectra of other samples and may correspond to translational and librational modes of HF [34].
The assignment of Raman bands is given in Table 4. For the assignment of the bands at 327-331 and 667-680 cm −1 to the vibrations of the S 4 − radical anion, see the Discussion section.

X-ray Diffraction Data and Crystal Structure
The studied sample of the F-rich sodalite-group mineral (Sample 9) retains the sodalite-type framework built by corner-linked TO 4 tetrahedra (T = Si, Al). Extra-framework cations (Na + and subordinate K + and Ca 2+ statistically replacing each other), anions (F − , Cl − , SO 4 2-) and neutral molecules (H 2 O, CO 2 ) occur in β-cages. The refined F content of 1.54 apfu is significantly higher than the value of 0.83 F apfu in the empirical formula. On the other hand, no separate sites were found for H 2 O and CO 2 molecules. For this reason, we assume that these components may replace F anions. Additional water molecules may occur with a very low occupancy around O atoms of SO 4 tetrahedron (OS). The Na site is split onto three subsites with partial occupancies. The site occupancy factors for all Na subsites were refined using Na scattering curve and U anis restrained to be equal; then, K (K = K + Ca) admixture was added according to chemical data and the refined number of electrons. Atom coordinates and displacement parameters for Sample 9 are given in Table 5 and interatomic distances in Table 6. The crystal structure of Sample 9 is shown in Figure 12.   * The Na2 subsite is connected with F and Cl atoms with short distances of 2.11 Å (Na2-F) and 2.57 Å (Na2-Cl); for this reason, these bonds were excluded from the coordination sphere of Na2.

Discussion
The empirical formulae of the studied sodalite-group minerals were calculated based on chemical data (Table 3) and taking into account the above data on the infrared and Raman spectra. According to the spectroscopic data, the anomalously high-sulfur Sample 7 from Sar-e Sang (the type locality of lazurite) has a high content of the radical anions S3 -and a low content of the SO4 2groups; other polysulfide anions are absent, and the presence of the monosulfide anion S 2-cannot be excluded.

Discussion
The empirical formulae of the studied sodalite-group minerals were calculated based on chemical data (Table 3) and taking into account the above data on the infrared and Raman spectra. According to the spectroscopic data, the anomalously high-sulfur Sample 7 from Sar-e Sang (the type locality of lazurite) has a high content of the radical anions S 3 − and a low content of the SO 4 [35] together with the reference [9]. Moreover, the latter formula contradicts the results of X-ray structural analysis of "lazurite" from [9], which correspond to a SO 4 -dominant mineral.
The charge-balanced empirical formula of another sample from Sar-e Sang (Sample 8) is (Na 6 [11], but this sample is characterized by a relatively low intensity of the stretching band of carbon dioxide and, therefore, most of the carbon in it belongs to carbonate anions.
In the review [57], it is stated that "The radical anion [S-S-S-S]* − has not yet been observed convincingly by Raman spectroscopy". However, it is to be noted that the structure and Raman spectrum of the tetrasulfide radical anion is discussed in detail in [54]. The authors of this paper concluded that (S 4 ) − has the flat star shape, D3h. The calculated wavenumbers of Raman active bands ν 1 (A1) = 329, ν 3 (E) = 680, and ν 4 (E) = 345 cm −1 are in excellent agreement with experimental data. The intensities of the bands ν 1 and ν 3 are nearly equal, whereas ν 4 is observed as a weak shoulder. In our Raman spectra of Samples 3 and 6, the weak bands ν 1 and ν 3 also have nearly equal intensities, and the band ν 4 is not observed. In the Raman spectra of F-rich and S-depleted Samples 9 and 10, the bands at 673 and 665, which are not accompanied by the bands in the range 320-340 cm −1 , are tentatively assigned to the vibrations of the HF molecule as a whole, in accordance with [34].
The IR and Raman spectra indicate that F-rich Sample 9 contains sulfate groups, H 2 O and CO 2 molecules. As noted above, the band at 1350-1358 cm −1 observed in both IR and Raman spectra may correspond to vibrations of the H + cation in the β-cage. Unlike the synthetic F-rich sodalite-type compound (Sample 10), Sample 9 does not show IR and Raman bands of the AlF 6 3anion. Consequently, the empirical formula of the latter sample is H + 0.12 (Na 5.86 K 0.82 Ca 0.10 )(Si 6.34 Al 5.66 O 24 )F 0.83 Cl 0.17 (SO 4 ) 0.17 (CO 2 ) 0.36 ·nH 2 O. The addition of H + in the empirical formula is formally required for the charge balance, and is in agreement with the above assumption that Sample 10 contains additional H + cation. The most interesting feature of Sample 9 is a significant predominance of F − over all other extra-framework anions, which makes it possible to consider this mineral as a potentially new mineral species with the idealized formula Na 7 (Si 6 Al 6 O 24 )F − ·nH 2 O. Most probably, this mineral crystallized at high fugacities of HF and CO 2 .
Hypothetically, anionic and neutral extra-framework components (Cl − , OH − , F − , AlF 6 3-, SO 4 2-, S 2-, S 3 − , CO 3 2-, C 2 O 4 2-, CO 2 , H 2 O, CH 4 etc.) occurring in β-cages of minerals belonging to the sodalite and cancrinite groups could be used as important geochemical markers, provided that the relationships between their contents and conditions of mineral formation will be investigated in detail experimentally. In particular, one can suppose that the ratios SO

Conflicts of Interest:
The authors declare no conflict of interest.