Relationships between the Structural, Vibrational, and Optical Properties of Microporous Cancrinite

: The crystal-chemical, vibrational, and optical properties of microporous aluminosilicate cancrinite have been investigated by combining electron probe microanalysis, single-crystal X-ray diffraction, infrared (IR) absorption, Raman, UV-Visible absorption, and electron spin resonance spectroscopy. The behavior of the peaks in the IR spectra was also studied during the dehydration of the sample. The analyzed sample has the following unit cell parameters ( P 6 3 ): a = 12.63189(14) Å, c = 5.13601(7) Å. The empirical formula, based on 12(Si + Al), is Na 6.47 Ca 1.23 K 0.01 [Al 5.97 Si 6.03 O 24 ] (CO 3 ) 1.45 (SO 4 ) 0.03 Cl 0.01 · 2H 2 O. The Al-Si framework of AB-type is formed by columns of based-shared “cancrinite” (CAN) cages, containing Na and H 2 O positions located on the 3-fold axis, and channels with CO 3 groups, lying in two mutually exclusive and partially occupied positions in the center of the channel, and split Na/Ca cation sites. The revealed characteristics are somewhat different in comparison with the cancrinite structural features previously described in the literature. Studied crystals change color from grayish-pink to blue after X-ray irradiation (10 4 Gy). The blue color of the irradiated cancrinite is caused by the formation (CO 3 ) −• radicals in the crystals. Combining the results obtained using the selected methods will provide a better understanding of the relationships between the structural, chemical, and optical-physical properties of microporous aluminosilicates.


Introduction
Cancrinite is a widespread mineral, having white, light blue, gray, yellowish, greenish, or bright yellow color. Its name corresponds to the surname of Count Georg Ludwig Cancrin, Russian Minister of Finance (XIX century). Cancrinite is an aluminosilicate belonging to the feldspathoid family, characterized by layers of six-membered rings of Si-and Al-tetrahedra stacked along c to form a three-dimensional open-framework. The framework density, defined as the number of framework knots per 1000 Å 3 , is equals to 16.9 [1], a value lying in the range (from 14 to 22) found for zeolites and microporous mineral phases. Moreover, the mineral was considered as promising zeolite for accommodating various inorganic and organic species in the structural voids. That it is useful for the development of new pigments [2] and storage tanks containing high-level waste [3].
Cancrinite contains (CO 3 ) 2− as the main anionic component in the channel voids. The ideal chemical formula of cancrinite (according to IMA List of Minerals) can be written as (Na,Ca, ) 8  The crystal structure of cancrinite was first solved by Pauling (1930) [4] and then refined by Jarchow (1965) [5]. To obtain a better understanding of the crystal chemistry of cancrinite-group minerals, electron probe microanalysis (EPMA), single-crystal X-ray X-ray diffraction analysis of the selected single crystal was performed using a Bruker AXS D8 VENTURE automated diffractometer (Bruker, Berlin, Germany) equipped with a Photon 100 detector, and MoKα radiation. Operating conditions were: 50 kV and 1 mA, crystal-to-detector distance of 40 mm. A set of 24 frames was used for initial cell determination with a strategy optimized by the APEX2 suite package [15]. Complete data collection was then accomplished by several ϕ and ω scans with 0.3 • rotation and 5.5 s exposure time per frame. The half of the Ewald sphere (±h, ±k, ±l) was recorded in theta ranges up to~40 • . Data reduction was performed using CrysAlisPro Version 1.171. 39.46 [16]. Least-squares refinement was performed using the program CRYSTALS [17] in the space group P6 3 . The refined parameters were scale factor, atom positions, anisotropic displacement parameters, and occupancies for extra framework cations and anions. The relevant details of structure refinement are reported in Table 1. The CIFs were deposited with the Cambridge Crystallographic Data Centre (CCDC 2064244).
Figures showing structural details were prepared using the program VESTA (version 4.3.0) [18].

Raman and Infrared (IR) Spectroscopy
Raman spectra of randomly oriented cancrinite grains were obtained using a WITec alpha300R confocal Raman spectroscopic system (WITec GmbH, Ulm, Germany) coupled with frequency-doubled 532 nm Nd:YAG laser at room temperature. The spectra were recorded with a diffraction grating 1800 gr mm −1 with spectral resolution 3 cm −1 . The output power of the laser beam was 15 mW. The diameter of the focal spot on the sample was 5-10 µm. The backscattered Raman signal was collected with a Zeiss 50×/NA 0.55 objective throughput UHTS300 spectrometer equipped with a Peltier-cooled, front-illuminated CCD camera; signal acquisition time for a single scan of the spectral range was 1 s, and the signal was averaged over 100 scans. Crystalline silicon was used as a standard.
In order to obtain IR absorption spectra of the cancrinite, powdered samples were mixed with anhydrous KBr, pelletized, and analyzed using an FT-801 spectrometer (Simex, Novosibirsk, Russia) at a resolution of 1 cm −1 . Thirty-two scans were collected for each spectrum. The IR spectrum of an analogous pellet of pure KBr was used as a reference. The dehydration procedure with subsequent IR spectra acquisition is the same as described in [19,20]. The heating rate was 10 K/min.

Optical Absorption Spectroscopy and Electron Spin Resonance (ESR)
Diffuse-light optical absorption spectra of the cancrinite were measured at room temperature with a PerkinElmer Lambda 950 spectrophotometer (Perkin-Elmer, Shelton, CT, USA) in an integrating sphere. For measurements, the samples were placed in a quartz test tube, which is transparent in the range of 270-830 nm (37,000-12,000 cm −1 ). The light beam was completely concentrated on the sample [21][22][23].
Electron spin resonance (ESR) spectra were recorded using a RE-1306 X-band spectrometer (KBST, Smolensk, Russia) with a frequency of 9.257 GHz. Small single crystal grain of the cancrinite was mounted in a quartz dewar in the resonator. A magnetic field was applied along the c axis of the cancrinite. The measurements were carried out at 77 K [23].
Irradiation of the cancrinite samples was performed by an X-ray tube with Pd-anode operated at 40 kV, 20 mA for 30 min (10 4 Gy) at room temperature.

Chemical Composition and Structure Description
Chemical composition results, obtained from 10 analyses, show that the samples examined are relatively homogeneous, particularly regarding Al 2 O 3 (29.5-29.9  Final atomic coordinates and displacement parameters are given in Tables 2 and 3, selected interatomic bond distances and angles are listed in Table 4.  The crystal structure of cancrinite is illustrated in Figure 1. The framework of cancrinite is schematically shown in Figure 2a. It is formed through an ABAB...-stacking sequence (AB-type), describing the relative translations of the layers, containing rings of Al-and Si-centered tetrahedra. On the basis of observed <T-O> distances for Si and Al-centered tetrahedra (Table 4), it is possible to state that Si and Al regularly alternate on the T sites. The Si-Al framework contains columns of based-shared "cancrinite" cages (CAN) and channels, formed by 12-membered Al-Si-rings. Cages and channels are connected by distorted 6-membered and 4-membered windows. Cancrinite cage and channel arranged along the c axis and will be described separately. The structural cavities and channels host extra-framework alkaline (Na, minor K) and earth-alkaline (Ca) cations, water molecules, carbonate, and minor sulfate and chlorine anionic groups. All Ca was located at the M2 site.
along the c axis and will be described separately. The structural cavities and channels host extra-framework alkaline (Na, minor K) and earth-alkaline (Ca) cations, water molecules, carbonate, and minor sulfate and chlorine anionic groups. All Ca was located at the M2 site.   along the c axis and will be described separately. The structural cavities and channels host extra-framework alkaline (Na, minor K) and earth-alkaline (Ca) cations, water molecules, carbonate, and minor sulfate and chlorine anionic groups. All Ca was located at the M2 site.

Cancrinite Cage
The cancrinite cage, [4 6 6 5 ] according to the IUPAC rules [24], has a thickness along the c corresponding to two layers. The cage is hendecahedral and composed of six fourmembered and five six-membered rings (Figure 2b). In the refined structure, chains of cancrinite cages occur at (1/3, 2/3, z) and (2/3, 1/3, z) (Figures 1 and 2a). Each CAN cage contains Na1 and H 2 O positions located on the 3-fold axis. The Ow bonded strongly to the Na1 cation on the one side and to a second Na1 cation of neighboring cancrinite cage on another side, with bond distances of 2.345(8) Å and 2.791(8) Å, respectively ( Table 4). The H1 position was found to have the statistical distribution of corresponding hydrogen atoms with a probability of 1/3. Ow-H1 distances is 1.055 Å (H1-Ow-H1 angle is 118.0 • ), the shortest H1···O distance is 2.962(1) Å.

Channel
The channel (Figure 2c), denoted as [6 6 12 2/2 ] [24], occurs at (00z) ( Figure 1) and contains CO 3 groups, lying in two mutually exclusive and partially occupied positions in the center of the channel, and cation site, distributed into two split positions: Na2 and Ca2 with occupancies of 0.58 and 0.33, respectively. Carbon atoms in the channel are located over four levels along the c axis, with C1-C2 distances of 1.16(1) Å on the one side and 1.41(1) Å on another side, and occupancies equal 0.38 (C1, and the corresponding O C1 ) and 0.47 (C2, and the corresponding O C2 ). Sodium and calcium cations inside the channel can be coordinated either by oxygen atoms belonging to triangle C1O (C1)3 (case (1) in Table 4), or, in the second case ((2) in Table 4), by oxygens of C2O (C2)3 group.

Infrared Spectroscopy
The infrared absorption spectrum of the cancrinite is presented in Figure 4. The spectrum consists of a group of overlapping strong bands of stretching vibrations with absorption peaks in the following spectral regions (a) 980 and 1080 cm −1 -stretching vibrations of the aluminosilicate framework; (b) three relatively narrow bands corresponding to mixed vibrations of the framework of tetrahedra (in the ranges 578 cm −1 , 621 cm −1 and 675-685 cm −1 ); (c) asymmetric stretch vibrations ν 3 of (CO 3 ) 2− in the range 1360-1590 cm −1 and out-of-plane ν 2 (CO 3 ) 2− mode at 855 cm −1 ; (d) bending vibration at 1628 cm −1 and stretching vibration in the range 3000-3800 cm −1 of water molecules located in the cancrinite cages [12]. Modes related to (SO 4 ) 2− radicals located at 585 cm −1 in Raman [19] and 1160-1180 cm −1 in infrared absorption spectra [12] are not observed in the studied cancrinite. Intense mode ν 1 of (SO 4 ) 2− group at 990-1000 cm −1 are overlapped with the bands of the aluminosilicate framework, but ν 4 mode located at 585 cm −1 in Raman [19] and ν 3 mode 1160-1180 cm −1 in infrared absorption spectra [12] are not observed in the studied cancrinite.  The absorption bands of mixed vibrations of the aluminosilicate framework in the range 570-700 cm -1 are sensitive to the spatial structure of feldspathoids. Chukanov et al. (2011) [12] found a correlation between positions of the framework mixed vibration bands and types of the framework in different cancrinites. Samples with an AB-type framework show three distinct bands in this range; changes in the stacking of layers are characterized by an additional splitting of the bands attributed to framework mixed vibrations. In the studied sample, the bands at 578 and 685 cm −1 correspond spectra of cancrinite solid-solutions in [12] with AB-type framework that is slightly disturbed by water molecules in structure.
As pointed out above, the carbonate anion (CO3) 2-is the most common carbon-containing extra-framework species in cancrinite minerals. Mostly the anions are planar and have close to D3h symmetry. They have three active modes in an IR absorption spectrum ( Figure 4). The most surround sensitive mode is asymmetric stretch vibrations ν3 of (CO3) 2− in the range 1360-1590 cm −1 . This mode is doubly degenerate and it splits into two bands  [12] found a correlation between positions of the framework mixed vibration bands and types of the framework in different cancrinites. Samples with an AB-type framework show three distinct bands in this range; changes in the stacking of layers are characterized by an additional splitting of the bands attributed to framework mixed vibrations. In the studied sample, the bands at 578 and 685 cm −1 correspond spectra of cancrinite solidsolutions in [12] with AB-type framework that is slightly disturbed by water molecules in structure. As pointed out above, the carbonate anion (CO 3 ) 2− is the most common carboncontaining extra-framework species in cancrinite minerals. Mostly the anions are planar and have close to D 3h symmetry. They have three active modes in an IR absorption spectrum ( Figure 4). The most surround sensitive mode is asymmetric stretch vibrations ν 3 of (CO 3 ) 2− in the range 1360-1590 cm −1 . This mode is doubly degenerate and it splits into two bands at about 1400 and 1500 cm −1 . The band at 1400 cm −1 has a slightly resolved structure with peaks at 1385, 1398, 1409, 1434 cm −1 . The band at about 1500 cm −1 also has a structure with peaks at 1450, 1483, 1503,1515, 1520 cm −1 . As it was revealed from structural refinement, in the investigated cancrinite two non-equivalent positions of (CO 3 ) 2− exist. Chukanov et al. (2011) [12] pointed, that the content of Ca in cancrinite was the most important factor in determining splitting and position of the second band related to ν 3 mode. The average position of the second band of ν 3 mode at about 1500 cm −1 is usually observed in Ca-rich cancrinite minerals with Ca content 1.19-1.6 apfu. That is in agreement with crystal-chemical data (1.2 apfu). The splitting and position of the second band can be explained by the generation of chain vibrations due to the resonance between ν 3 -type vibrations of neighboring Na 2 Ca(CO 3 ) clusters [12].

Optical Absorption Spectroscopy and ESR
Studied crystals have light grayish-pink color and opaque. Therefore, the optical absorption spectrum is measured in a diffuse light integration sphere. The absorption spectrum of the initial sample is given in Figure 5 The irradiated samples become blue color ( Figure 6). The absorption spectrum of the irradiated cancrinite is given in Figure 5. Strong bands in the region 12,500-23,900 cm −1 (800-435 nm) and 25,000-37,000 cm −1 (400-270 nm) appear. The band at 12,500-23,900 cm −1 has a slightly resolved structure with peaks at 14,800 and 15,500 cm −1 (675 and 645 nm respectively). In addition, an intense ESR signal having g⊥=2.015 is registered in the irradiated sample (Figure 7). The intensities of absorption band and ESR signal decrease with the heating of the sample above 400 K and the samples remain initial color and ESR signal disappear at 550 K.   The irradiated samples become blue color ( Figure 6). The absorption spectrum of the irradiated cancrinite is given in Figure 5. Strong bands in the region 12,500-23,900 cm −1 (800-435 nm) and 25,000-37,000 cm −1 (400-270 nm) appear. The band at 12,500-23,900 cm −1 has a slightly resolved structure with peaks at 14,800 and 15,500 cm −1 (675 and 645 nm respectively). In addition, an intense ESR signal having g ⊥ =2.015 is registered in the irradiated sample (Figure 7). The intensities of absorption band and ESR signal decrease with the heating of the sample above 400 K and the samples remain initial color and ESR signal disappear at 550 K.   Raman spectrum of the irradiated sample is also changed (Figure 3, red curve). Intensities of the ν1 (CO3) 2− related bands are decreased and the 195 cm −1 band is disappeared, but intensities of the 770, 460, and 345 cm −1 bands are strongly increased in the irradiated sample. After heating the irradiated sample to 550 K the Raman spectrum becomes the same as before irradiation.

Crystal-Chemical Features
The sample selected for the present work has a chemical composition close to most of the previously studied cancrinite minerals. Chemical data, represented in Table 5, show that cancrinite is substantially free of chlorine (~0.03 wt.% Cl), potassium (~0.06 wt.% K2O), and sulfur (~0.3 wt.% SO3). The main difference between the chemistry of the reference minerals represented in Table 5 and the studied one is that our cancrinite contains significant amounts of calcium (~6.8 wt.% CaO vs. 0.20-5.32, excluding the sample analyzed by [6]; see Table 5). The structural formulas of the studied sample of cancrinite and those from literature are represented in Table 6. Concerning the chemical composition of the cationic and anionic filling of the "cancrinite" (CAN) cage and the channel, the samples represented in Table 6 do not have particularly large differences. It can be noted that three main structural features of cancrinite are distinguished: (1) Localization of the water molecule (on the third-order axis or outside it); (2) disordering of the CO3-group into several sub-positions (from 2 to 4) and (3) disordering of the cationic position, designated М2, which in most cases has a mixed occupancy by Na and Ca atoms. Comparing the literature data, one can conclude that the studied sample is close to two cancrinites: A mineral from Ilmeny Mtns (Russia) [30] and a sample from Bancroft (Canada) [31]. However, none of Raman spectrum of the irradiated sample is also changed (Figure 3, red curve). Intensities of the ν 1 (CO 3 ) 2− related bands are decreased and the 195 cm −1 band is disappeared, but intensities of the 770, 460, and 345 cm −1 bands are strongly increased in the irradiated sample. After heating the irradiated sample to 550 K the Raman spectrum becomes the same as before irradiation.

Crystal-Chemical Features
The sample selected for the present work has a chemical composition close to most of the previously studied cancrinite minerals. Chemical data, represented in Table 5, show that cancrinite is substantially free of chlorine (~0.03 wt.% Cl), potassium (~0.06 wt.% K 2 O), and sulfur (~0.3 wt.% SO 3 ). The main difference between the chemistry of the reference minerals represented in Table 5 and the studied one is that our cancrinite contains significant amounts of calcium (~6.8 wt.% CaO vs. 0.20-5.32, excluding the sample analyzed by [6]; see Table 5). The structural formulas of the studied sample of cancrinite and those from literature are represented in Table 6. Concerning the chemical composition of the cationic and anionic filling of the "cancrinite" (CAN) cage and the channel, the samples represented in Table 6 do not have particularly large differences. It can be noted that three main structural features of cancrinite are distinguished: (1) Localization of the water molecule (on the third-order axis or outside it); (2) disordering of the CO 3 -group into several subpositions (from 2 to 4) and (3) disordering of the cationic position, designated М2, which in most cases has a mixed occupancy by Na and Ca atoms. Comparing the literature data, one can conclude that the studied sample is close to two cancrinites: A mineral from Ilmeny Mtns (Russia) [30] and a sample from Bancroft (Canada) [31]. However, none of the previously described cancrinite samples have all three structural features similar to studied cancrinite from Zhidoi. In the two mentioned samples of the mineral [30,31], the M2 position is ordered.   Geometric data and distortion parameters for the cancrinite sample are given in Table 7. Analyzing the structural model of cancrinite, several features can be noted. The Al2 site appears to have a valence sum that is too high (3.26 vu), indicating the possibility of partial entry into this position of a more highly charged cation, i.e., Si. The Al2 tetrahedron also displays the higher TAV (16.5480) parameter with respect to Si1 (TAV = 6.9838). The BLD, ELD, and TQE values are similar for Si1 and Al2. For sodium and calcium polyhedra, the distortion parameters are also close. Table 7. Calculated geometrical and distortion parameters for polyhedra in the crystal structure of the studied cancrinite sample. An effective coordination number (ECoN) and the volume of the coordination polyhedron (Vp) were calculated the program VESTA (version 4.3.0) [18]. Bond valence calculations (BVS) were performed using the parameters by [35]. Bond length distortion (BLD) and edge length distortion (ELD) were calculated according to [36]. Tetrahedral angle variance (TAV) and tetrahedral quadratic elongation (TQE) were calculated according to [37].

Si1
Al2 Na1 However, exceptional values were obtained for some anionic positions. The oversaturation of the O C1 and O C2 sites, when they coordinate calcium (2.24 and 2.22 vu, respectively), indicates local structural disordering. One or two of the three cation atoms are filled with sodium or one of the three calcium atoms is replaced by a vacancy, which has~9% occupancy in this Na2/Ca2 position. The anionic CO 3 -group position within the channel also has an incomplete occupancy (the vacancy is 15%).
The measure of the strain of the crystal structure is expressed in the global instability index (GII), defined by [38]. Values of the GII < 20% suggest that no or reasonable strain is presented, while values > 20% indicate that structure is so strained as to be unstable [39]. To analyze stability, we considered several models: (1) and (2) sodium occupies Na2 position and bonded to two C1O (C1)3 or C2O (C2)3 triangles, respectively; (3) and (4) calcium occupies Ca2 positions; (5) and (6) sodium occupies Na2 site and bonded to one C1O (C1)3 or C2O (C2)3 triangle, respectively; (7) and (8) calcium occupies Ca2 site and bonded to one C1O (C1)3 or C2O (C2)3 triangle, respectively. Due to the Ca2 position being undersaturated, configurations Ca2 + C1 + V and Ca2 + C2 + V have become extremely strained (GII total = 20.83 and 20.46%, Table 8). Cancrinite shows a significantly increased index for Al2 (25.90%) and low value for Si1 (1.20%), but Na, O, and C are still in the medium range. Although the GII value for C in model 6 increases (24.32%), the index for GII total is getting acceptable (16.57%). In the cancrinite crystal structure, calcium is an unstable position. In case 5, significant local relaxation of Na is noted. The overall stress quantified by the global instability index (GII) significantly varies across the models. Assuming only GII total values, structure models 1-6 can be considered stable (GII total ranges from 13.85 to 16.87%, Table 8). The most relaxed are the models in which sodium occupies Na2 position and bonded to two C;1-carbonate groups or one C1O (C1)3 group and vacancy. Stability analysis indicates that the coordination of Na2/Ca2 and the environment of CO 3 -triangles is the driving force for the stabilization of the crystal structure model. According to [6,40,41] substitutional or positional ordering (or both) of interframework cations and anions may be the reason for the appearance of a superstructure, which should be destroyed on heating [31]. Table 8. The global instability index (GII, %), calculated for crystal structure models of studied cancrinite. V-vacancy.

GII (%) C GII (%) O GII (%) Total
( The CAN cage is built from three types of rings: Two horizontal (perpendicular to the c axis) 6-membered rings with aperture dimension of 5.281(1) × 5.281(1) × 5.281(1) Å (ecw = 2.58 × 2.58 × 2.58 Å), three lateral 6-membered rings with cross-section diameters Channels are extended along the c-axis and delimited by 12-membered rings of tetrahedra (Figure 2 a,c). The shortest distances between oppositely located oxygen atoms in the ring are 8.456(1) × 9.232(1) Å. A fundamental characteristic of a channel described the accessibility of the pore system to guest species, is effective channel width (ecw), which is defined as the distance between oxygen atoms in the smallest n-ring or smallest free aperture subtracted by 2.7 Å, when the oxygen ionic radius is assumed to be 1.35 Å. Therefore, ecw for the studied cancrinite channel is 5.76 × 6.53 Å, while a minimum ecw of 3.2 Å is required to be defined as a microporous mineral [42]. Thus, the channels of cancrinite have larger dimensions with respect to the CAN cage aperture and may comprise large guest atoms and molecular groups. The dehydration of cancrinite was studied with X-ray diffraction methods by [31,43]. They found that the dehydration of cancrinite occurred in temperature interval 470-850 K. In this study, we monitor the dehydration process using IR absorption spectroscopy. The wide band peaked at about 3320 cm −1 begin to decrease at about 400 K and it is completely disappeared at 650 K ( Figure 8, curve 1). The intensities of absorption bands at 3540 and 3605 cm −1 related to stretching vibrations and 1628 cm −1 band belonging to bending vibration of H 2 O molecules decrease in the range 400-850 K (Figure 8, curve 2). Therefore, these H 2 O molecules are located in the structural cages (CAN-cages), similarly to [10]. However, H 2 O molecules related to the 3320 cm −1 band could be located in the channels with a larger aperture [12]. K. In this study, we monitor the dehydration process using IR absorption spectroscopy. The wide band peaked at about 3320 cm −1 begin to decrease at about 400 K and it is completely disappeared at 650 K ( Figure 8, curve 1). The intensities of absorption bands at 3540 and 3605 cm −1 related to stretching vibrations and 1628 cm −1 band belonging to bending vibration of H2O molecules decrease in the range 400-850 K (Figure 8, curve 2). Therefore, these H2O molecules are located in the structural cages (CAN-cages), similarly to [10]. However, H2O molecules related to the 3320 cm −1 band could be located in the channels with a larger aperture [12]. During the dehydration we find the changing the shape and splitting of the absorption band at about 1500 cm −1 attributed to ν3 of (CO3) 2− mode. The modification of the band in the region 1480-1510 cm −1 is started at about 540 K and completed at 850 K together with full dehydration of the cancrinite (Figure 9). The structure of the ν3 band of dehydrated cancrinite is similar to H2O-free cancrinite discovered by us in effusive rocks of Bellerberg, Eifel volcanic area, Germany [12]. According to [31], the ordering of the [Ca-CO3] clusters and vacancies in the channels is destroyed at 504 °C (781 K). Therefore, the structure of the 1500 cm −1 band becomes poor due to the disordering of [Ca-CO3] clusters and their vacancies. During the dehydration we find the changing the shape and splitting of the absorption band at about 1500 cm −1 attributed to ν 3 of (CO 3 ) 2− mode. The modification of the band in the region 1480-1510 cm −1 is started at about 540 K and completed at 850 K together with full dehydration of the cancrinite (Figure 9). The structure of the ν 3 band of dehydrated cancrinite is similar to H 2 O-free cancrinite discovered by us in effusive rocks of Bellerberg, Eifel volcanic area, Germany [12]. According to [31], the ordering of the [Ca-CO 3 ] clusters and vacancies in the channels is destroyed at 504 • C (781 K). Therefore, the structure of the 1500 cm −1 band becomes poor due to the disordering of [Ca-CO 3 ] clusters and their vacancies. During dehydration positions of two peaks related to aluminosilicate framework vibration modes at 622 and 1036 cm −1 are shifted. The maximum of the band at 1036 cm −1 is shifted to the lower wavenumbers region at about 10 cm −1 , but the peak at 622 cm −1 is moved in the opposite direction to 626 cm −1 after heating to 700 K. The temperature dependencies of these peak positions are given in Figure 10. The dependences are in agreement with the temperature dependence of angles Si-O4-Al and Si-O3-Al [31], respectively. However, Gatta et al. (2014) pointed that the loss of H2O at 748 K seems to influence only the Si-O2-Al intertetrahedral angle [43]. Fechtelkord et al. (2001) also showed a decrease in Na1-O1 and a constant Na1-O2 [44]. Furthermore, an inversion of the Si-O2-Al vs. temperature behavior following the dehydration process was pointed out by [30]. Therefore, we can conclude that Al-O2-Si atoms participate in the vibrations with the 622 and 1036 cm −1 modes. Figure 9. Infrared absorption spectra of the initial (293 K) and dehydrated at 473; 573; 673; 773; and 873 K sample of cancrinite in the spectral region of asymmetric stretch vibrations ν 3 . All spectra were recorded at 293 K.

Dehydration of Cancrinite
During dehydration positions of two peaks related to aluminosilicate framework vibration modes at 622 and 1036 cm −1 are shifted. The maximum of the band at 1036 cm −1 is shifted to the lower wavenumbers region at about 10 cm −1 , but the peak at 622 cm −1 is moved in the opposite direction to 626 cm −1 after heating to 700 K. The temperature dependencies of these peak positions are given in Figure 10. The dependences are in agreement with the temperature dependence of angles Si-O4-Al and Si-O3-Al [31], respectively. However, Gatta et al. (2014) pointed that the loss of H 2 O at 748 K seems to influence only the Si-O2-Al intertetrahedral angle [43]. Fechtelkord et al. (2001) also showed a decrease in Na1-O1 and a constant Na1-O2 [44]. Furthermore, an inversion of the Si-O2-Al vs. temperature behavior following the dehydration process was pointed out by [30]. Therefore, we can conclude that Al-O2-Si atoms participate in the vibrations with the 622 and 1036 cm −1 modes.

Color of Cancrinite
Natural cancrinite very rarely has a blue color. We found only one photography of blue color cancrinite from Farnese, Viterbo Province, Lazio, Italy in the Mindat database

Color of Cancrinite
Natural cancrinite very rarely has a blue color. We found only one photography of blue color cancrinite from Farnese, Viterbo Province, Lazio, Italy in the Mindat database (https: //www.mindat.org/photo-269943.html, accessed on 12 March 2021). However, the usually blue color of sodalities, lazurites, and kyanoxalite is attributed to the presence of (S 3 ) −• radicals in cages of the minerals [19,21]. Synthetic cancrinite containing sulfur radicals was studied by [2]. It was found that slightly yellow-blue colored samples contained (S 3 ) −• radicals and the ESR signal of this radical was registered with g-factor 2.029. In the optical absorption spectrum of synthetic cancrinite, weak intensity structureless band peaked at 629 nm (at about 15900 cm −1 ) is attributed to transition in (S 3 ) −• centers. Observed in synthetic cancrinite ESR and optical absorption spectra are close to observed ones in lazurite and sulfur-rich sodalities [19,21], where the wide structureless band peaked at 600 nm and ESR signal with g-factor 2.030 corresponded to (S 3 ) −• radicals and during heating intensities of (S 3 ) −• ESR and absorption signal were increased upon heating to 800 K.
Another nature of blue color in carbonate-containing minerals was observed in blue irradiated calcite [45] and Maxixe type beryl [46][47][48]. The radiation-induced blue color of these minerals was due to the formation of a hole (CO 3 ) −• radical. The (CO 3 ) −• radicals demonstrate optical absorption and magnetic circular dichroism in the 12,500-23,900 cm −1 spectral region [46,49,50]. In samples containing (CO 3 ) −• radicals, a strong ESR signal with g-factor 2.016-2.020 is also observed. Therefore, the blue color of irradiated cancrinite is caused by the formation (CO 3 ) −• radicals in the crystals. A lower energy absorption band could be interpreted as basically a 2 A 2 '-2 E' electron transition, and higher energy absorption band could be attributed to higher energy transition in (CO 3 ) −• radical from 2 A 2 ' to 2 E" state or transitions in vacancy trapped electron centers. The disordering of CO 3 groups yields no vibrational fine structure of the (CO 3 ) −• absorption band and broadening of the ESR signal.
Changing the Raman spectrum of the irradiated sample requires a more detailed investigation. However, in the region of 418-487 cm −1 , antisymmetric modes ν 3 of H 2 O-(CO 3 ) −• radicals are expected [51,52]. On the other hand, the origin of electron traps in cancrinite is unknown. Probably, an electron is captured by a vacancy in the framework, which could also lead to changing some modes of the Raman spectrum.

Conclusions
The structural features of cancrinite revealed in this work are unique since they have not previously been found in such a combination among the structural characteristics of cancrinite samples from other localities. In the crystal structure of cancrinite from Zhidoi massif (Russia), the CO 3 groups occupy the center of the channel in two mutually exclusive and iso-oriented configurations. The second characteristic feature of the studied sample is the disordering of the M2 cation position into two sites: Na2 and Ca2. In addition to these two distinctive features, in our sample, Ow position (oxygen of H 2 O molecule) is placed on the threefold axis, recently noted only by [30] and [31] for cancrinites from Ilmen Mountains (Russia) and Bancroft (Canada), respectively. Displacement of the Ow from the ideal position at (2/3, 1/3, z) was observed in most published structural models ( Table 6).
The revealed structural peculiarities are reflected in the position and splitting of characteristic bands associated with vibrations of CO 3 groups in the IR absorption and Raman spectra. The IR absorption spectroscopy was used to clarify the dehydration mechanism, i.e., the releasing of water molecules from structural cages. The mechanism confirmed the previously published results of structural experiments.
Under X-ray irradiation, the cancrinite became blue color. The origin of this color was established by optical spectroscopy and ESR methods. The radiation-induced blue color of the mineral is due to the formation of the hole (CO 3 ) −• radicals.