Crystal Chemistry, Isomorphism, and Thermal Conversions of Extra-Framework Components in Sodalite-Group Minerals

: Isomorphic substitutions of extra-framework components in sodalite-group aluminosilicate minerals and their thermal conversions have been investigated using infrared, Raman, electron spin resonance (ESR), as well as ultraviolet, visible and near infrared (UV–Vis–near IR) absorption spectroscopy methods and involving chemical and X-ray diffraction data. Sodalite-related minerals from gem lazurite deposits (haüyne, lazurite, and slyudyankaite) are characterized by wide variations in S-bearing extra-framework components including SO 42 − and various polysulﬁde groups (S 2 •− , S 3 •− , S 4 •− radical anions, and S 4 and S 6 neutral molecules) as well as the presence of CO 2 molecules. Heating at 700 ◦ C under reducing conditions results in the transformation of initial S-bearing groups SO 42 − and S 3 •− to a mixture of S 2 − , HS − , S 2 •− , and S 4 •− and transformation of CO 2 to a mixture of CO 32 − and C 2 O 42 − or HC 2 O 4 − anionic groups. Further heating at 800 ◦ C in air results in the decomposition of carbonate and oxalate groups, restoration of the SO 42 − and S 3 •− groups, and a sharp transformation of the framework. The HS − anion is stable only under reducing conditions, whereas the S 3 •− radical anion is the most stable polysulﬁde group. The HS − -dominant sodalite-group mineral sapozhnikovite forms a wide solid-solution series with sodalite. The conditions required for the formation of HS − - and CO 20 -bearing sodalite-group minerals are discussed.


Introduction
Minerals belonging to the sodalite group are microporous cubic or pseudo-cubic alumino-and beryllosilicates which occur in different kinds of alkaline magmatic and metasomatic rocks. Some of them (sodalite, haüyne, and nosean) belong to important rock-forming minerals.
The unit cell parameters of the "idealized sodalite (SOD-type) framework" deposited in the Database of Zeolite Structures are a = 8.9561 Å; V = 718.4 Å 3 ; and space group Im-3m (aristotype) [1,2]. However, due to different schemes of the Si/Al ordering as well as specific features of the occupation of the intra-framework cavities (sodalite cages) by extra-framework components, the observed symmetry is usually lower. A distinctive feature of sodalite-type compounds is a three-dimensional system of channels consisting of sodalite cages and running along different directions. The extra-framework constituents in sodalite-group minerals are more diverse than in the members of the related cancrinite Table 1. Chemical composition (wt.%) of haüyne from the Malo-Bystrinskoe gem lazurite deposit, Baikal Lake area, Siberia, Russia obtained in this work. Representative chemical analyses of sodalitesapozhnikovite series minerals (Sample 12) from Karnasurt Mt., Lovozero massif, Kola Peninsula, Russia obtained in this work. a CO 2 contents corresponding to CO 2 molecules were determined from the IR spectra using a procedure described in [3]; b All sulfur is given as SO 3 , which results in high total sums for the samples bearing sulfide groups. * Recalculated from values for S measured by electron microprobe. Dash means Fe content below detection limit. Analyses are ordered by increase of the S: Cl ratio.
Sample 1 is bluish lilac S 4 -bearing haüyne. The mineral forms granular aggregate (4 cm across) embedded in calciphyre. The associated minerals are calcite, diopside, and pyrite. The charge-balanced empirical formula is ( Table 1; the contents of CO 2 and HS − were estimated from the IR spectrum; for the identification of H 2 S and S 4, see below): Na 6.24 Ca 1. 49  Sample 2 is deep blue haüyne. It forms single-crystal grains up to 1 cm across in association with calcite, phlogopite, and accessory sphalerite, Zn-bearing spinel, and grossular. The empirical formula of Sample 2 is (Table 1): Na 6.28 Ca 1.47 K 0.17 (Si 6.05 Al 5.92 Fe 0.03 O 24 )(SO 4 2− ) 1.68 S 0 0.40 Cl 0.06 (CO 2 ) 0.06 ·nH 2 O, where S 0 is all sulfide sulfur provided that it occurs in neutral molecules. Sample 3 is lilac haüyne forming a rim around light gray fine-grained aggregates of earlier silicates up to 2 cm across. The associated minerals are lazurite, sodalite, diopside, and calcite. The empirical formula is Na 6.39 K 0.06 Ca 1.57 (Si 6.08 Al 5.92 O 24 )(SO 4 ) 1.78 (S 4 ) 0.03 (S 2− ) 0.02 Cl 0.07 (CO 2 ) 0.15 ·nH 2 O (Z = 1) [4]. The mineral is cubic, with the unit cell parameter a = 9.076 Å.
Sample 4 is light blue SO 3 •− -bearing haüyne with the empirical formula Na 6 [4]. The unit cell parameter is a = 9.067 Å. The mineral forms grains up to 5 mm in calciphyre, in association with pyrite.
The diffuse absorption spectra in the near infrared, visible, and ultraviolet (NIR/Vis/UV) ranges were measured at room temperature using a Lambda 950 spectrophotometer (Perkin-Elmer, Shelton, CT, USA). In particular, the absorption spectra of platelets of Sample 1 (about 1 mm thick) were measured in the transmission mode through a circular diaphragm 0.8 mm in diameter. The spectra were measured in an integrating sphere at room temperature. To do this, small crystals of the studied mineral were placed in a quartz test tube, transparent in the region of 250-2000 nm, and the test tube was placed in an integrating sphere.
The ESR spectra were measured with a RE-1306 X-band spectrometer (KBST, Smolensk, Russia) with a frequency of 9.3841 GHz at room temperature and 9.1841 GHz at 77 K. For low-temperature measurements, a quartz ampoule with the sample was placed in a flooded cryostat.
Heating of Samples 3 and 4 was carried out according to the following scheme. The samples were placed in a muffle furnace in air and sequentially heated every 50 • C for 10 min. After holding at each of the temperatures, ESR and NIR/Vis/UV diffuse absorption spectra were measured.
Thermal conversions of other studied samples were carried out in two stages. In the first stage, the samples were heated for three days at 700 • C in reducing atmosphere (over the Fe-FeS buffer). Thereafter, parts of the preheated samples were annealed at 800 • C in air for 24-36 h.
The powder X-ray diffraction study was carried out at room temperature with an automatic powder diffractometer (D8 ADVANCE, Bruker, Germany) equipped with a Göbel mirror. The powder X-ray diffraction patterns were obtained in step scan mode (in the 2θ range of 10 to 80 • ) using CuKα radiation at an accelerating voltage of 40 kV, with a current of 40 mA, time per step of 1 s, and a 2θ step of 0.02 • . Calculations of interplanar distances were performed using the software that was delivered with the diffractometer (DIFFRAC Plus Evaluation package EVA, Bruker AXS, Bruker, Germany). Unit cell parameters were refined using the (440), (611), (622), and (721) reflections.

Isomorphism of Extra-Framework Components in Cubic Sodalite-Group Minerals
Raman spectra are very sensitive to anionic groups and neutral molecules containing sulfide sulfur. Representative Raman spectra of cubic sodalite-group minerals are given in Figures 1-3.
Except for a very weak band at 285 cm −1 (combination of low-frequency lattice modes involving Na + cations), all bands in the Raman spectrum of lazurite (Sample 9: curve b in Figure 1) correspond to the S 3 •− radical anion [5]. Weak bands of sulfate groups are not observed in the Raman spectrum of Sample 9 because of their overlapping with the bands of S 3 •− . Analogous bands of the trisulfide radical anion are present in the spectrum of bluish lilac haüyne (Sample 1), which contains trace amounts of S 3 •− (curve a in Figure 1). Additionally, in the Raman spectrum of Sample 1, bands of SO 4 2− [1,2,48]. The presence of hydronium in Sample 1 is in agreement with the deficit of extra-framework cations determined by the electron microprobe analysis (Table 1). distances were performed using the software that was delivered with the diffractometer (DIFFRAC Plus Evaluation package EVA, Bruker AXS, Bruker, Germany). Unit cell parameters were refined using the (440), (611), (622), and (721) reflections.
In the NIR/Vis/UV absorption spectrum of Sample 1, strong bands with the maxima at 0.65, 0.88, 2.05, and 2.38 eV (respectively, 1900, 1400, 605, and 520 nm) as well as absorption growth above 2.7 eV (460 nm) and a weak band at 1.33 eV (920 nm) are observed ( Figure 4). The ESR spectrum of Sample 1 contains a sextet of Mn 2+ ions in the low-spin state (S = ½ [51]) with g = 2.001, a strong single band with g = 2.002, and a weak band with g = 2.036 ( Figure 5). Most probably, Mn 2+ ions occur in calcite forming inclusions in haüyne individuals.  (Figures 2 and 3). The only significant distinctive feature of sapozhnikovite is a strong Raman band at 2553 cm −1 corresponding to stretching vibrations of HS − anions. Bolotinaite is the member of this solid-solution system most enriched in impurities (Table 1). In particular, it contains S 2 •− , which is the cause of the yellow color of this mineral and its strong luminescence under laser beams ( Figure 2).
In the NIR/Vis/UV absorption spectrum of Sample 1, strong bands with the maxima at 0.65, 0.88, 2.05, and 2.38 eV (respectively, 1900, 1400, 605, and 520 nm) as well as absorption growth above 2.7 eV (460 nm) and a weak band at 1.33 eV (920 nm) are observed ( Figure 4). The ESR spectrum of Sample 1 contains a sextet of Mn 2+ ions in the low-spin state (S = 1 /2 [51]) with g = 2.001, a strong single band with g = 2.002, and a weak band with g = 2.036 ( Figure 5). Most probably, Mn 2+ ions occur in calcite forming inclusions in haüyne individuals.
In the NIR/Vis/UV absorption spectrum of Sample 1, strong bands with the maxima at 0.65, 0.88, 2.05, and 2.38 eV (respectively, 1900, 1400, 605, and 520 nm) as well as absorption growth above 2.7 eV (460 nm) and a weak band at 1.33 eV (920 nm) are observed ( Figure 4). The ESR spectrum of Sample 1 contains a sextet of Mn 2+ ions in the low-spin state (S = ½ [51]) with g = 2.001, a strong single band with g = 2.002, and a weak band with g = 2.036 ( Figure 5). Most probably, Mn 2+ ions occur in calcite forming inclusions in haüyne individuals.  The absorption bands with maxima at 0.65 and 0.88 eV are associated with the vibrational modes of water in the studied mineral. The absorption bands with maxima at 2.05 and 2.38 eV, which impart a blue color to the studied mineral, can be associated with the presence of S3 •− radical anions, which are easily detected by ESR [4,5], while the lilac The absorption bands with maxima at 0.65 and 0.88 eV are associated with the vibrational modes of water in the studied mineral. The absorption bands with maxima at 2.05 and 2.38 eV, which impart a blue color to the studied mineral, can be associated with the presence of S 3 •− radical anions, which are easily detected by ESR [4,5], while the lilac hue can be associated with the presence of S 4 molecules (red chromophore). In the samples under study, the ESR signal characteristic of S 3 •− (with g = 2.036) is practically not observed, which indicates a very low concentration of S 3 •− radical anions in Sample 1. The absorption spectrum of S 3 •− radical anions is characterized by a plateau at about 1.77 eV. However, the shape of the absorption spectrum of Sample 1 differs from the characteristic absorption of S 3 •− of lazurite [4,5]. Thus, the absorption band at 2.05 eV is not related to S 3 •− radical anions. On the other hand, in a number of works [40,41,52,53] it is noted that absorption bands at 2.05 and 2.38 eV, as well as a much less intense band about 1.33 eV, can be attributed to the absorption of transand cis-conformers of the neutral S 4 molecule. The authors of [40] calculated the oscillator strengths f for these conformals, which are equal to 0.07 and 0.047, respectively.
The observed intense ESR signal with a g-factor of 2.002 can be attributed to the presence of some amount of O •− radical anions ( Figure 5). Previously, blue sodalite, in which a similar ESR signal and absorption spectrum were observed, was studied in [54,55], but the nature of the color of the mineral was not fully elucidated in this work.
When this sample is heated above a temperature of 500 • C, the studied sodalite loses its color. Along with this, an absorption band appears at 3.1 eV (400 nm) upon excitation, in which intense luminescence is observed with a maximum at 1.9 eV (650 nm). Such luminescence is characteristic of S 2 •− radical anions [4].

Crystal Chemistry, Isomorphism, and Thermal Conversions of Haüyne
The IR spectra of initial and heated haüyne (Samples 2 and 5) are given in      Heating results in the lowering of intensities of the bands of H2O and disappearance of the bands of SO4 2− and CO2. The samples preheated under reducing conditions change color from blue to yellow. Their IR spectra contain a weak band at 2559-2560 cm −1 , which is due to stretching vibrations of the HS − anion [39]. The weak bands at 1405-1411 and 1498 cm −1 in the IR spectra of the preheated samples correspond to asymmetric stretching vibrations of carbonate groups required for the charge compensation. The weak band at 1954 cm −1 (curve a in Figure 8) is an overtone of stretching vibrations of the aluminosili- Heating results in the lowering of intensities of the bands of H 2 O and disappearance of the bands of SO 4 2− and CO 2 . The samples preheated under reducing conditions change color from blue to yellow. Their IR spectra contain a weak band at 2559-2560 cm −1 , which is due to stretching vibrations of the HS − anion [39]. The weak bands at 1405-1411 and 1498 cm −1 in the IR spectra of the preheated samples correspond to asymmetric stretching vibrations of carbonate groups required for the charge compensation. The weak band at 1954 cm −1 (curve a in Figure 8) is an overtone of stretching vibrations of the aluminosilicate framework.
It is worth noting that bands of the isolated S 2− anion are absent in the middle IR range, but the transformation of SO 4 2− into S 2− as a result of annealing of lazurite at 800 • C was reported [56,57]. Thus, the main scheme of transformations in the first step includes reduction of sulfate anions, and formation of S 2− and HS − .
Splittings and shifts of IR bands in the preheated samples are mainly due to transformations of the framework. However, the band at 461 cm −1 may be partly due to vibrations of the trans-S 4 molecule (C 2h conformation) [40].
Annealing of the preheated haüyne samples at 800 • C in air results in the change of color from yellow to blue, disappearance or weakening of the band of HS − and in the case of Sample 5, disappearance of the bands of CO 3 2− (Figures 1-3). As noted above, unlike IR spectra, Raman spectra are sensitive to anionic groups and neutral molecules containing sulfide sulfur. The Raman spectra of Samples 2 and 5 are given in Figures   The assignment of bands in the Raman spectra of Samples 2 and 5 and products of their thermal conversions made is given in Tables 3 and 4. The Raman spectra confirm the presence of HS − and CO 3 2− anions in the products of the heating of both samples at 700 • C, over the Fe-FeS buffer. Additionally, the Raman spectra indicate the formation of oxalate C 2 O 4 2− and monosulfide S 2− anions, S 2 •− and S 4 •− radical anions, and S 4 molecules in the preheated samples. The Raman spectra of both initial haüyne samples and products of their annealing at 800 • C in air contain sets of characteristic bands of S 3 •− radical anions. . Raman spectra of (a) initial Sample 2, (b) Sample 2 preheated for three days at 700 C, over the Fe-FeS buffer, and (c) preheated Sample 2 additionally annealed at 800 C in air for one day. The assignment of bands in the Raman spectra of Samples 2 and 5 and products of their thermal conversions made is given in Tables 3 and 4. The Raman spectra confirm the presence of HS − and CO3 2− anions in the products of the heating of both samples at 700 Note: w-weak band, s-strong band.
In the original Sample 3, no S 3 •− centers were observed before heating. Two absorption bands with maxima at 525 and 585 nm are associated with S 4 •− centers in different configurations, and a weak absorption band at 400 nm corresponds to S 2 •− centers ( Figure 11) [4,58]. Being excited in the 400 nm region, the samples show intense luminescence with a maximum in the 650 nm region associated with these S 2 •− radical anions. In the ESR spectrum, in addition to six lines associated with Mn 2+ centers in calcite inclusions, there is a signal with g-tensor components of 2.034 and 2.021, associated with S 4 •− centers ( Figure 12) [58]. As a result of heating the sample, the intensity of this signal decreases, together with a weakening of the absorption bands. At temperatures above 400 • C, the S 4 •− -centers disappear.      The intensities of the absorption band and luminescence associated with S 2 •− centers increase in this case. At temperatures above 700 • C, a signal with g 1 = 2.056, g 2 = 2.041, and g 3 = 2.008, associated with S 3 •− centers, appears in the ESR spectrum. At the same time, the shape of the absorption band in the region of 600 nm changes, it acquires a table-like shape, and its intensity also begins to grow. The sample acquires an intense blue color, also associated with the S 3 •− radical anion [4]. In this case, the absorption band in the region of 400 nm and the luminescence associated with the S 2 •− centers begin to decrease. The temperature dependences of the observed effects are shown in Figure 13. The absorption spectrum of initial sample 4 shows a broad band at 600 nm, which is also associated with S3 •− radical anions, as well as a sharp rise in the region of 280 nm, which can be caused by the absorption of S6 •− radical anions ( Figure 14) [39,58,59]. In the original sample 4, a weak signal with the g-tensor components g1 = 2.045, g2 = 2.034, and g3 = 2.008, associated with S3 •− radical anions [4], as well as a signal with the g-tensor components g1 = 2.011 and g2 = 2.006, associated with the radical anion S6 •− [45,58,60], are observed ( Figure 15). The absorption spectrum of initial sample 4 shows a broad band at 600 nm, which is also associated with S 3 •− radical anions, as well as a sharp rise in the region of 280 nm, which can be caused by the absorption of S 6 •− radical anions ( Figure 14) [39,58,59]. In the original sample 4, a weak signal with the g-tensor components g 1 = 2.045, g 2 = 2.034, and g 3 = 2.008, associated with S 3 •− radical anions [4], as well as a signal with the g-tensor components g 1 = 2.011 and g 2 = 2.006, associated with the radical anion S 6 •− [45,58,60], are observed ( Figure 15).
When the sample was heated above 500 • C, the ESR signal with g 1 = 2.011 and g 2 = 2.017 decreases together with a decrease in the rise intensity in the region of 280 nm. Along with this, the sample acquires a saturated blue color, which is accompanied by an increase in the ESR signal with g 1 = 2.045, g 2 = 2.034, and g 3 = 2.008, as well as absorption bands in the region of 600 nm associated with S 3 •− radical anions. The temperature dependences built on the basis of the analysis of the absorption and EPR spectra are shown in Figure 16.     When the sample was heated above 500 C, the ESR signal with g1 = 2.011 and g2 = 2.017 decreases together with a decrease in the rise intensity in the region of 280 nm. Along with this, the sample acquires a saturated blue color, which is accompanied by an increase in the ESR signal with g1 = 2.045, g2 = 2.034, and g3 = 2.008, as well as absorption bands in the region of 600 nm associated with S3 •-radical anions. The temperature dependences built on the basis of the analysis of the absorption and EPR spectra are shown in Figure 16. The blue color of Sample 2 is due to the presence of the S3 •− radical anions, which are easily identified by an ESR signal with g1 = 2.049, g2 = 2.035, and g3 = 2.010 ( Figure 17, curve 1). After heating in reducing atmosphere using the Fe/FeS buffer at 700 C, the color of the sample changes to yellow, and the ESR signal associated with the S3 •− radical anion disappears ( Figure 17, curve 2). During subsequent heating at 800 C, the blue color is partially restored and an ESR signal with g1 = 2.049, g2 = 2.035, and g3 = 2.010 appears ( Figure 17, curve 3). However, its intensity is less than that of the original sample. The blue color of Sample 2 is due to the presence of the S 3 •− radical anions, which are easily identified by an ESR signal with g 1 = 2.049, g 2 = 2.035, and g 3 = 2.010 ( Figure 17, curve 1). After heating in reducing atmosphere using the Fe/FeS buffer at 700 • C, the color of the sample changes to yellow, and the ESR signal associated with the S 3 •− radical anion disappears ( Figure 17, curve 2). During subsequent heating at 800 • C, the blue color is partially restored and an ESR signal with g 1 = 2.049, g 2 = 2.035, and g 3 = 2.010 appears ( Figure 17, curve 3). However, its intensity is less than that of the original sample.
The observed results show that various polysulfide radical anions are converted into S 2 •− and S 4 •− radical anions and S 4 neutral molecules during heating at 700 • C in a reducing atmosphere, whereas the S 3 •− radical anion is stable during heating at 800 • C in air. These data do not contradict the experimental results obtained in [61,62], where it was shown that various sulfur radicals are converted into S 2 •− and S 3 •− radical anions when heated.
It should be noted that the values of the g-tensor components of the S 3 •− radical anions formed during heating of most S-bearing sodalite-group minerals studied in this work and elsewhere [58] are close to those of the g tensor of S 3 •− in haüyne and some synthetic sodalites [4,63]. The only exception is S 3 •− radical anion in Sample 3 heated in air: its values of the g-tensor components are close to those of S 3 •− or S 3 •− occurring in minerals of the cancrinite group [58,63].
Annealing of preheated powdered Sample 5 at 800 • C in air results in gradual enhancement of its unit cell parameter from 8.04 to 8.97 Å, accompanied by its transformation into a new cubic phase with the a parameter growing from 9.05 to 9.08 Å during 32 h (Figure 18). This transformation is accompanied by broadening and shifts of reflections. The observed results show that various polysulfide radical anions are converted into S2 •− and S4 •− radical anions and S4 neutral molecules during heating at 700 C in a reducing atmosphere, whereas the S3 •− radical anion is stable during heating at 800 C in air. These data do not contradict the experimental results obtained in [61,62], where it was shown that various sulfur radicals are converted into S2 •− and S3 •− radical anions when heated.
It should be noted that the values of the g-tensor components of the S3 •− radical anions formed during heating of most S-bearing sodalite-group minerals studied in this work and elsewhere [58] are close to those of the g tensor of S3 •− in haüyne and some synthetic sodalites [4,63]. The only exception is S3 •− radical anion in Sample 3 heated in air: its values of the g-tensor components are close to those of S3 •− or S3 •− occurring in minerals of the cancrinite group [58,63].
Annealing of preheated powdered Sample 5 at 800 C in air results in gradual enhancement of its unit cell parameter from 8.04 to 8.97 Å, accompanied by its transformation into a new cubic phase with the a parameter growing from 9.05 to 9.08 Å during 32 h (Figure 18). This transformation is accompanied by broadening and shifts of reflections.
The initial phase with a smaller a parameter disappears five hours after the start of annealing. The final product of annealing is pale blue. Annealing of preheated single-crystal grains of Sample 5 at 800 C during 32 h results in the formation of a greenish blue cubic phase. Based on the above spectroscopic data, one can conclude that the enhancement of the unit cell parameter during annealing is caused by the transformation of small extra-framework anions (S 2− and HS − ) into larger ones (SO4 2− and, to a lesser extent, S3 − ). Thus, the process observed during annealing has the character of a phase transition, and both in the initial and in the final phase, the degree of sulfur oxidation (and, consequently, the unit cell parameter) can vary smoothly within relatively narrow limits. The observed abrupt increase in the unit cell parameter may indicate that the formation of the new phase proceeds by the nucleation mechanism. Partial oxidation of sulfur in the preheated sample at the initial stage of calcination leads to a broadening of the lines in the diffraction patterns, which may indicate the inhomogeneity of the composition and associated mechanical stresses in the crystal.

Thermal Conversions of Slyudyankaite
Slyudyankaite, ideally Na28Ca4(Si24Al24O96)(SO4)6(S6)1/3(CO2)·2H2O, is a triclinic soda- The initial phase with a smaller a parameter disappears five hours after the start of annealing. The final product of annealing is pale blue. Annealing of preheated singlecrystal grains of Sample 5 at 800 • C during 32 h results in the formation of a greenish blue cubic phase. Based on the above spectroscopic data, one can conclude that the enhancement of the unit cell parameter during annealing is caused by the transformation of small extraframework anions (S 2− and HS − ) into larger ones (SO 4 2− and, to a lesser extent, S 3 •− ). Thus, the process observed during annealing has the character of a phase transition, and both in the initial and in the final phase, the degree of sulfur oxidation (and, consequently, the unit cell parameter) can vary smoothly within relatively narrow limits.
The observed abrupt increase in the unit cell parameter may indicate that the formation of the new phase proceeds by the nucleation mechanism. Partial oxidation of sulfur in the preheated sample at the initial stage of calcination leads to a broadening of the lines in the diffraction patterns, which may indicate the inhomogeneity of the composition and associated mechanical stresses in the crystal.

Thermal Conversions of Slyudyankaite
Slyudyankaite, ideally Na 28 Ca 4 (Si 24 Al 24 O 96 )(SO 4 ) 6 (S 6 ) 1/3 (CO 2 )·2H 2 O, is a triclinic sodalite-group mineral containing sodalite cages of two kinds. Cages of the first type contain Na + and Ca 2+ cations and SO 4 2− anions as well as trace amounts of S 3 •− radical anions. Cages of the second type contain only neutral molecules (cyclic S 6 having chair-like conformation, CO 2 , H 2 O, and subordinate S 4 ).
The IR spectrum of slyudyankaite (curve a in Figure 19) contains bands of O-H stretching vibrations of hydrogen bonds of medium strength (3380 to 3610 cm −1 ), strong hydrogen bond or strong dipole-dipole interactions with CO 2 molecules (shoulder at 3240 cm −1 ), antisymmetric stretching vibrations of free 12 CO 2 molecules and 12 CO 2 molecules which are acceptors of strong hydrogen bonds (2341 and 2385 cm −1 , respectively), antisymmetric stretching vibrations of the 13 4 2− and extraframework molecules (below 550 cm −1 ). An indistinct shoulder at~465 cm −1 may be partly due to stretching vibrations of the cyclic S 6 molecule having D 3d symmetry [41].
Changes in the IR spectrum of slyudyankaite that occur as a result of its calcination are basically similar to those observed during calcination of the haüyne samples described above: the bands of O-H stretching vibrations, CO 2 , S 4, and S 6 molecules, and SO 4 2− anions disappear or become weaker. In the IR spectrum of Sample 6 preheated at 700 • C under reducing conditions, a weak band of HS − is observed at 2561 cm −1 . Both heated samples (preheated and annealed at 800 • C in air) contain bands of CO 3 2− anions in the range of 1390-1500 cm −1 . Unlike IR spectra of heated haüyne (Samples 2 and 5), spectral curves of both preheated and annealed at 800 • C, slyudyankaite samples have a weak shoulder at 1275 cm −1 , which may correspond to the BO 3 3− anion. If this assumption is correct, then it follows that initial slyudyankaite contained boron in the four-fold coordination, which could not be detected by means of IR spectroscopy because IR bands of the BO 4 tetrahedron overlap with bands of the aluminosilicate framework.
Raman spectra of slyudyankaite and products of its heating are presented in Figure 20. The assignment of Raman bands is given in Table 5. shoulder at ~465 cm −1 may be partly due to stretching vibrations of the cyclic S6 molecule having D3d symmetry [41]. Changes in the IR spectrum of slyudyankaite that occur as a result of its calcination are basically similar to those observed during calcination of the haüyne samples described above: the bands of O-H stretching vibrations, CO2, S4, and S6 molecules, and SO4 2− anions disappear or become weaker. In the IR spectrum of Sample 6 preheated at 700 C under reducing conditions, a weak band of HS − is observed at 2561 cm −1 . Both heated samples (preheated and annealed at 800 C in air) contain bands of CO3 2− anions in the range of 1390-1500 cm −1 . Unlike IR spectra of heated haüyne (Samples 2 and 5), spectral curves of both preheated and annealed at 800C, slyudyankaite samples have a weak shoulder at 1275 cm −1 , which may correspond to the BO3 3− anion. If this assumption is correct, then it follows that initial slyudyankaite contained boron in the four-fold coordination, which could not be detected by means of IR spectroscopy because IR bands of the BO4 tetrahedron overlap with bands of the aluminosilicate framework.
Raman spectra of slyudyankaite and products of its heating are presented in Figure  20. The assignment of Raman bands is given in Table 5.
The Raman spectrum of the initial slyudyankaite sample contains bands of SO 4 2− anions, S 3 •− radical anions, and different neutral molecules (cis-and trans-S 4 , S 6 , H 2 S, and CO 2 ). Preheating of slyudyankaite at 700 • C under reducing conditions results in the transformation of S-bearing species into HS − , S 2 •− , and S 4 •− and, possibly, monosulfide anion S 2− , which could not be detected by means of vibrational spectroscopy methods but is required for the charge balance. Note that S 2− was identified as the main extraframework anion in the crystal structure of the product of thermal conversions of lazurite under the same conditions [57]. Reduction of CO 2 occurring in initial slyudyankaite results in the formation of carbonate and acid oxalate anions.
Further annealing of preheated slyudyankaite at 800 • C in air results in the disappearance of HS − , S 2 •− , and S 4 •− , restoration of the SO 4 2− and S 3 •− anionic groups, and transformation of acid oxalate groups into neutral ones. Bands of S 3 •− in the Raman spectrum of the annealed slyudyankaite are shifted towards higher frequencies as compared to analogous bands of the initial sample, which may indicate their occurrence in a straitened state.

Sodalite-Sapozhnikovite Solid-Solution Series
Sodalite, ideally Na 8 (Al 6 Si 6 O 24 )Cl 2 , and sapozhnikovite, its hydrosulfide-dominant structural analogue with the formula of the hypothetical end-member Na 8 (Table 1; Figure 21). Samples with Cl > S (in atom proportions) belong to the mineral species sodalite, whereas samples with S > Cl belong to sapozhnikovite. Slight deficiency of the Cl + S sum in comparison with the ideal value 2.00 atoms per formula unit (Table 1; Figure 21) is caused by the presence of minor amounts of H 2 O in these minerals [39], probably together with Cl − and HS − anions in sodalite cages. The presence of water also explains slight deficiency of analytical totals in all electron-microprobe analyses of sodalite-sapozhnikovite series minerals (Table 1). In the interval [Cl1.11-0.38(HS)0.84-1.57], this homovalent isomorphous series is continuous ( Figure 21). We suggest this interval is caused by geochemical features rather crystal chemical constraints.

General Remarks on the Isomorphism of Extra-Framework Components in Sodalite-Group Minerals
The general common formula of cubic alkaline aluminosilicates belonging to the sodalite group is ABNa 6 (Si 6 Al 6 O 24 )XY, where the pairs of cationic and anionic extraframework components AB + XY are Na 2 + Cl 2 for sodalite, Na 2 +(HS) 2 for sapozhnikovite, K  (Figures 2 and 3). The nt distinctive feature of sapozhnikovite is a strong Raman band at 2553 cm −1 g to stretching vibrations of HS − anions. Bolotinaite is the member of this system most enriched in impurities (Table 1). In particular, it contains S2 − , cause of the yellow color of this mineral and its strong luminescence under Figure 2). IR/Vis/UV absorption spectrum of Sample 1, strong bands with the maxima 2.05, and 2.38 eV (respectively, 1900, 1400, 605, and 520 nm) as well as abth above 2.7 eV (460 nm) and a weak band at 1.33 eV (920 nm) are observed e ESR spectrum of Sample 1 contains a sextet of Mn 2+  •− )(SO 4 ) for lazurite. However, the application of a complex of spectroscopic methods in this work and elsewhere [3,4,64] shows that the crystal chemistry of subordinated components in cubic sodalite-group aluminosilicates is much more complex. In particular, H + , H 3 O + , CO 3 2− , OH − , S 2− , S 2 •− , S 3 , cisand trans-S 4 •− , SO 3 •2− , SO 4 •2− , cis-, transand gauche-S 4 , S 6 , CO 2 , COS, and HF were identified as extra-framework species in these minerals. Experiments with annealing show that these components take part in complex thermal conversions involving some additional components, e.g., oxalate anions. Similar transformations can occur in natural mineral assemblages. Thus, the composition of sodalite-group minerals may serve as a marker of mineral-forming media including fugacities of volatile components (H 2 O, O 2 , CO 2 , HF, SO 2 , and polysulfide compounds).
In general, similar processes were observed in sulfur-enriched fluids [61,62]. Apparently, when heated, the channel widths increase, which allows complex anion radicals to migrate and disproportionate with the formation of the most stable radical anion radicals (S 3 •− or S 2 •− , depending on the redox conditions). Along with this, dehydration of the studied samples also occurs, which also facilitates the migration of polysulfide species.

Sapozhnikovite as a Marker of Reducing Conditions
Experiments with heating of haüyne under reducing conditions (in the presence of Fe-FeS buffer) show that the main scheme of transformations of extra-framework components is SO 4 2− → S 2− + 2O 2 (gas), and a subordinate process is CO 2 + 2SO 4 •− , and 2CO 2 + 2e → C 2 O 4 2− (e = electron) are the additional channels of haüyne transformations during their heating at 700 • C under reducing conditions. Thus, HS − and C 2 O 4 2− anions are stable only under reducing conditions. Data on the occurrence of sapozhnikovite Na 8 (Al 6 Si 6 O 24 )(HS) 2 confirm this conclusion. Crystallization of aegirine NaFe 3+ Si 2 O 6 , which precedes formation of sapozhnikovite, could be accompanied by the following redox reactions: Na 2 O + 2FeO + 4SiO 2 + CO 2 → 2NaFeSi 2 O 6 + CO; Na 2 O + 2FeO + 4SiO 2 + 0.5CO 2 → 2NaFeSi 2 O 6 + 0.5C, where C is carbon with the oxidation degree of 0 [39]. During the crystallization of aegirine, iron occurring as Fe 2+ in the fluid is fixed as Fe 3+ in the solid phase. Such processes are possible only in peralkaline media with a high Na:Ca ratio (otherwise, hedenbergite CaFe 2+ Si 2 O 6 is formed instead of aegirine). Similar reactions, e.g., 2Na 2 O + 4FeO + 4SiO 2 + 2SO 3 + 2OH − + O 2 → 4NaFeSi 2 O 6 + 2HS − , could lead to the formation of reduced forms of sulfur, including HS − anions.
The redox processes in sodalite-group minerals discussed above involve species hosted by sodalite cages as well as gaseous O 2 . However, in the case of sapozhnikovite, an alternative mechanism including incorporation of HS − as a result of the exchange reaction of sodalite with a late reduced fluid is not excluded.
Unlike CO 2 molecules, carbonate anions rarely occur in sodalite-group minerals in significant amounts. An unusual CO 3 2− -bearing mineral with the sodalite-type framework, empirical formula (Na 5.28 K 0.40 Ca 1.23 Fe 0.01 )(Si 5.93 Al 6.07 O 24.01 )(SO 4 ) 0.77 Cl 0.94 (CO 3 ) 0.30 and unit cell parameter a = 9.0352 Å has been discovered in a metasomatic ejectum from Mount Vesuvio, Italy and its crystal structure has been solved [65]. Taking into account abovedescribed thermal transformations of CO 2 in haüyne and the fact that the CO 3 -bearing sodalite-group mineral from Vesuvio underwent heating during metasomatism, it is reasonable to suppose that CO 3 2− anions in this mineral were formed as a result of the transformation CO 2 + 2SO 4 2− + H 2 O → 2HS − + CO 3 2− + 4O 2 (gas) in an initial sodalitegroup mineral. In the experiments with heating of haüyne under reducing conditions, CO 2 molecules occurring in sodalite cages partly transformed into neutral or acid oxalate anions. In this regard, the association of sapozhnikovite with kyanoxalite (a cancrinite-group mineral containing neutral or acid oxalate anions) is indicative.
Experiments with annealing of sulfate sodalite-group minerals show that under hightemperature oxidizing conditions (in air, at 800 • C) S 3 •− is the most stable sulfide species. Annealing of the preheated samples in air at 800 • C results in partial reverse transformations: S 2 •− + S 2− + 2O 2 (gas) → SO 4 2− , S 4 •− + S 2 •− → 2S 3 •− , as well as subordinate processes S 3 •− + 5e +6O 2 (gas) → 3SO 4 2− and C 2 O 4 2− → 2CO 2 (gas) + 2e. The oxidation of S 3 •− into SO 4 2− can proceed only partly due to the charge-balance requirement. As noted above, S 3 •− is a very strong blue chromophore and even trace amounts of this radical anion are detectable by Raman spectroscopy. However, S 3 •− was not detected in sapozhnikovite. The possible cause of this fact is a high concentration of lithium in peralkaline rocks of the Lovozero massif (55 ppm, in average [66], whereas, e.g., in peralkaline rocks of the Khibiny massif, the mean Li content is only 20 ppm). Unlike other metal cations, Li + reacts with S 3 •− to form a cyclic radical anion LiS 3 • with strong covalent Li-S bonds [67], which results in the immobilization of S 3 •− . Kyanoxalite containing trace amounts of S 3 •− crystallized after sapozhnikovite at the hydrothermal zeolite stage with a lowered activity of lithium and containing trace amounts of S 3 •− [58].

Color Centers in Sodalite-group Minerals
Sodalite-group minerals show a wide range of colors. In particular, yellow, pink, violet, lilac, green, and blue samples are known. In most cases, coloration of sodalite-group minerals is caused by S-bearing extra-framework components. The S 3 •− radical anion is a very strong chromophore [45] which causes the deep blue color of lazurite [5]. Blue color of haüyne is usually due to trace amounts of S 3 •− [3,4,44,48]. The main yellow chromophore in sodalite-group minerals is the S 2 •− radical anion, which occurs in significant amounts in yellow varieties of bolotinaite (IMA 2021-088) earlier described as an "F-rich sodalite-group mineral" [4] as well as in above-described yellow products of heating of haüyne under reducing conditions.
Green chromophores are unknown among extra-framework components in sodalitegroup minerals. Green color of some varieties of haüyne and slyudyankaite appears as a result of the presence of both S 3 •− and a yellow chromophore (S 2 •− and/or S 6 ). However, S 6 is a rather weak chromophore [41].
Pink color of sodalite-group minerals (in particular, sodalite and tugtupite) may have different causes, including the presence of S 4 or S 4 •− [41,45], Cr 3+ or F-centers [68]. Lilac and violet tints of some haüyne samples from gem lazutite deposits (Sample 1 in this work; see [3,4]) is due to simultaneous presence of S 4 or S 4 •− and trace amounts of S 3 •− . Color coordinates in the CIE1931 color space chromaticity diagram for the samples studied in this work using NIR/Vis/UV spectroscopy are shown in Figure 22. Coordinates of coloration were calculated for daylight illuminant D with 4500 K temperature.
amounts of S3 − crystallized after sapozhnikovite at the hydrothermal zeolite stage with a lowered activity of lithium and containing trace amounts of S3 − [58].

Color Centers in Sodalite-group Minerals
Sodalite-group minerals show a wide range of colors. In particular, yellow, pink, violet, lilac, green, and blue samples are known. In most cases, coloration of sodalite-group minerals is caused by S-bearing extra-framework components. The S3 radical anion is a very strong chromophore [45] which causes the deep blue color of lazurite [5]. Blue color of haüyne is usually due to trace amounts of S3 − [3,4,44,48]. The main yellow chromophore in sodalite-group minerals is the S2 radical anion, which occurs in significant amounts in yellow varieties of bolotinaite (IMA 2021-088) earlier described as an "F-rich sodalite-group mineral" [4] as well as in above-described yellow products of heating of haüyne under reducing conditions.
Green chromophores are unknown among extra-framework components in sodalite-group minerals. Green color of some varieties of haüyne and slyudyankaite appears as a result of the presence of both S3 − and a yellow chromophore (S2 − and/or S6). However, S6 is a rather weak chromophore [41].
Pink color of sodalite-group minerals (in particular, sodalite and tugtupite) may have different causes, including the presence of S4 or S4 − [41,45], Cr 3+ or F-centers [68]. Lilac and violet tints of some haüyne samples from gem lazutite deposits (Sample 1 in this work; see [3,4]) is due to simultaneous presence of S4 or S4 − and trace amounts of S3 − .
Color coordinates in the CIE1931 color space chromaticity diagram for the samples studied in this work using NIR/Vis/UV spectroscopy are shown in Figure 22. Coordinates of coloration were calculated for daylight illuminant D with 4500 K temperature.
Lilac-blue color of Sample 1 may be partly due to the presence of two conformers of S4 that absorb mainly in the middle part of the visible range with band maximums shifted in opposite directions (towards blue and red ranges). However, the Raman spectrum of Sample 1 shows a series of characteristic bands of the S3 •− radical anion, which may be an additional cause of the blue tint.  Lilac-blue color of Sample 1 may be partly due to the presence of two conformers of S 4 that absorb mainly in the middle part of the visible range with band maximums shifted in opposite directions (towards blue and red ranges). However, the Raman spectrum of Sample 1 shows a series of characteristic bands of the S 3 •− radical anion, which may be an additional cause of the blue tint.
The color change of hackmanite and tugtupite upon irradiation with ultraviolet light from colorless or pale pink to purple is explained by the transfer of an electron from the impurity S 2 2− anions to the Cl vacancy, with the formation of color centers (F-centers). The absorption band of the F-centers is located at about 550 nm. The resulting S 2 •− radical anions practically do not affect the color due to their low concentration and relatively low oscillator strength. When irradiated with visible light, the F-centers disappear due to the phototransfer of an electron back to S 2 •− , and the crystals become colorless. This reversible color change is called tenebrescence or photochromism. It is noteworthy that the presence of significant amounts of potassium in the mineral leads to the disappearance of photochromism: in hackmanite and tugtupite, where sodium sharply predominates among the extraframework cations, photochromism is observed, unlike haüyne, despite hauine usually containing a fairly large number of S 2 •− centers. The causes of this are still being discussed. Perhaps this phenomenon is due to the fact that when sodium is replaced by potassium, either the value of the energy barrier for electron phototransfer from sulfur to a vacancy changes, or the excited state of the F-center enters the conduction band. Radiation coloring of haüyne does not change the visible color caused by F-centers. The presence of Fe 3+ and Ti 3+ impurities also affects the intensity of photochromic color and luminescence of S 2 •− radical anions [69].
Other possible radical anions in minerals of the sodalite group are SO 4 •− и SO 3 •− . The most intense absorption band of SO 4 •− is located at about 450 nm and the second band is at about 300 nm in Na 2 S 2 O 8 [70]. It was noted in [71] that there is an electron transfer between Fe 2+ и SO 4 •− with the formation of Fe 3+ и SO 4 2− . Thus, the presence of SO 4 •− in the mineral can lead to the loss of the blue color. The radical anion SO 3 •− absorbs in the region of about 280 nm and does not affect the color [70].

Conclusions
Application of a complex of spectroscopic methods to sodalite-group minerals and products of their thermal conversions made it possible to identify various S-and C-bearing extra-framework components, including SO 4  2− (e = electron). Subsequent annealing in air at 800 • C results in the partial reverse transformations S 2 •− + S 2− + 2O 2 (gas) → SO 4 2− , S 4 •− + S 2 •− → 2S 3 •− , S 3 •− + 5e +6O 2 (gas) → 3SO 4 2− , and C 2 O 4 2− → 2CO 2 (gas) + 2e. Based on these data, it can be concluded that natural HS − -and CO 2 0 -bearing sodalitegroup minerals can be formed under reducing conditions and as a result of thermal transformation of an initial mineral, respectively. In this respect, the occurrence of the HS −dominant mineral sapozhnikovite in association with the oxalate-rich cancrinite-type mineral kyanoxalite in agpaitic rocks [58] and the occurrence of a CO 3 2− -bearing sodalite-group mineral in a metasomatic volcanic ejectum [65] are indicative.
Normally, the oxalate group is unstable at temperatures higher than 500 • C. However, it was shown that the occurrence of oxalate anions in wide channels of cancrinite-group minerals is a stabilizing factor. In particular, oxalate anions in kyanoxalite decompose at 700 • C [72]. New data obtained in this work show that oxalate groups in sodalite cages are relatively stable up to 700 • C and decompose at 800 • C. Earlier we have shown that similar stabilization of nitrate anion takes place in cancrinite channels [73].
Color variations of sodalite-group minerals are mainly caused by the presence of S-bearing chromophores, S 3 •− (blue), S 2 •− and/or S 6 (yellow), S 4 •− and/or S 4 (red). Green color is due to the simultaneous presence of blue and yellow chromophores. Funding: All spectroscopic studies, as well as full investigation of the sodalite-sapozhnikovite series of minerals from the Lovozero massif, were supported by the Russian Science Foundation, grant No. 22-17-00006, https://rscf.ru/project/22-17-00006/. Collecting of sodalite-group minerals and identification and chemical analyses of minerals from the Malo-Bystrinskoe deposit were carried out in accordance with the state task, state registration No. AAA-A19-119092390076-7.
Data Availability Statement: Not applicable.

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