Crystal Chemistry, Isomorphism, and Thermal

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1. 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 Å³; 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 group and include different cations (Na⁺, [N(CH₃)₄]⁺, Ca²⁺, Mn²⁺, Fe²⁺, and Zn²⁺), anions (Cl⁻, F⁻, OH⁻, HS⁻, S²⁻, and SO₄²⁻), radical anions (S₃^●−), and neutral molecules (H₂O, CO₂, S₆) as species-defining components. Some subordinate, admixed components (K⁺, H⁺, S₂^●−, S₄^●−, SO₄^●2−, S₃, S₄, MoO₄²⁻, WO₄²⁻, AsO₄³⁻, and COS) were identified in sodalite-group minerals using a multianalytical approach involving different spectroscopic methods [3,4,5,6].

To date, many dozens of chemically different microporous compounds with the SOD-type framework have been synthesized. Interest in such compounds is caused by their specific properties, which make it possible to consider them as advanced materials with technologically important properties. The hydrothermal method was applied to obtain sodalite-type compounds with aluminosilicate frameworks as well as frameworks containing W-, Mo-, Ga-, Be-, Ge-, P-, or As-centered tetrahedra, with a wide variety of extra-framework cations and anions [7]. Microporous compounds belonging to the topological type of sodalite are advanced materials that can be used as pigments, sorbents for water purification and gas sorption, matrices for immobilization of radioactive isotopes and heavy metals, hydrogen and methane storage, superconductors, catalysts, membranes for separation of gases and desalination of seawater, etc. [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37].

This paper provides new data on the isomorphism of extra-framework components in some sodalite-group minerals as well as mechanisms for their thermal conversions.

2. Materials and Methods

The studied samples are listed below. Their empirical formulae are partly taken from literature sources. Chemical data for newly analyzed samples are given in Table 1. Samples 1–6 and 9 were collected at the Malo-Bystrinskoe gem lazurite deposit, Baikal Lake area, Siberia, Russia.

Sample 1 is bluish lilac S₄-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 CO₂ and HS⁻ were estimated from the IR spectrum; for the identification of H₂S and S_4, see below): Na_6.24Ca_1.49K_0.05(Si_5.99Al_6.01O₂₄)(SO₄)_1.50(H₂S)_0.075(S₄)_0.09Cl_0.10F_0.24(CO₂)_0.16·nH₂O (Z = 1).

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_6.28Ca_1.47K_0.17(Si_6.05Al_5.92Fe_0.03O₂₄)(SO₄²⁻)_1.68S⁰_0.40Cl_0.06(CO₂)_0.06·nH₂O, where S⁰ 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_6.39K_0.06Ca_1.57(Si_6.08Al_5.92O₂₄)(SO₄)_1.78(S₄)_0.03(S²⁻)_0.02Cl_0.07(CO₂)_0.15·nH₂O (Z = 1) [4]. The mineral is cubic, with the unit cell parameter a = 9.076 Å.

Sample 4 is light blue SO₃^●⁻-bearing haüyne with the empirical formula Na_6.45K_0.03Ca_1.35(Si_6.07Al_5.93O₂₄)(SO₄)_1.35(SO₃^●⁻)_1.35S₂^●⁻_0.02Cl_0.16(CO₂)_0.02·nH₂O (Z = 1) [4]. The unit cell parameter is a = 9.067 Å. The mineral forms grains up to 5 mm in calciphyre, in association with pyrite.

Sample 5 is bright blue haüyne with the empirical formula Na_6.45K_0.01Ca_1.36(Si_6.06Al_5.94O₂₄)(SO₄)_1.58S⁰_0.43Cl_0.09(CO₂)_0.02·nH₂O, where S⁰ is all sulfide sulfur provided that it occurs in neutral molecules. The unit cell parameter is a = 9.071 Å. The mineral forms grains in coarse-grained calciphyre consisting mainly of calcite, with subordinate diopside.

Sample 6 is the holotype specimen of slyudyankaite Na₂₈Ca₄(Si₂₄Al₂₄O₉₆)(SO₄)₆(S₆)_1/3(CO₂)·2H₂O [38]. The mineral is triclinic, space group: P1, a = 9.0523 (4) Å, b = 12.8806 (6) Å, c = 25.681 (1) Å, α = 89.988(2)°, β = 90.052(1)°, γ = 90.221(1)°, and V = 2994.4 (2) Å³. The empirical formula is Na_27.57Ca_4.05K_0.11(Si_24.52Al_23.48O₉₆)(SO₄)_6.06S⁰_2.42Cl_0.12(CO₂)_1.43·2.21H₂O (Z = 1), where S⁰_2.42 is total sulfide sulfur, mainly occurring as S₆ and subordinate S₄ neutral molecules, according to the structural data. The color of slyudyankaite is nonuniform: from blue-green in the major part of grains to pink and yellow in thin peripheral zones. The mineral forms isolated anhedral equant grains up to 0.5 cm across. The associated minerals are diopside, calcite, fluorapatite, phlogopite, lazurite, and pyrite.

Sample 7 is gray nosean forming crystals up to 0.5 cm across and elongate twins on (111) up to 1 cm long in cavities of nosean sanidinite. The sample originates from the In den Dellen (Zieglowski) pumice quarry, 1.5 km NE of Mendig, Laach Lake (Laacher See) paleovolcano, Eifel region, Rhineland-Palatinate, Germany. The associated minerals are sanidine, augite, annite, and zircon. The empirical formula of Sample 7 is [3]: (H₃O)_xNa_5.72K_0.64Ca_0.33(Si_6.43Al_5.51Fe_0.06O₂₄)(SO₄)_1.21Cl_0.11F_0.09(CO₂)_0.11·nH₂O (Z = 1).

Sample 8 is the holotype specimen of sapozhnikovite Na₈(Al₆Si₆O₂₄)(HS)₂ from a hydrothermally altered urtite-like rock at Karnasurt Mountain, Lovozero alkaline massif, Kola Peninsula, Russia [39]. The mineral forms isolated colourless to pale grayish anhedral equant grains up to 0.5 cm across. The empirical formula is Na_7.73Al_6.08Si_5.97O₂₄(HS)_1.25Cl_0.60·0.16H₂O (Z = 1). Sapozhnikovite is cubic, P $\bar{4}$ 3n, with a = 8.91462(7) Å.

Sample 9 is the neotype specimen of lazurite with the empirical formula (Na_6.97Ca_0.88K_0.10)_7.96(Si_6.04Al_5.96O₂₄)(SO₄)_1.09(S₃^●⁻)_0.55S²⁻_0.05Cl_0.04·0.72H₂O [5]. The associated minerals are calcite and diopside as well as accessory dolomite, forsterite, and phlogopite. The crystal structure of Sample 9 is characterized by commensurate and incommensurate modulations; the a parameter of the cubic sub-cell is equal to 9.087(3) Å.

Sample 10 is the holotype specimen of the recently IMA-approved new F-dominant sodalite-group mineral bolotinaite (IMA 2021-088). The mineral occurs as isolated yellow elongate interpenetration twins up to 1 mm long in cavities of a volcanic ejectum of trachitoid sanidinite collected in the In den Dellen pumice quarry, Eifel region, Rhineland-Palatinate, Germany. The associated minerals are sanidine, nepheline, annite, and zircon. The empirical formula of Sample 10 is (Na_5.92K_0.82Ca_0.10H_0.08)(Si_6.33Al_5.67O₂₄)(SO₄)_0.17F_0.84Cl_0.16(CO₂)_0.38·3.96H₂O.

Sample 11 is blue sodalite forming grains up to 3 cm across in a cancrinite-sodalite pegmatite uncovered at the western slope of Mount Kobelikha, Vishnevye Gory, Vishnevogorskiy syenite-miaskite complex, South Urals, Russia. The associated minerals are potassic feldspar, cancrinite, annite, muscovite, different zeolites, as well as accessory zircon, thorite, ilmenite, columbite-(Fe), Nb-bearing rutile, a pyrochlore-group mineral, various sulfides, etc. The composition of Sample 10 is close to the end-member formula Na₈(Si₆Al₆O₂₄)Cl₂ [3].

Sample 12 was collected from the same area as sapozhnikovite (Sample 8), north slope of Karnasurt Mountain located in the northern part of the Lovozero alkaline massif. This sample includes several similar but not identical to each other pieces of hydrothermally altered highly agpaitic urtite-like rocks containing rock-forming minerals of the sodalite–sapozhnikovite solid-solution series. Their share in a rock varies from 5 to 20 vol.%. Other rock-forming minerals are nepheline (the major constituent: >60 vol.%), aegirine, albite, potassic feldspar, natrolite, and kyanoxalite in different proportions; accessory minerals are murmanite (sometimes with lomonosovite relics), fluorapatite, and loparite-(Ce). Sodalite–sapozhnikovite series minerals form white or colourless, with strong orange fluorescence in longwave UV light (λ = 330 nm), grains up to 1 cm across. The representative chemical compositions of these minerals are given in Table 1.

Chemical analyses of haüyne Samples 1 and 2 (Table 1) were carried out using a Tescan VEGA-II XMU INCA Energy 450 (TESCAN, Brno, Czech Republic) (EDS mode, 20 kV, 120–150 pA, beam size 120 nm, excitation zone < 5 μm). The following standards were used: CaF₂ for F, albite for Na, synthetic Al₂O₃ for Al, wollastonite for Ca, potassium feldspar for K, SiO₂ for Si, Fe metal for Fe, and FeS₂ for S. Chemical analyses of sodalite–sapozhnikovite series minerals (Sample 12, Table 1) were carried out 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, Geological Faculty, Moscow State University), with an acceleration voltage of 20 kV, a beam current of 10 nA, and a 10 μm beam diameter. The standards used are as follows: jadeite for Na, Al and Si, FeS₂ for Fe, ZnS for S, and NaCl for Cl. Contents of other elements with atomic numbers >6 are below detection limits.

Additionally, chemical compositions of 55 samples of minerals belonging to the sodalite–sapozhnikovite solution series were investigated.

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.

3. Results

3.1. 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 Figure 1, Figure 2 and Figure 3.

The assignment of bands in the Raman spectra of cubic sodalite-group minerals shown in Figure 1, Figure 2 and Figure 3 (Table 2) was made in accordance with [3,4,38,39,40,41,42,43,44,45,46,47,48,49,50].

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 S₃^●− 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 S₃^●−.

Analogous bands of the trisulfide radical anion are present in the spectrum of bluish lilac haüyne (Sample 1), which contains trace amounts of S₃^●− (curve a in Figure 1). Additionally, in the Raman spectrum of Sample 1, bands of SO₄²⁻ groups are observed at 987 cm⁻¹ [symmetric stretching vibrations, A₁(ν₁) mode], 616 cm⁻¹ [bending vibrations, F₂(ν₄) mode], and 442 cm⁻¹ [bending vibrations, the E(ν₂) mode]. A series of distinct Raman bands (at 327, 650, and 682 cm⁻¹) corresponding to S₄ molecules having cis-, gahche- and trans-conformations, respectively, as well as bands of carbonate groups (at 1953 and 1460 cm⁻¹), H₂S molecules (at 2580 cm⁻¹), and hydronium cations H₃O⁺ (at 3005 and 3280 cm⁻¹) (Table 2) are the specific feature of Sample 1 distinguishing it from all other haüyne samples studied in this work and elsewhere [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).

Ideally, sodalite, bolotinaite and sapozhnikovite are the end-members of the solid-solution system with the general formula [Na₇(▯,Na)](Si₆Al₆O₂₄){Cl,HS,[F_0.5(H₂O)₂]}₂. However, natural samples of these minerals contain various extra-framework impurities (K, H₃O⁺, SO₄²⁻, S₂^●−, CO₂, S₄, HF, etc.) The Raman spectrum of sapozhnikovite in the range of 200–2500 cm⁻¹ is very close to the spectrum of typical sodalite (Figure 2 and Figure 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 S₂^●−, 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 Mn²⁺ 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, Mn²⁺ 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 hue can be associated with the presence of S₄ molecules (red chromophore). In the samples under study, the ESR signal characteristic of S₃^●− (with g = 2.036) is practically not observed, which indicates a very low concentration of S₃^●− radical anions in Sample 1. The absorption spectrum of S₃^●− 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 S₃^●− of lazurite [4,5]. Thus, the absorption band at 2.05 eV is not related to S₃^●− 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 trans- and cis-conformers of the neutral S₄ 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 S₂^●− radical anions [4].

3.2. Crystal Chemistry, Isomorphism, and Thermal Conversions of Haüyne

The IR spectra of initial and heated haüyne (Samples 2 and 5) are given in Figure 6, Figure 7 and Figure 8. The spectra of initial samples contain bands of O–H stretching and bending vibrations of H₂O molecules (in the ranges of 3300–3700 and 1640–1670 cm⁻¹, respectively), antisymmetric vibrations of the CO₂ molecules (at 2341 cm⁻¹), asymmetric stretching [F₂(ν₃) mode 1135–1137 cm⁻¹] and bending [F₂(ν₄) mode, 616 cm⁻¹] vibrations of the SO₄ tetrahedra, as well as stretching (1002–1004 cm⁻¹), O–(Si,Al)–O (650–730 cm⁻¹), and Si–O–Al (in the range of 390–450 cm⁻¹) modes of the aluminosilicate framework.

Heating results in the lowering of intensities of the bands of H₂O and disappearance of the bands of SO₄²⁻ 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 aluminosilicate framework.

It is worth noting that bands of the isolated S²⁻ anion are absent in the middle IR range, but the transformation of SO₄²⁻ into S²⁻ 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 S²⁻ 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-S₄ 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 CO₃²⁻ (Figure 1, Figure 2 and Figure 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 Figure 9 and Figure 10.

The assignment of bands in the Raman spectra of Samples 2 and 5 and products of their thermal conversions made is given in Table 3 and Table 4. The Raman spectra confirm the presence of HS⁻ and CO₃²⁻ 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 C₂O₄²⁻ and monosulfide S²⁻ anions, S₂^●− and S₄^●− radical anions, and S₄ 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 S₃^●− radical anions.

In the original Sample 3, no S₃^●− centers were observed before heating. Two absorption bands with maxima at 525 and 585 nm are associated with S₄^●− centers in different configurations, and a weak absorption band at 400 nm corresponds to S₂^●− 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 S₂^●− radical anions.

In the ESR spectrum, in addition to six lines associated with Mn²⁺ centers in calcite inclusions, there is a signal with g-tensor components of 2.034 and 2.021, associated with S₄^●− 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 S₄^●−—centers disappear.

The intensities of the absorption band and luminescence associated with S₂^●− centers increase in this case. At temperatures above 700 °C, a signal with g₁ = 2.056, g₂ = 2.041, and g₃ = 2.008, associated with S₃^●− 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 S₃^●− radical anion [4]. In this case, the absorption band in the region of 400 nm and the luminescence associated with the S₂^●− 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).

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 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 (S²⁻ and HS⁻) into larger ones (SO₄²⁻ 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.

3.3. Thermal Conversions of Slyudyankaite

Slyudyankaite, ideally Na₂₈Ca₄(Si₂₄Al₂₄O₉₆)(SO₄)₆(S₆)_1/3(CO₂)·2H₂O, is a triclinic sodalite-group mineral containing sodalite cages of two kinds. Cages of the first type contain Na⁺ and Ca²⁺ cations and SO₄²⁻ anions as well as trace amounts of S₃^●− radical anions. Cages of the second type contain only neutral molecules (cyclic S₆ having chair-like conformation, CO₂, H₂O, and subordinate S₄).

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 CO₂ molecules (shoulder at 3240 cm⁻¹), antisymmetric stretching vibrations of free ¹²CO₂ molecules and ¹²CO₂ molecules which are acceptors of strong hydrogen bonds (2341 and 2385 cm⁻¹, respectively), antisymmetric stretching vibrations of the ¹³CO₂ molecules (2275 cm⁻¹), C–O stretching vibrations of trace admixture of O=C=S molecules (a very weak band at 2040 cm⁻¹), bending vibrations of H₂O molecules (at 1632 cm⁻¹ with an indistinct shoulder at ~1680 cm⁻¹ which may correspond to H₂O molecules forming strong hydrogen bonds), asymmetric stretching vibrations of the SO₄²⁻ anionic groups [the degenerate F₂(ν₃) mode] (1138 and 1107 cm⁻¹), stretching vibrations of the aluminosilicate framework (1002 cm⁻¹), mixed vibrations of the aluminosilicate framework (in the range of 650–720 cm⁻¹), stretching vibrations of the neutral S₄ molecule having cis conformation (641 cm⁻¹), bending vibrations of the SO₄²⁻ anionic groups [the degenerate F₂(ν₄) mode] (614 cm⁻¹), and lattice modes involving bending vibrations of the aluminosilicate framework and librations of SO₄²⁻ and extra-framework molecules (below 550 cm⁻¹). An indistinct shoulder at ~465 cm⁻¹ may be partly due to stretching vibrations of the cyclic S₆ 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, CO₂, S_4, and S₆ molecules, and SO₄²⁻ 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 CO₃²⁻ 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 BO₃³⁻ 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 SO₄²⁻ anions, S₃^●⁻ radical anions, and different neutral molecules (cis- and trans-S₄, S₆, H₂S, and CO₂). Preheating of slyudyankaite at 700 °C under reducing conditions results in the transformation of S-bearing species into HS⁻, S₂^●⁻, and S₄^●⁻ and, possibly, monosulfide anion S²⁻, which could not be detected by means of vibrational spectroscopy methods but is required for the charge balance. Note that S²⁻ was identified as the main extra-framework anion in the crystal structure of the product of thermal conversions of lazurite under the same conditions [57]. Reduction of CO₂ 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⁻, S₂^●−, and S₄^●−, restoration of the SO₄²⁻ and S₃^●− anionic groups, and transformation of acid oxalate groups into neutral ones. Bands of S₃^●− 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.

3.4. Sodalite—Sapozhnikovite Solid-Solution Series

Sodalite, ideally Na₈(Al₆Si₆O₂₄)Cl₂, and sapozhnikovite, its hydrosulfide-dominant structural analogue with the formula of the hypothetical end-member Na₈(Al₆Si₆O₂₄)(HS)₂ form a solid-solution (isomorphous) series in which the Cl⁻:HS⁻ (simply, Cl:S) ratio is the only significant variable. In the studied specimens from Karnasurt Mt. (Lovozero alkaline massif), this ratio varies from [Cl_1.11(HS)_0.84]_Σ1.95 to [(HS)_1.57Cl_0.38]_Σ1.95 (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 H₂O 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 [Cl_1.11-0.38(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.

4. Discussion

4.1. 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 ABNa₆(Si₆Al₆O₂₄)XY, where the pairs of cationic and anionic extra-framework components AB + XY are Na₂ + Cl₂ for sodalite, Na₂+(HS)₂ for sapozhnikovite, K▯+F(H₂O)₄ for bolotinaite, Na₂+(SO₄)(H₂O) for nosean, Ca₂+(SO₄)₂ for haüyne, and NaCa+(S₃^●−)(SO₄) 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⁺, H₃O⁺, CO₃²⁻, OH⁻, S²⁻, S₂^●−, S₃, cis- and trans-S₄^●−, SO₃^●2−, SO₄^●2−, cis-, trans- and gauche-S₄, S₆, CO₂, 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 (H₂O, O₂, CO₂, HF, SO₂, 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 (S₃^●− or S₂^●−, depending on the redox conditions). Along with this, dehydration of the studied samples also occurs, which also facilitates the migration of polysulfide species.

4.2. 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 SO₄²⁻ → S²⁻ + 2O₂(gas), and a subordinate process is CO₂ + 2SO₄²⁻ + H₂O → 2HS⁻ + CO₃²⁻ + 4O₂(gas). The conversions 3SO₄²⁻ → S₃^●− +5e +6O₂(gas), 2S₃^●− → S₂^●− + S₄^●−, and 2CO₂ + 2e → C₂O₄²⁻ (e = electron) are the additional channels of haüyne transformations during their heating at 700 °C under reducing conditions.

Thus, HS⁻ and C₂O₄²⁻ anions are stable only under reducing conditions. Data on the occurrence of sapozhnikovite Na₈(Al₆Si₆O₂₄)(HS)₂ confirm this conclusion. Crystallization of aegirine NaFe³⁺Si₂O₆, which precedes formation of sapozhnikovite, could be accompanied by the following redox reactions: Na₂O + 2FeO + 4SiO₂ + CO₂ → 2NaFeSi₂O₆ + CO; Na₂O + 2FeO + 4SiO₂ + 0.5CO₂ → 2NaFeSi₂O₆ + 0.5C, where C is carbon with the oxidation degree of 0 [39]. During the crystallization of aegirine, iron occurring as Fe²⁺ in the fluid is fixed as Fe³⁺ in the solid phase. Such processes are possible only in peralkaline media with a high Na:Ca ratio (otherwise, hedenbergite CaFe²⁺Si₂O₆ is formed instead of aegirine). Similar reactions, e.g., 2Na₂O + 4FeO + 4SiO₂ + 2SO₃ + 2OH⁻ + O₂ → 4NaFeSi₂O₆ + 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 O₂. 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 CO₂ molecules, carbonate anions rarely occur in sodalite-group minerals in significant amounts. An unusual CO₃²⁻-bearing mineral with the sodalite-type framework, empirical formula (Na_5.28K_0.40Ca_1.23Fe_0.01)(Si_5.93Al_6.07O_24.01)(SO₄)_0.77Cl_0.94(CO₃)_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 above-described thermal transformations of CO₂ in haüyne and the fact that the CO₃-bearing sodalite-group mineral from Vesuvio underwent heating during metasomatism, it is reasonable to suppose that CO₃²⁻ anions in this mineral were formed as a result of the transformation CO₂ + 2SO₄²⁻ + H₂O → 2HS⁻ + CO₃²⁻ + 4O₂(gas) in an initial sodalite-group mineral. In the experiments with heating of haüyne under reducing conditions, CO₂ 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 high-temperature oxidizing conditions (in air, at 800 °C) S₃^●− is the most stable sulfide species. Annealing of the preheated samples in air at 800 °C results in partial reverse transformations: S₂^●− + S²⁻ + 2O₂(gas) → SO₄²⁻, S₄^●− + S₂^●− → 2S₃^●−, as well as subordinate processes S₃^●− + 5e +6O₂(gas) → 3SO₄²⁻ and C₂O₄²⁻ → 2CO₂(gas) + 2e. The oxidation of S₃^●− into SO₄²⁻ can proceed only partly due to the charge-balance requirement.

As noted above, S₃^●− is a very strong blue chromophore and even trace amounts of this radical anion are detectable by Raman spectroscopy. However, S₃^●− 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 S₃^●− to form a cyclic radical anion LiS₃^● with strong covalent Li–S bonds [67], which results in the immobilization of S₃^●−. Kyanoxalite containing trace amounts of S₃^●− crystallized after sapozhnikovite at the hydrothermal zeolite stage with a lowered activity of lithium and containing trace amounts of S₃^●− [58].

4.3. 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], Cr³⁺ 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.

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 S₂²⁻ 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 S₂^●− 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 S₂^●−, 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 S₂^●− 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 Fe³⁺ and Ti³⁺ impurities also affects the intensity of photochromic color and luminescence of S₂^●− radical anions [69].

Other possible radical anions in minerals of the sodalite group are SO₄^●− и SO₃^●−. The most intense absorption band of SO₄^●− is located at about 450 nm and the second band is at about 300 nm in Na₂S₂O₈ [70]. It was noted in [71] that there is an electron transfer between Fe²⁺ и SO₄^●− with the formation of Fe³⁺ и SO₄²⁻. Thus, the presence of SO₄^●− in the mineral can lead to the loss of the blue color. The radical anion SO₃^●− absorbs in the region of about 280 nm and does not affect the color [70].

5. 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 SO₄²⁻, HS⁻, S²⁻, CO₃²⁻, C₂O₄²⁻, HC₂O₄⁻, Cl⁻, and F⁻ anions S₂^●−, S₃^●−, S₄^●−, and S₆^●− radical anions as well as CO₂, H₂O, and S₄ neutral molecules. The composition of extra-framework species in sodalite-group minerals may serve as a marker of mineral-forming media including fugacities of volatile components (H₂O, O₂, CO₂, HF, SO₂, and polysulfide compounds). Complex mutual transformations of these components are observed during heating at high temperatures. The scheme of transformations of extra-framework components in SO₄²⁻-bearing members of the sodalite group during their heating at 700 °C under reducing conditions includes the elementary processes SO₄²⁻ → S^2– + 2O₂(gas), CO₂ + 2SO₄²⁻ + H₂O → 2HS⁻ + CO₃²⁻ + 4O₂(gas). 3SO₄²⁻ → S₃^●− +5e +6O₂(gas), 2S₃^●− → S₂^●− + S₄^●−, and 2CO₂ + 2e → C₂O₄²⁻ (e = electron). Subsequent annealing in air at 800 °C results in the partial reverse transformations S₂^●− + S²⁻ + 2O₂(gas) → SO₄²⁻, S₄^●− + S₂^●− → 2S₃^●−, S₃^●− + 5e +6O₂(gas) → 3SO₄²⁻, and C₂O₄²⁻ → 2CO₂(gas) + 2e.

Based on these data, it can be concluded that natural HS⁻- and CO₂⁰-bearing sodalite-group 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 CO₃²⁻-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, S₃^●− (blue), S₂^●− and/or S₆ (yellow), S₄^●− and/or S₄ (red). Green color is due to the simultaneous presence of blue and yellow chromophores.

Author Contributions

Conceptualization, N.V.C., R.Y.S. and I.V.P.; methodology, N.V.C., R.Y.S., M.F.V., V.D.S. and D.A.V.; collecting of minerals, A.N.S.; investigation, R.Y.S., M.F.V., V.D.S., I.V.P. and D.A.V.; original manuscript draft preparation, N.V.C., A.N.S. and R.Y.S.; manuscript review and editing, R.Y.S., N.V.C., I.V.P. and D.A.V.; figures, N.V.C., R.Y.S. and V.D.S. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. Raman spectra of (a) S4-bearing haüyne (Sample 1) and (b) lazurite (Sample 9).

Figure 2. Raman spectra of (a) bolotinaite (Sample 10) and (b) sodalite (Sample 11).

Figure 3. Raman spectra of (a) sapozhnikovite (Sample 8) and (b) nosean (Sample 7).

View Image - Figure 4. Absorption spectrum of haüyne (Sample 1, dotted curve). The black and red solid lines show the deconvolution of the doublet in the region of 2.0–2.5 eV into two peaks, and the total decomposition curve is shown with green color.

Figure 4. Absorption spectrum of haüyne (Sample 1, dotted curve). The black and red solid lines show the deconvolution of the doublet in the region of 2.0–2.5 eV into two peaks, and the total decomposition curve is shown with green color.

Figure 5. ESR spectrum of haüyne (Sample 1) measured at room temperature.

View Image - Figure 6. Infrared spectra of (a) initial haüyne (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.

Figure 6. Infrared spectra of (a) initial haüyne (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.

View Image - Figure 7. Infrared spectra of (a) initial haüyne (Sample 5), (b) Sample 5 preheated for three days at 700 °C over the Fe-FeS buffer, and (c) preheated Sample 5 additionally annealed at 800 °C in air for one day.

Figure 7. Infrared spectra of (a) initial haüyne (Sample 5), (b) Sample 5 preheated for three days at 700 °C over the Fe-FeS buffer, and (c) preheated Sample 5 additionally annealed at 800 °C in air for one day.

View Image - Figure 8. Infrared spectra in the 1350–3600 region of (a) haüyne (Sample 5) preheated for three days at 700 °C over the Fe-FeS buffer, and (b) preheated Sample 5 additionally annealed at 800 °C in air for one day.

Figure 8. Infrared spectra in the 1350–3600 region of (a) haüyne (Sample 5) preheated for three days at 700 °C over the Fe-FeS buffer, and (b) preheated Sample 5 additionally annealed at 800 °C in air for one day.

View Image - Figure 9. 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.

Figure 9. 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.

View Image - Figure 10. Raman spectra of (a) initial haüyne (Sample 5), (b) Sample 5 preheated for three days at 700 °C, over the Fe-FeS buffer, and (c) preheated Sample 5 additionally annealed at 800 °C in air for one day.

Figure 10. Raman spectra of (a) initial haüyne (Sample 5), (b) Sample 5 preheated for three days at 700 °C, over the Fe-FeS buffer, and (c) preheated Sample 5 additionally annealed at 800 °C in air for one day.

Figure 11. Spectra of diffuse absorption of haüyne (Sample 3): unheated sample (1) and samples heated at 200 °C (2), 400 °C (3), 600 °C (4), and 800 °C (5).

View Image - Figure 12. The ESR spectra of unheated haüyne (Sample 3) (1) and Sample 3 heated at a temperature of 800 °C (2). The dots indicate the bands of S4●− and the vertical lines show the bands of S3●−.

Figure 12. The ESR spectra of unheated haüyne (Sample 3) (1) and Sample 3 heated at a temperature of 800 °C (2). The dots indicate the bands of S4●− and the vertical lines show the bands of S3●−.

View Image - Figure 13. Relative amounts of various sulfur radical anions in haüyne (Sample 3) heated at different temperatures. The values of the relative amounts are normalized to the maximum number of each of the radical anions.

Figure 13. Relative amounts of various sulfur radical anions in haüyne (Sample 3) heated at different temperatures. The values of the relative amounts are normalized to the maximum number of each of the radical anions.

Figure 14. Diffuse absorption spectra of original haüyne (Sample 4) (1) and Sample 4 heated at 800 °C (2).

View Image - Figure 15. The ESR spectra of original haüyne (Sample 4) (1) and Sample 4 heated at 800 °C (2). The black triangles show the bands of the S6●− radical anions bands, and the vertical blue lines indicate the bands of S3●−.

Figure 15. The ESR spectra of original haüyne (Sample 4) (1) and Sample 4 heated at 800 °C (2). The black triangles show the bands of the S6●− radical anions bands, and the vertical blue lines indicate the bands of S3●−.

View Image - Figure 16. Relative amounts of polysulfide radical anions in haüyne (Sample 3) heated at different temperatures. The values of the relative amounts are normalized to the maximum number of each of the radical anions.

Figure 16. Relative amounts of polysulfide radical anions in haüyne (Sample 3) heated at different temperatures. The values of the relative amounts are normalized to the maximum number of each of the radical anions.

Figure 17. ESR spectra of initial haüyne (Sample 2) (1), and Sample 2 heated at 700 °C under buffer (2) and at 800 °C in air (3).

View Image - Figure 18. Kinetics of changes of the cubic unit cell parameter of the initial phase of the preheated haüyne (Sample 5, circles) and product of its annealing at 800 °C (squares).

Figure 18. Kinetics of changes of the cubic unit cell parameter of the initial phase of the preheated haüyne (Sample 5, circles) and product of its annealing at 800 °C (squares).

View Image - Figure 19. Infrared spectra of (a) initial slyudyankaite (Sample 6), (b) Sample 6 preheated for three days at 700 °C, over the Fe-FeS buffer, and (c) preheated Sample 6 additionally annealed at 800 °C in air for one day. The inset shows the IR spectrum of initial Sample 6 in the range of 1500–3800 cm−1.

Figure 19. Infrared spectra of (a) initial slyudyankaite (Sample 6), (b) Sample 6 preheated for three days at 700 °C, over the Fe-FeS buffer, and (c) preheated Sample 6 additionally annealed at 800 °C in air for one day. The inset shows the IR spectrum of initial Sample 6 in the range of 1500–3800 cm−1.

View Image - Figure 20. Raman spectra of (a) initial slyudyankaite (Sample 6), (b) Sample 6 preheated for three days at 700 °C, over the Fe-FeS buffer, and (c) preheated Sample 6 additionally annealed at 800 °C in air for one day. The inset shows the Raman spectrum of initial Sample 6 in the range of 1200–3750 cm−1.

Figure 20. Raman spectra of (a) initial slyudyankaite (Sample 6), (b) Sample 6 preheated for three days at 700 °C, over the Fe-FeS buffer, and (c) preheated Sample 6 additionally annealed at 800 °C in air for one day. The inset shows the Raman spectrum of initial Sample 6 in the range of 1200–3750 cm−1.

View Image - Figure 21. Variations of the contents of Cl and S atoms per formula unit (apfu) in minerals belonging to the sodalite–sapozhnikovite solid-solution series from Karnasurt Mt., Lovozero massif, Kola Peninsula, Russia.

Figure 21. Variations of the contents of Cl and S atoms per formula unit (apfu) in minerals belonging to the sodalite–sapozhnikovite solid-solution series from Karnasurt Mt., Lovozero massif, Kola Peninsula, Russia.

Figure 22. Color space chromaticity diagram for sodalite-group minerals.

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 sodalite–sapozhnikovite series minerals (Sample 12) from Karnasurt Mt., Lovozero massif, Kola Peninsula, Russia obtained in this work.

Sample No.			1			2
Color			Bluish lilac			Deep blue
Na₂O			18.04			17.92
K₂O			0.21			0.74
CaO			7.83			7.60
Al₂O₃			28.63			27.79
Fe₂O₃			0			0.19
SiO₂			33.65			33.46
CO₂ ^a			0.67			0.25
SO₃ ^b			14.58			15.33
F			0.42			0
Cl			0.32			0.31
–O=Cl,F			−0.25			−0.07
Total			104.10			103.52
Analysis No.	1	2	3	4	5	6	7	8
Contents, wt. %
Na₂O	23.60	24.20	24.60	24.28	24.49	24.34	24.01	24.21
Al₂O₃	30.84	31.50	31.37	31.38	31.69	31.29	31.16	31.33
Fe₂O₃	0.36	-	0.27	-	-	0.13	-	-
SiO₂	36.03	36.88	36.95	36.61	36.82	36.50	36.48	36.26
HS *	2.78	3.10	3.52	3.74	4.17	4.48	4.86	5.27
Cl	3.97	3.41	3.32	3.07	2.68	2.31	1.85	1.35
–O=(Cl,HS)	–1.57	–1.52	–1.60	–1.59	–1.61	–1.60	–1.60	–1.58
Total	96.01	97.57	98.43	97.49	98.24	97.45	96.76	96.84
Formula Calculated on the Basis of Al + Fe + Si = 12 Atoms Per Formula Unit
Na	7.56	7.61	7.72	7.68	7.68	7.71	7.56	7.70
Al	6.00	6.02	5.99	6.03	6.04	6.02	6.02	6.05
Fe	0.04	-	0.03	-	-	0.02	-	-
Si	5.96	5.98	5.98	5.97	5.96	5.96	5.98	5.95
S	0.84	0.91	1.03	1.11	1.22	1.33	1.45	1.57
Cl	1.11	1.06	0.91	0.85	0.73	0.64	0.51	0.38
S + Cl	1.95	1.97	1.94	1.96	1.95	1.97	1.96	1.95

^a CO₂ contents corresponding to CO₂ molecules were determined from the IR spectra using a procedure described in [3]; ^b All sulfur is given as SO₃, 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.

Table 2

Assignment of Raman bands of cubic sodalite-group minerals.

Raman Shift (cm⁻¹)						Assignment
Sample No.
1	7	8	9	10	11
202w	-	-	-	210	-	Combination of low-frequency lattice modes and/or trans-S₄ bending mode
255	-		257	-	-	S₃^●− bending mode (ν₂)
-	-	260	-	-	266	Bending vibrations of the [ClNa₄]³⁺ and [(HS)Na₄]³⁺ clusters
-	-	294	285w	283w	-	Combination of low–frequency lattice modes involving Na⁺ cations and/or S₆ bending mode
327	-	-	-	-	-	cis-S₄ mixed (bending + stretching) ν₄ mode
-	-	417w	-	-	410w	Bending vibrations of the aluminosilicate framework
442	439		-	441s	-	SO₄²⁻ [the E(ν₂) mode] and/or δ[O–Si(Al)–O] bending vibrations
-	-	459s	-	-	463s	Stretching vibrations of the [ClNa₄] and [(HS)Na₄] clusters
544s	-	-	546s	548w	-	S₃^●− symmetric stretching (ν₁) mode
582w	-	-	585	-	-	S₃^●− antisymmetric stretching (ν₃), possibly, overlapping with the stretching band of S₂^●−
-	-	-	-	580w, 605w	-	S₂^●− stretching mode
-	-	611w	-	-	611w	Overtone of vibrations involving Na⁺ cations?
616	621	-	-	-	-	SO₄²⁻ bending vibrations [F₂(ν₄) mode]
650	-	-	-	-	-	gauche-S₄ symmetric stretching A₁(ν₁) mode
682	-	-	-	673w	-	trans-S₄ symmetric stretching ν₃ mode
-	724w	732w	-	-	732w	Mixed vibrations of the aluminosilicate framework
987s	983s	978, 989	-	986s	970, 986	SO₄²⁻ symmetric stretching vibrations [A₁(ν₁) mode]
1053w	1049	1062	-	-	1060	Stretching vibrations of the framework and/or CO₃²⁻ symmetric stretching vibrations
-	-	-	-	1074	-	HF libration
1085	-	-	1093s	-	-	S₃^●− overtone (2 × ν₁)
-	1138	-	-	-	-	SO₄²⁻ asymmetric stretching vibrations [F₂(ν₃) mode], possibly, overlapping with S₂^●− overtone (2 × ν₁)
-	-	-	-	1271	-	CO₂ Fermi resonance
1332	-	-	-	-	-	Overtone of the cis-S₄ antisymmetric stretching mode (2 × ν₃)
-	-	-	-	1350	-	H⁺ translation
-	-	-	1363	-	-	S₃^●− combination mode (2ν₁ + ν₂)
-	-	-	-	1381	-	CO₂ Fermi resonance
1460w	-	-	-	-	-	CO₃ asymmetric stretching mode
1632	-	-	1638	-	-	S₃^●− overtone (3 × ν₁)
1895	-	-	-	-	-	S₃^●− combination mode (3 × ν₂ + ν₁)
2175	-	-	2181	-	-	S₃^●− overtone (4 × ν₁)
-	-	-	2440w	-	-	S₃^●− combination mode (4 × ν₂ + ν₁)
-	-	2553	-	-	-	HS⁻ stretching mode
2580	-	-	-	-	-	H₂S symmetric stretching mode
2725	-	-	2720	-	-	S₃^●− overtone (5 × ν₁)
3005, 3280	-	-	-	-	3130	H₃O⁺ stretching mode
3475, 3615	-	-	-	3540	-	H₂O stretching vibrations

Table 3

Assignment of Raman bands of Sample 2 and products of its thermal conversions.

Raman Shift (cm⁻¹)			Assignment
Initial Sample	Preheated Sample	Sample Heated at 800 °C in Air	Assignment
-	194w	168w	Combination of low-frequency lattice modes
-	213w	219	trans-S₄ bending mode
-	254	-	Bending vibrations of the [(HS)⁻Na₄]³⁺ cluster
260	-	260	S₃^●− bending mode (ν₂)
287w	290	-	Low–frequency lattice modes involving Na⁺ cations and/or S₄^●− bending vibrations
-	-	-	cis-S₄ mixed ν₄ mode (combined symmetric bending + stretching vibrations)
327w	355w	-	cis-S₄^●− mixed ν₃ mode
-	-	380	cis-S₄ mixed ν₃ mode
448w	-	-	SO₄²⁻ [E(ν₂) mode] and/or δ[O–Si(Al)–O] bending vibrations
-	454s	-	Stretching vibrations of the [(HS)⁻Na₄]³⁺ cluster
-	474	473	S₆ stretching mode and/or mixed ν₄ mode of trans–S₄
547s	-	546s	S₃^●− symmetric stretching (ν₁) mode
	555	-	S₃ neutral molecule and/or gauche-S₄
580	-	583	S₃^●− antisymmetric stretching (ν₃), possibly, overlapping with the stretching band of S₂^●−
-	590	-	Stretching vibrations of the [(S²⁻)Na₄]²⁺ cluster and/or S₂^●− stretching mode
-	733	-	O–C–O bending vibrations of oxalate anions
-	-	720	Mixed vibrations of the aluminosilicate framework
804	-	804	S₃^●− combination mode (ν₁ + ν₂)
-	845w	-	C–C stretching vibrations of oxalate anions
990	996	981w	SO₄²⁻ symmetric stretching vibrations [A₁(ν₁) mode] and/or framework stretching vibrations
-	1061	-	CO₃²⁻ symmetric stretching vibrations
1091s	-	1092s	S₃^●− overtone (2 × ν₁)
-	1335	-	Symmetric C–O stretching vibrations of oxalate anions
1353	-	1345	S₃^●− combination mode (2ν₁ + ν₂)
-	1613	-	Antisymmetric C–O stretching vibrations of oxalate anions
1639	-	1638	S₃^●− overtone (3 × ν₁)
1900w	-	1910w	S₃^●− combination mode (3 × ν₂ + ν₁)
2180	-	2185	S₃^●− overtone (4 × ν₁)
2443w	-	-	S₃^●− combination mode (4 × ν₂ + ν₁)
-	2553s	-	HS⁻ stretching mode
-	2581w	-	H₂S symmetric stretching mode
2721	-	2725	S₃^●− overtone (5 × ν₁)
2950w	-	-	S₃^●− combination mode (5 × ν₁ + ν₂)

Note: w—weak band, s—strong band.

Table 4

Assignment of Raman bands of Sample 5 and products of its thermal conversions.

Raman Shift (cm⁻¹)			Assignment
Initial Sample	Preheated Sample	Sample Heated at 800 °C in Air	Assignment
-	166	-	Combination of low-frequency lattice modes
-	219w	217w	trans-S₄ bending mode
-	260	-	Bending vibrations of the [(HS)⁻Na₄]³⁺ cluster
256	-	265	S₃^●− bending mode (ν₂)
282w	-	-	Low–frequency lattice modes involving Na⁺ cations and/or S₄^●− bending vibrations
-	308s	297s	S₄^●− bending vibrations and/or cis-S₄ mixed ν₄ mode (combined symmetric bending + stretching vibrations)
-	453s	-	Stretching vibrations of the [(HS)⁻Na₄]³⁺ cluster
-	471	-	S₆ stretching mode and/or mixed ν₄ mode of trans–S₄
543s	547	546s	S₃^●− symmetric stretching (ν₁) mode
570	-	587	S₃^●− antisymmetric stretching (ν₃), possibly, overlapping with the stretching band of S₂^●−
-	596	-	Stretching vibrations of the [(S²⁻)Na₄]²⁺ cluster and/or S₂^●− stretching mode
-	728	-	O–C–O bending vibrations of oxalate anions
798	-	805	S₃^●− combination mode (ν₁ + ν₂)
-	850w	-	C–C stretching vibrations of oxalate anions
983w	989	-	SO₄²⁻ symmetric stretching vibrations [A₁(ν₁) mode] and/or framework stretching vibrations
-	1064	-	CO₃²⁻ symmetric stretching vibrations
1087s	-	1097s	S₃^●− overtone (2 × ν₁)
-	1350	-	Symmetric C–O stretching vibrations of oxalate anions
1348	-	1365	S₃^●− combination mode (2ν₁ + ν₂)
-	1605	-	Antisymmetric C–O stretching vibrations of oxalate anions
1634	-	1643	S₃^●− overtone (3 × ν₁)
1894w	-	1900w	S₃^●− combination mode (3 × ν₂ + ν₁)
-	1909w	-	Overtone of SO₄²⁻ symmetric stretching vibrations?
2173	-	2185	S₃^●− overtone (4 × ν₁)
2434w	-	2430w	S₃^●− combination mode (4 × ν₂ + ν₁)
-	2556s	-	HS⁻ stretching mode
2713	-	2725	S₃^●− overtone (5 × ν₁)
2971w	-	-	S₃^●− combination mode (5 × ν₁ + ν₂)
3233w, 3470w	3232, 3605w	3220w	O–H stretching modes

Table 5

Assignment of Raman bands of slyudyankaite and products of its thermal conversions.

Raman Shift (cm⁻¹)			Assignment
InitialSample	Preheated Sample	Sample Heated at 800 °C in Air	Assignment
219	-	-	trans-S₄ bending
260	-	262	S₃^●⁻ bending A₂ (ν₂) and S₆ (with D_3d symmetry) bending
283	-	-	Framework bending vibrations (resonance with a S₆ bending mode?)
298	302s	-	S₄^●⁻ bending vibrations
330	-	-	cis-S₄ mixed ν₄ mode (symmetric bending + stretching)
380w	-	-	cis-S₄ mixed ν₃ mode
437	-	440w	SO₄ [bending E (ν₂) mode] and/or S₆ (mixed mode)
-	461s	-	[(HS)Na₄]³⁺ stretching vibrations
477	-	474	S₆ stretching mode and/or mixed ν₄ mode of trans-S₄
503	-	-	Bending vibrations of the framework
545s	-	548s	S₃^●⁻ symmetric stretching (ν₁) (possibly, overlapping with the stretching band of gauche-S₄)
580	-	586	S₃^●⁻ antisymmetric stretching mode (ν₃)
-	603	-	S₂^●⁻ stretching mode
614	-	-	SO₄^2– [bending F₂ (ν₄) mode]
645	-	-	cis-S₄ stretching
682	-	-	trans-S₄ symmetric stretching ν₃ mode
-	724, 756w	722w	O–C–O bending vibrations of oxalate anions
807	-	810	S₃^●⁻ combination mode (ν₁ + ν₂)
985s	983	-	SO₄²⁻ [symmetric stretching A₁ (ν₁) mode] (possibly, overlapping with the weak band of framework stretching vibrations)
-	1077w	-	CO₃²⁻ symmetric stretching mode
1088s	-	1097s	S₃^●⁻ overtone (2 × ν₁) [possibly, overlapping with the SO₄^●⁻ stretching band (ν₃ − F₂)]
-	1163	-	S₂^●− overtone (2 × ν₁)
1279, 1381	-	-	Symmetric stretching vibrations of CO₂ molecules (Fermi doublet, resonance with the overtone of bending vibrations).
1340	1342	1360	Symmetric C–O stretching vibrations of CO₂ molecules—involved in strong dipole-dipole interactions and/or symmetric C–O stretching vibrations of acid oxalate anions
-	1480w	-	CO₃²⁻ asymmetric stretching mode
-	1609s	-	Antisymmetric C–O stretching vibrations of acid oxalate anions
1631	-	-	S₃^●⁻ overtone (3 × ν₁)
-	-	1651	Symmetric C–O stretching vibrations of oxalate anions
-	1768, 1832	-	C=O stretching vibrations of acid oxalate groups
1891	-	1915	S₃^●⁻ combination mode (3 × ν₁ + ν₂)
2172	-	2191	S₃^●⁻ overtone (4 × ν₁)
2428w	-	-	S₃^●⁻ combination mode (4 × ν₂ + ν₁)
-	2553s	-	HS⁻ stretching mode
2575w	-	-	H₂S symmetric stretching mode
2710	-	2737	S₃^●⁻ overtone (5 × ν₁)
2964w	-	-	S₃^●⁻ combination mode (5 × ν₁ + ν₂)
3025	-	-	O–H stretching vibrations
3247w	-	-	S₃^●⁻ overtone (6 × ν₁)

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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 SO₄²⁻ and various polysulfide groups (S₂^●−, S₃^●−, S₄^●− radical anions, and S₄ and S₆ neutral molecules) as well as the presence of CO₂ molecules. Heating at 700 °C under reducing conditions results in the transformation of initial S-bearing groups SO₄²⁻ and S₃^●− to a mixture of S²⁻, HS⁻, S₂^●−, and S₄^●− and transformation of CO₂ to a mixture of CO₃²⁻ and C₂O₄²⁻ or HC₂O₄⁻ anionic groups. Further heating at 800 °C in air results in the decomposition of carbonate and oxalate groups, restoration of the SO₄²⁻ and S₃^●− groups, and a sharp transformation of the framework. The HS⁻ anion is stable only under reducing conditions, whereas the S₃^●− 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 CO₂⁰-bearing sodalite-group minerals are discussed.

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

Author

Chukanov, Nikita V¹; Roman Yu Shendrik²

; Vigasina, Marina F³

; Pekov, Igor V⁴; Sapozhnikov, Anatoly N²; Shcherbakov, Vasily D³; Varlamov, Dmitry A⁵

¹ Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, 142432 Moscow, Russia; [email protected]; Faculty of Geology, Moscow State University, 119991 Moscow, Russia; [email protected] (M.F.V.); [email protected] (I.V.P.); [email protected] (V.D.S.)
² Vinogradov Institute of Geochemistry, Siberian Branch of Russian Academy of Sciences, 664033 Irkutsk, Russia; [email protected] (R.Y.S.); [email protected] (A.N.S.)
³ Faculty of Geology, Moscow State University, 119991 Moscow, Russia; [email protected] (M.F.V.); [email protected] (I.V.P.); [email protected] (V.D.S.)
⁴ Faculty of Geology, Moscow State University, 119991 Moscow, Russia; [email protected] (M.F.V.); [email protected] (I.V.P.); [email protected] (V.D.S.); Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 119991 Moscow, Russia
⁵ Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, 142432 Moscow, Russia; [email protected]; Institute of Experimental Mineralogy RAS, Chernogolovka, 142432 Moscow, Russia

First page

887

Publication year

2022

Publication date

2022

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MDPI AG

e-ISSN

2075163X

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Scholarly Journal

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English

DOI

https://doi.org/10.3390/min12070887

ProQuest document ID

2694023859

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

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