Isomorphism and Mutual Transformations of S-Bearing Components in Feldspathoids with Microporous Structures

Abstract

The isomorphism of S-bearing feldspathoids belonging to the cancrinite, sodalite, tugtupite, vladimirivanovite, bystrite, marinellite and scapolite structure types has been investigated using a multimethodical approach based on infrared, Raman and 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. Sapozhnikovite Na₈(Al₆Si₆O₂₄)(HS)₂ and sulfite and thiosulfate analogues of cancrinite are synthesized hydrothermally and characterized by means of electron microprobe analyses, powder X-ray diffraction and Raman spectroscopy. The possibility of the incorporation of significant amounts of SO₄²⁻, S₄ and SO₃²⁻ in the crystal structures of cancrisilite, sulfhydrylbystrite and marinellite, respectively, has been established for the first time. Thermal conversions of S-bearing groups in the synthetic sulfite cancrinite and sapozhnikovite analogues as well as natural vladinirivanovite and S₄-bearing haüyne under oxidizing and reducing conditions have been studied using the multimethodical approach. The SO₄²⁻ and S²⁻ anions and the S₃^•– radical anion are the most stable S-bearing species under high-temperature conditions (in the range of 700–800 °C); their ratio in the heated samples is determined by the redox conditions and charge-balance requirement. The HS⁻ and S₅²⁻ anions are stable only under highly reducing conditions.

1. Introduction

Feldspathoids are defined as framework aluminosilicates (tectosilicates) which are anhydrous (leucite and nepheline-group minerals) or contain extra-framework anions (microporous aluminosilicates belonging to the sodalite, cancrinite and scapolite groups) [1,2]. Some feldspathoids are among the main components of different kinds of magmatic and metasomatic rocks. The extra-framework components in feldspathoids which have microporous structures are very diverse and include alkali and alkaline–earth cations (mainly, Na⁺, K⁺ and Ca²⁺), anions (SO₄²⁻, SO₃²⁻, PO₄³⁻, CO₃²⁻, C₂O₄²⁻, Cl⁻, HS⁻, OH⁻, F⁻ and S_n²⁻ with n from 1 to 5, MoO₄²⁻, WO₄²⁻, AsO₄³⁻, etc.), radical anions (S₂^•–, S₃^•–, cis- and trans-S₄^•–, SO₄^•–) and neutral molecules (H₂O, CO₂, COS, cis- and trans-S₄, S₆, etc.) [3,4,5,6,7,8,9,10,11,12,13,14] (the components playing a species-defining role in some minerals are given in bold type; “^•” means an unpaired electron).

The extra-framework components in feldspathoids are important markers of redox conditions and fugacities of volatile components during the formation of rock [6,13,15,16]. On the other hand, the microporous compounds belonging to the topological types of sodalite and, to a less extent, cancrinite are considered to be advanced materials that can be used in different technologies [4,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46].

As one can see from the above list of extra-framework components in feldspathoids, S-bearing species are most diverse: the oxidation degree of sulfur, the polymerization degree of sulfide sulfur and the charge of S-bearing species in feltspathoids varies from 0 to 6, from 1 to 6 and from 0 to −2, respectively. To date, a theoretical and methodological basis has been created for the study of various forms of sulfur and the mechanisms of their mutual transformations [4,5,6,8,12,13,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63]. In this paper, we provide new data on the formation, isomorphism and mutual conversions of S-bearing extra-framework components in some feldspathoids and their synthetic counterparts.

2. Materials and Methods

2.1. Natural Samples

The studied samples are listed below. Their empirical formulae are partly taken from literature sources. Chemical data for the newly analyzed samples of feldspathoids are given in

Table 1. Chemical composition (wt.%) of S- and P-bearing cancrisilite (Sample 3).

Component	Spot Analysis No.				Mean
	1	2	3	4
Na2O	23.02	22.60	22.78	22.69	22.77
K2O	0.18	0.16	0.15	0.10	0.15
Al2O3	25.85	25.23	25.80	25.64	25.63
Fe2O3	0.72	0.52	0.38	0.35	0.49
SiO2	39.66	41.34	40.40	40.82	40.56
P2O5	0.24	0.42	0.45	0.26	0.34
SO3	2.27	2.23	2.87	2.67	2.51
Cl	0.22	0.11	0.14	0.09	0.14
−O≡Cl	−0.05	−0.02	−0.03	−0.02	−0.03
Total	92.11	92.59	92.94	92.60	92.56

,

Table 2. Chemical composition (wt.%) of bystrite (Sample 9) and sulfhydrylbystrite (Sample 11).

Component	Sulfhydrylbystrite (for 19 Spot Analyses)			Bystrite (for 5 Spot Analyses)
	Mean	Ranges	Standard Deviation	Mean	Ranges	Standard Deviation
Na2O	11.89	10.59–13.14	0.63	18.70	18.38–19.31	0.28
K2O	9.20	8.00–10.54	0.76	0.16	0.08–0.29	0.08
CaO	5.79	5.23–7.38	0.49	5.55	4.91–5.95	0.41
Al2O3	26.29	25.17–27.27	0.59	26.95	26.20–27.43	0.46
Fe2O3	0.07	0–0.35	0.11	0.03	0–0.11	0.05
SiO2	32.35	31.52–34.72	0.74	32.68	31.88–33.64	0.73
S52−	14.08	15.68–17.34 (for total sulfur)	0.43 (for total sulfur)	14.87	14.69–15.50 (for total sulfur)	0.31 (for total sulfur)
HS−	2.21			0.51
S2−	0.28			-
S0	-			-
Cl−	0.27	0–0.65	0.19	2.70	2.39–2.90	0.19
–O≡ S52−	−1.41	-	-	−1.48	-	-
–O≡ HS−	−0.53	-	-	−0.12	-	-
–O≡ S2−	−0.14	-	-	-	-	-
–O≡Cl−	−0.06	-	-	−0.61	-	-
Total	100.29	-	-	99.94	-	-

and

Table 3. Chemical composition (wt.%) of scapolites.

Constituent	Sample 15	Sample 16
	Content (wt.%)
Na2O	3.05	5.21
K2O	0.64	0.31
CaO	17.57	14.75
Al2O3	27.91	27.32
SiO2	43.61	48.19
SO3	1.97	0.10
Cl	0.58	0.68
–O=Cl	–0.13	–0.15
Total	95.20	96.41
Formula coefficients based on Si + Al = 12 apfu
Na	0.93	1.51
K	0.13	0.05
Ca	2.95	2.36
Al	5.16	4.81
Si	6.84	7.19
S	0.23	0.01
Cl	0.15	0.16

. For samples with unknown contents of CO₃²⁻ and unknown ratios of different S-bearing species, these values were calculated based on the charge-balance requirement. All anionic groups and neutral molecules in the empirical formulae are confirmed by structural data or by using spectroscopic methods.

Sample 1 is cancrinite from the Mica mine, Kovdor alkaline complex, Kola Peninsula, Russia. Its empirical formula is [12]: Na_6.7Ca_0.7(Si_6.5Al_5.5O₂₄)(CO₃)_1.3·nH₂O (Z = 1). The mineral forms grey short-prismatic crystals up to 3 cm long in association with pectolite, potassic feldspar and aegirine-diopside.

Sample 2 is a CO₃²⁻-bearing variety of vishnevite from Loch Borrolan, Scotland, GB [64]. Its empirical formula is Na_6.46Ca_0.88K_0.08(Si_6.18Al_5.82O₂₄)(SO₄)_0.74(CO₃)_0.50·nH₂O (Z = 1).

Sample 3 is lilac S- and P-bearing cancrisilite from Alluaiv Mt., Lovozero massif, Kola Peninsula, Russia. The sample originates from a peralkaline pegmatite and associates with HS⁻-bearing sodalite. Its chemical composition is given in Table 1. The empirical formula of Sample 3 is Na_7.45K_0.03(Si_6.84Al_5.10Fe³⁺_0.05O₂₄)(CO₃)_0.775(SO₄)_0.32(PO₄)_0.05Cl_0.04·nH₂O (the content of the carbonate groups was calculated from the charge balance requirement).

Sample 4 is a common lilac S-free cancrisilite sample from Alluaiv Mt. Its empirical formula is Na_7.17(Si_7.39Al_4.61O₂₄)(CO₃)_1.28⋅nH₂O [64].

Sample 5 is bright blue S₄-bearing haüyne with the empirical formula Na_6.45K_0.01Ca_1.36(Si_6.06Al_5.94O₂₄)(SO₄)_1.56(S₄)_0.09(S₃^•–)_0.03Cl_0.09(CO₂)_0.02·nH₂O [5,13]. The unit cell parameter is a = 9.071 Å. The mineral forms grains in coarse-grained calciphyre consisting mainly of calcite, with subordinate diopside, scapolite and relics of plagyoclase. Sample 5 has been collected at the Pokhabinskoe lapis lazuli deposit situated in the basin of the Pokhabikha river, Slyudyanka district, Irkutsk region, Baikal Lake area, Siberia, Russia. A 30 cm thick tectonic zone of fractured metasomatic rocks with sodalite-group minerals occurs there at the contact of marbles and igneous rocks (leucomonzonites and anorthosites).

Sample 6 is the neotype specimen of lazurite with the empirical formula (Na_6.97Ca_0.88K_0.10)(Si_6.04Al_5.96O₂₄)(SO₄)_1.09(S₃^•–)_0.55S²⁻_0.05Cl_0.04·0.72H₂O from the Malo-Bystrinskoe gem lazurite deposit, Baikal Lake area, Siberia, Russia. [6]. The associated minerals are calcite and diopside as well as accessory dolomite, forsterite and phlogopite. The crystal structure of Sample 6 is characterized by commensurate and incommensurate modulations; the a parameter of the cubic sub-cell is equal to 9.087(3) Å.

Sample 7 is the holotype specimen of vladimirivanovite, an orthorhombic (space group Pnaa) lazurite-related mineral from the Tultui gem lazurite deposit, Baikal Lake area, Siberia, Russia [65]. The mineral forms dark blue zones around lazurite grains and separate elongate individuals up to 4 mm long. The other associated minerals are calcite, afghanite, tounkite, phlogopite and marialite. The Raman spectrum of Sample 7 shows the presence of significant amounts of the S₃^•– radical anions. The charge-balanced empirical formula written under the assumption that all sulfide sulfur belongs to S₃^•– is (Na_6.36Ca_1.52)(Si_6.03Al_5.97O₂₄)(SO₄)_1.08(S₃^•–)_0.22Cl_0.04·0.62H₂O.

Sample 8 is tugtupite, Na₈(Si₈Al₂Be₂O₂₄)Cl₂, originating from its type locality, Tugtup Agtakorfia, Ilimaussaq alkaline complex, Greenland. The mineral forms rose-colored equant grains up to 1 cm across in peralkaline pegmatite. Tugtupite is not considered as a feldspathoid, but it belongs to the sodalite group and was studied in this work for a comparison.

Sample 9 is bystrite from the Malo-Bystrinskoe gem lazurite deposit. The mineral forms yellow anhedral equant grains up to 0.5 mm across in partly recrystallized lazurite calciphyre. The associated minerals are calcite, lazurite, sodalite, fluorapatite, phlogopite, diopside and dolomite (Figure 1 and Figure 2a). Bystrite grains are embedded in calcite granular aggregate. Lazurite and bystrite do not show any reaction relations. Sodalite forms inclusions in bystrite and lazurite, and dolomite occurs as inclusions in calcite. Locally, bystrite occurs as a component of fine-grained polymineral aggregates, in which grains of earlier minerals are partly substituted with phlogopite (Figure 2b). The chemical composition of Sample 9 is given in Table 2. The empirical formula of Sample 9 is Na_6.75K_0.04Ca_1.11(Si_6.09Al_5.91O₂₄)(S₅)²⁻_1.04(HS⁻)_0.17Cl_0.85·nH₂O (Z = 2; sulfur is divided among S₅²⁻ and HS⁻, taking into account the charge-balance requirement). The presence of HS⁻ in this sample is confirmed by the Raman spectrum (see below).

Sample 10 is orange–yellow bystrite from the Malo-Bystrinskoe gem lazurite deposit. Its chemical composition is (wt.%): Na₂O 19.51, K₂O 0.16, CaO 4.97, Al₂O₃ 27.61, SiO₂ 32.72, S₅²⁻ 13.46, SO₃ 1.08, Cl⁻ 2.66, –O=(Cl⁻, S₅²⁻) –1.94, total 100.23 (sulfur is divided among S₅²⁻ and SO₃, taking into account the charge-balance requirement). The presence of a minor amount of sulfate sulfur in Sample 10 is confirmed by Raman spectroscopy. The empirical formula of Sample 10 is Na_6.97K_0.04Ca_0.98(Si_6.03Al_5.97O₂₄)(S₅²⁻)_0.93(SO₄²⁻)_0.15Cl_0.83 (Z = 2).

Sample 11 is orange–yellow sulfhydrylbystrite from the Malo-Bystrinskoe gem lazurite deposit. The mineral forms anhedral grains up to 0.2 mm across and partial pseudomorphs after lazurite in a metasomatic rock mainly composed of calcite, diopside, nepheline, lazurite, phlogopite, Sr-bearing plagioclase and Ca- and Ba-bearing stronalsite Na₂(Sr,Ca,Ba)(Si₄Al₄O₁₆)·nH₂O (Figure 3 and Figure 4). Plagioclase and stronalsite occur in the rock in minor amounts. For the chemical analyses of Sample 11, see Table 2. The empirical formula of Sample 11 based on (Si,Al,Fe)₁₂(S₅²⁻) is Na_4.37K_2.22Ca_1.17(Si_6.12Al_5.87Fe_0.01O₂₄)(S₅)²⁻(HS⁻)_0.76S²⁻_0.10Cl_0.09·nH₂O (Z = 2, n << 1; the excess of sulfur over 5 atoms per formula unit is divided among HS⁻ and S²⁻, taking into account the charge-balance requirement). The presence of a minor amount of H₂O molecules in Sample 11 was established using IR spectroscopy (see below).

Sample 12 is sulfhydrylbystrite from the Malo-Bystrinskoe deposit with the composition (wt.%): Na₂O 14.72, K₂O 6.89, CaO 4.85, Al₂O₃ 27.44, SiO₂ 31.49, S 15.60, Cl 0.15, –O=Cl −0.03, total 101.11, where S is the total sulfur belonging to S₅²⁻, HS⁻ and S₄, in accordance with the Raman spectrum (see below). The oxygen equivalent for sulfur could not be calculated because the proportion of the sulfide groups is unknown. The empirical formula is H_x(Na_5.34K_1.65Ca_0.96)(Si_5.89Al_6.04Fe_0.07O₂₄)(S^total)_5.47Cl_0.06 (Z = 2). The mineral forms red–orange anhedral grains up to 0.4 mm across in a metasomatic rock mainly composed of calcite, diopside, nepheline, lazurite, afghanite, phlogopite, plagioclase and Ca- and Ba-bearing stronalsite Na₂(Sr,Ca,Ba)(Si₄Al₄O₁₆)·nH₂O.

Samples 13a and 13b are two varieties of the SO₃²⁻-bearing analogue of marinellite-forming pale violet and deep violet grains, respectively, up to 1 mm across in a subvolcanic rock mainly composed of sanidine with subordinate biotite and minor nepheline and leucite (Figure 5). The specimens originate from Magliano, Rome, Latium, Italy. The chemical composition of Sample 13b based on six spot analyses is (wt.%, all sulfur was calculated as SO₃): Na₂O 14.01, K₂O 8.44, CaO 5.52, Al₂O₃ 27.90, SiO₂ 32.79, SO₃ 9.72, Cl 1.09, H₂O (determined by the Alimarin method) 0.95, –O=Cl –0.26, total 100.18, which corresponds to the empirical formula (Na_29.78K_11.81Ca_6.49)(Si_35.05Al_36.05O₂₄)(SO₄,SO₃)_8.00Cl_2.03(OH)_0.49·3.25H₂O (Z = 1). The crystal structure of Sample 13b was refined to an R-factor of 5.71% for 6819 independent reflections with I > 2σ(I) [66]. The mineral is trigonal, in space group P31c and the unit cell parameters are: a = 12.8727(2), c = 31.7393(5) Å and V = 4554.75(15) ų. The crystal chemical formula is Na_29.78K_11.82Ca_6.40(Si₃₆Al₃₆O₁₄₄)(SO₄)₆(SO₃)₂Cl₂(OH)_0.40⋅3.98H₂O. The SO₃²⁻ groups dominate at an extra-framework site hosted by the liottite cage.

Sample 14 is the holotype specimen of the 30-layer cancrinite-group mineral biachellaite [67]. It occurs as colorless short-prismatic and tabular blocky crystals up to 1 cm (Figure 6) in a volcanic syenitic ejectum from the Biachella Valley, Sacrofano paleovolcano, Latium, Italy. The associated minerals are sanidine, diopside, andradite, leucite, haüyne, sacrofanite, steudelite, liottite and alloriite. The empirical formula of Sample 14 is (Na_3.76Ca_2.50K_1.44)(Si_6.06Al_5.94O₂₄)(SO₄)_1.84Cl_0.15(OH)_0.43·0.81H₂O (Z = 15). The mineral is trigonal, in space group P3 and the unit cell parameters are a = 12.913(1), c = 79.605(5) Å and V = 11495(1) ų.

Sample 15 is white S-bearing meionite from the Slyudyanka phlogopite deposit, Uluntuy gorge, Slyudyanka district. It is a fragment of a tetragonal prismatic crystal (4 cm long and 3 cm thick) from magnesian skarn mainly consisting of diopside and calcite. Its empirical formula is (Table 3, CO₃²⁻ calculated based on the charge balance requirement): (Ca_2.95Na_0.93K_0.13)(Si_6.84Al_5.16O₂₄)(CO₃)_0.605(SO₄)_0.23Cl_0.13·

Sample 16 is violet–blue meionite (“glaucolite”) from the Slyudyanka river valley, Slyudyanka district. It is a fragment of old (beginning of 20th century) specimen deposited in the Fersman Mineralogical Museum of the Russian Academy of Sciences (Moscow) with catalogue number 54,413. Meionite forms coarse-grained nest (8 cm across), rimmed by light golden-brown phlogopite, in calcite marble. Its empirical formula is (Table 3, CO₃²⁻ calculated based on the charge balance requirement): (Ca_2.36Na_1.51K_0.05)(Si_7.19Al_4.81O₂₄)(CO₃)_0.645(SO₄)_0.01Cl_0.16·

2.2. Synthesis and Thermal Conversions

All syntheses of sodalite- and cancrinite-type compounds were performed under mild hydrothermal conditions at temperatures of 180 °C to 230 °C within 16–24 h. In order to obtain the framework aluminosilicates with incorporated S-bearing anions, we utilized the traditional process of geopolymers production, that involves an aluminosilicate precursor and a soluble alkaline medium, preferably Na or K hydroxide solutions. This method also simulates the crystallization conditions of sodalite- and cancrinite-group minerals in postmagmatic alkaline rocks. S-bearing sodalite-type phases were synthesized using a highly dispersed powder of metakaoline as an aluminosilicate source. The purity of metakaolinite was verified by the powder X-ray diffraction. The chemical composition of starting kaolin used in the experiments is as follows (wt.%): SiO₂ 46.5, Al₂O₃ 39.0, FeO 0.5% and H₂O 14.0%.

Three salts with different sulfur oxidation degrees were used in the experiments: Na₂S·10H₂O, Na₂SO₃·5H₂O and Na₂S₂O₃. The mixtures of the metakaoline powder (3 g), S-bearing salts (3 g) and 8M NaOH (5 mL) aqueous solution were loaded to the hydrothermal reactor with a 25 mL polypropylene (PP) bottle, stirred for 10 min and placed into the muffle furnace. In the experiment with Na₂S₂O₃, 1.5 g of CaCl₂ was added to the charge. After the completion of the experiments, the samples were washed with distilled water and dried in a cabinet dryer oven.

The thermal transformations of S-bearing extra-framework anions in the synthesized materials under different redox conditions were studied by the heating of initial powders in a muffle furnace at 700–800 °C for 6 h in uncoated crucibles in contact with atmospheric air (oxidizing conditions) and crucibles with carbon tablets (reducing conditions). The thermal treatment conditions and crystallographic data of the treatment products are given in

Table 4. Data on the experimental conditions and crystallographic characteristics of the products.

Experiment	S-Bearing Salt	Treatment Conditions	Color of the Product	Space Group, Unit Cell Parameters
1	Na2S ·10H2O	Initial product of synthesis	Pale blue	P4-3n, a = 8.9217(3) Å, V = 710.14(1) Å3
1a	Na2S ·10H2O	t = 800 °C, 6 h, muffle furnace	White	P23, a = 9.0878(1) Å, V = 750.55(1) Å3
1b	Na2S ·10H2O	t = 700 °C, 6 h, muffle furnace with carbon tablets	Blue	P23, a = 9.0926(1) Å, V = 751.74(1) Å3
2	Na2SO3·5H2O	Initial product of synthesis	White	P63, a = 12.7004(7) Å, c = 5.1768(7) Å, V = 723.2(1) Å3 and P4-3n, a = 8.8867(3) Å, V = 701.8(1) Å3
2a	Na2SO3·5H2O	t = 800 °C, 6 h, muffle furnace	White	P23, a = 9.0928(1) Å, V = 751.79(1) Å3
2b	Na2SO3·5H2O	t = 700 °C, 6 h, muffle furnace with carbon tablets	White with pale blue hue	P23, a = 9.0913(1) Å, V = 751.41(1) Å3
3	Na2S2O3	Initial product of synthesis	Light green	P3, a = 12.6780(3) Å, c = 5.1905(2) Å, V = 722.5(1) Å3

. The cell parameters and especially the space group assignments were determined from powder X-ray diffraction patterns, taking into account the systematic absences of the reflections. All the compounds of the products belong to the sodalite and/or cancrinite structure types. The chemical data for the synthetic phases are presented in

Table 5. Representative chemical data (wt.%) of the products of synthesis and analyses. The analyses numbers correspond to Experiment 1 (1), Experiment 1a (2), Experiment 1b (3), Experiment 2 (4), Experiment 2a (5), Experiment 2b (6) and Experiment 3 (7).

Analysis No. → Component ↓	1	2	3	4	5	6	7
	Content (wt.%)
Na2O	24.3	15.36	19.23	19.14	21.43	22.68	16.00
K2O	-	0.67	5.15	-	-	1.54	-
CaO	-	-	-	-	-	0.18	4.69
Al2O3	33.32	33.60	37.51	32.98	30.79	32.71	30.62
Fe2O3	-	-	-	-	0.33	0.53	0.33
SiO2	38.46	37.94	42.22	38.52	38.19	41.46	40.66
HS−	5.39	-	-	-	-	-	-
SO3	-	12.79	14.29	-	8.10	3.66	-
SO2	-	-	-	6.35	-	-	-
S2O3	-	-	-	-	-	-	9.56
Total	100.24 *	100.36	118.40	94.99	98.85	102.75	100.57 *
	Formula coefficients calculated on 12 Si + Al + Fe atoms per formula unit
Na	7.27	4.61	5.18	6.15	6.67	6.56	4.83
K	-	0.13	0.91	-	-	0.29	-
Ca	-	-	-	-	-	0.03	0.78
Al	6.06	6.13	6.14	5.90	5.83	5.75	5.62
Fe	-	-	-	-	0.04	0.06	0.04
Si	5.94	5.87	5.86	6.10	6.13	6.19	6.34
Stotal	1.51	1.49	1.49	0.91	0.98	0.41	1.60

* Oxygen equivalents of S-bearing groups are subtracted.

. For the product of Experiment 1b, containing both SO₄²⁻ and polysulfide groups (mainly, S₃^•− and S₄) in unknown proportions (see below), the oxygen equivalent of polysulfide groups could not be calculated. All the sulfur in this sample is given as SO₃, which resulted in a high analytical total.

The thermal conversions of Sample 5 were carried out in two stages. In the first stage, the method of the double quartz tube was applied. The samples were heated for three days at 700 °C in a reducing atmosphere (over the Fe-FeS buffer). As a result, a SO₄²⁻-free and (HS⁻,S²⁻,S₂^•−,S₄^•−)-bearing sodalite-type material (so-called “S-sodalite”) was obtained [13]. On the next step, this material was annealed in air, in a muffle furnace at 600 °C and 700 °C, from 1 to 256 h together with CaF₂ (cubic, with a = 5.4634 Å) that was used as an internal standard for the determination of the unit cell parameter of the sample under investigation. The annealing in the air resulted in gradual transformations of the above-listed sulfide groups into SO₄^2— and S₃^•– [13].

2.3. Analytical Methods

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) [12,13]. In particular, the absorption spectra of the 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 [12,13]. For low-temperature measurements, a quartz ampoule with the sample was placed in a flooded cryostat.

In order to obtain the IR absorption spectra, the powdered samples were mixed with anhydrous KBr, pelletized and analyzed using an ALPHA FTIR spectrometer (Bruker Optics, Karlsruhe, Germany) at a resolution of 4 cm⁻¹ as described in [5,6,8,12,13]. Sixteen scans were collected for each spectrum. The IR spectrum of an analogous pellet of pure KBr was used as a reference.

The Raman spectra of the natural samples have been obtained for randomly oriented grains using an EnSpectr R532 spectrometer based on an OLYMPUS CX 41 microscope (Enhanced Spectrometry, San Jose, CA, USA) coupled with a diode laser (λ = 532 nm) at room temperature [5,8,12,13]. The spectra were recorded in the range from 100 to 4000 cm⁻¹ with a diffraction grating (1800 gr mm⁻¹) and spectral resolution of about 6 cm⁻¹. The output power of the laser beam was in the range from 5 to 13 mW. The diameter of the focal spot on the sample was 5–10 μm. The backscattered Raman signal was collected with a 40× objective; the signal acquisition time for a single scan of the spectral range was 1 s and the signal was averaged over 50 scans. Crystalline silicon was used as a standard.

The Raman spectra of the synthesized samples were acquired with a confocal Raman microscope JY Horiba XPloRA at the Department of Petrology and Volcanology (Geological Faculty, Lomonosov Moscow State University, Moscow, Russia). A laser beam with a 532 nm wavelength and 12 mW power was used as an excitation source. The spectra were collected in the range 100–3900 cm⁻¹ with a 1800 gr/mm diffraction grating, a 10× objective and 40 s accumulation time per each window.

The powder X-ray diffraction investigation of the annealed Sample 5 was carried out with a DRON-3 diffractometer (Burevestnik, St. Petersburg, Russia) using Fe-Kα (with a Mn filter) and Cu-Kα (with a Ni filter) radiations [6]. The sub-cell a parameter was determined using the (440) reflection with an accuracy within ±0.001 Å.

The single-crystal XRD studies were carried out using an Xcalibur S diffractometer (OXFORD DIFFRACTION, Oxford, UK) equipped with a CCD detector (MoKα radiation).

The powder XRD data for the synthesized powders of S-bearing feldspathoids were collected using a Bruker D8 Discover diffractometer in the Bragg-Brentano geometry, 2Θ range of 5–120° and 2Θ step of 0.02° using CuKα radiation (λ = 1.54018 Å), at an accelerating voltage of 40 kV, current of 40 mA and time per step of 1.5 s.

The chemical composition of sample 15 was analyzed by means of a Hitachi FlexSEM 1000 equipped with an Xplore Contact 30 EDS-detector and Oxford AZtecLive STD system of analysis (15 kV, 5 nA, 2 μm) [6]. The standards and analytical lines are: NaKα–albite, AlKα–kyanite, SiKα–SiO₂, SKα–BaSO₄, ClKα–TlCl, KKα–KBr and CaKα–wollastonite. The contents of other elements with an atomic number higher than that of Be are below the detection limits.

The EDS-mode electron microprobe analyses of the other samples were carried out on an analytical suite, including a digital scanning electron microscope Tescan VEGA-II XMU equipped with an energy-dispersive spectrometer (EDS) INCA Energy 450 with a semiconducting Si (Li) detector Link INCA Energy and wave-dispersive spectrometer (WDS) Oxford INCA Wave 700, produced by Tescan Orsay Hld., Brno, the Czech Republic. The analyses were performed at an accelerating voltage of 20 kV, a current of 120 to 150 pA and a beam diameter of 120 nm. The diameter of the excitation zone was below 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. The contents of the other elements with atomic numbers >6 are below the detection limits.

The proportion of sulfide and sulfate sulfur in the experiments with the thermal conversions of Sample 5 was determined in the Central Research Geological Exploration Institute of Nonferrous and Noble Metals (Moscow) by X-ray photoelectron spectroscopy (XPS) [6]. The spectra were acquired using a Riber LAS-3000 spectrometer with a hemispherical electron detector (Riber, Bezons, France), with a retention potential and nonmonochromatic Al radiation (AlKα = 1486.6 eV) at an anode current of 20 mA and an accelerating voltage of 10 kV. The C 1 s peak at 285 eV was used to correct the binding energies for the charging of the surface. The obtained S 2p peaks are 2p_3/2–2p_1/2 spin–orbit doublets. In order to obtain accurate binding energy values, the peaks were deconvoluted using a special program allowing for a nonlinear background and Voigt function used to approximate the peak shapes. The peaks were assigned to a particular sulfur species based on 2p_3/2 doublet components using the available published data [68].

3. Results

3.1. Minerals of the Cancrinite-Cancrisilite-Vishnevite Solid-Solution Series

The aluminosilicate frameworks of minerals belonging to the cancrinite-cancrisilite-vishnevite solid-solution series have a hexagonal symmetry and a cancrinite-type topology with wide channels and columns of cancrinite cages running along the c axis. There is a wide isomorphism between cancrinite and vishnevite, whereas cancrisilite is usually S-depleted [64]. In this paper, we report data on an unusual S- and P-bearing cancrisilite variety (Sample 3, see Table 1). Its IR and Raman spectra are presented in Figure 7 and Figure 8, respectively. The spectra of cancrinite (Sample 1) and CO₃-bearing vishnevite (Sample 2) are given in these figures for a comparison.

The IR spectra of S-rich and S-poor cancrisilites are similar. All bands in the range of 3200–3700 cm⁻¹ correspond to H₂O molecules. The S-rich Sample 3 differs from the common cancrisilite Sample 4 by the presence of additional bands of H₂O molecules at 3235 and 3432 cm⁻¹, the enhanced intensity of the peak at 1135 cm⁻¹ (overlapping bands of Si−O−Si stretching vibrations and asymmetric stretching vibrations of SO₄²⁻ anions), as well as lowered intensities of the bands of asymmetric stretching and out-of-plane bending vibrations of the CO₃²⁻ anions (at 1462 and 865 cm⁻¹, respectively). In the crystal structure of cancrisilite, CO₃²⁻ anions occupy three independent sites. The bands at 1462 and 1400–1413 cm⁻¹ in the IR spectra of cancrisilite correspond to common vibrations of two identical CO₃²⁻ groups in which changes in the dipole moment are parallel and antiparallel, respectively [69]. The IR spectrum of cancrinite in this region (curve c in Figure 7) is more complex, which reflects the presence of numerous independent sites of CO₃²⁻ anions in the crystal structure of this mineral [70].

The Raman spectrum of S-rich cancrisilite (curve a in Figure 8) is more informative. It contains bands of bending vibrations of SO₄²⁻ at 454 and 465 cm⁻¹ [the E(ν₂) mode] and 634 cm⁻¹ [the F₂(ν₄) mode] as well as stretching modes of SO₄²⁻ at 978 cm⁻¹ [symmetric vibrations, the A₁(ν₁) mode] and 1095 cm⁻¹ with a shoulder at 1155 cm⁻¹ [asymmetric vibrations, the F₂(ν₃) mode]. The nondegenerate A₁(ν₁) band is not split, which indicates that the SO₄²⁻ anions occupy a single site. The Raman bands of in-plane bending, symmetric stretching and asymmetric stretching vibrations of the CO₃²⁻ anionic groups are observed at 703, 1044 + 1054 and 1412 + 1472 cm⁻¹, respectively. The doublet 1044 + 1054 cm⁻¹ corresponding to a nondegenerate mode may indicate that the CO₃²⁻ groups are locally nonequivalent and SO₄²⁻ substitutes the CO₃²⁻ groups only at one site. A comparison of the Raman spectra of Samples 1, 2 and 3 shows that the band at 1044 cm⁻¹ in the Raman spectrum of Sample 3 may correspond to the CO₃²⁻ anions, whose local environment contains SO₄²⁻.

The assignment of the band at 525 cm⁻¹ in the Raman spectrum of S-rich cancrisilite is ambiguous. This band is absent in the Raman spectra of other minerals with the cancrinite-type framework. The strong band at 520 cm⁻¹ in the Raman spectrum of hannebachite, Ca(SO₃)·0.5H₂O was tentatively assigned to the E(ν₂) mode of SO₃²⁻ [71]. On the other hand, a strong Raman band at 523–530 cm⁻¹ is a characteristic feature of the S₇ molecule, along with the bands at 237–239, 355–358, 400 and 481 cm⁻¹ [48].

3.2. Isomorphism of Bystrite and Sulfhydrylbystrite

Bystrite was first described as a four-layer cancrinite-group mineral with the simplified formula Ca(Na,K)₇(Si₆Al₆O₂₄)(S₃)_1.5·H₂O [72]. Subsequent investigations have shown that the samples were initially considered as Cl-bearing K-deficient and Cl-poor K-rich varieties of bystrite are different mineral species, bystrite and sulfhydrilbystrite, with the idealized formulae Na₇Ca(Al₆Si₆O₂₄)(S₅²⁻)Cl⁻ and Na₅K₂Ca(Al₆Si₆O₂₄)(S₅²⁻)(SH⁻), respectively [73,74].

The IR spectra of these minerals published earlier are of a poor quality and contain bands of different impurities (mainly, calcite and adsorbed water). The IR spectra of bystrite and sulfhydrylbystrite obtained in this work (Figure 9) are similar. The distinctive features of sulfhydrylbystrite are the bands at 3565, 3460 and 1645 cm⁻¹ corresponding to the stretching and bending vibrations of the H₂O molecules as well as the weak band at 2556 cm⁻¹ corresponding to the stretching vibrations of the HS⁻ anion. Weak bands at 3430 and 1628 cm⁻¹ in the IR spectrum of bystrite are related to the adsorbed water.

According to the data of the X-ray structural analysis [73,74], bystrite and sulfhydrylbystrite contain the S₅²⁻ anion that is most thermodynamically favored among the S_n²⁻ anions, with the n ranging from 2 to 8 (the order of decreasing stability in the aqueous solution is S₅²⁻ >> S₆²⁻ > S₄²⁻ >> S₇²⁻ > S₃²⁻>> S₈²⁻ > S₂²⁻ [63]). Neither the neutral cyclic S₅ molecule nor the radical anion S₅^•− have definitely been observed in the solution or in the solid state.

The wavenumbers of fundamental S–S stretching vibrations predicted for S₅²⁻ coordinated by Li⁺ are 471, 463 and 416 cm⁻¹ [63]. Similar bands are observed in the IR spectra of bystrite and sulfhydrylbystrite in the ranges of 413–422 and 454–466 cm⁻¹. No bands in these ranges are in the IR spectrum of carbobystrite, a carbonate cancrinite-group mineral with the bystrite-type framework [75].

Unlike IR spectroscopy, Raman spectroscopy is very sensitive to the presence of the HS⁻ anions. The wavenumbers of the Raman band of the H–S stretching vibrations of HS⁻ in sodalite-group minerals and products of their heating under reducing conditions are in the range of 2550–2580 cm⁻¹ [7,13]. Thus, the band at 2562 cm⁻¹ in the Raman spectrum of sulfhydrylbystrite (Figure 10) should be assigned to the HS⁻ anion occurring in the cancrinite cage, in accordance with the structural data [73].

The doublets 447 + 508 and 189 + 255 cm⁻¹ in the Raman spectrum of sulfhydrylbystrite are close to the bands of bending modes of the aluminosilicate framework and lattice acoustic modes, respectively, but taking into account the high intensities of these peaks, they should be considered as S–S stretching and S–S–S bending bands of S₅²⁻, respectively, maybe overlapping with the much weaker bands of the framework vibrations. The weak band at 2237 cm⁻¹ in the Raman spectrum of sulfhydrylbystrite may correspond to trace amounts of hydrated proton complexes with strong hydrogen bonds and the O···O distances of 2.5–2.6 Å [76]. Such complexes could be formed as a result of the partial dissociation of HS⁻ and the subsequent solvatation of H⁺ by the H₂O molecule which are present in the structure of the studied sample, according to the IR spectroscopy data. The weak band at 607 cm⁻¹ is due to the stretching vibrations of the S₂^•− radical anion occurring in sulfhydrylbystrite in trace amounts. Similar bands (at 609 and 605 cm⁻¹) observed in the Raman spectra of sapozhnikovite (Figure 10) and bolotinaite (described as a F-rich sodalite-group mineral [8]) also correspond to S₂^•−, that is the cause of the strong luminescence of all these minerals under a laser beam. An additional source of luminescence of feldspathoids is Fe³⁺ [77]. In particular, a broad Raman band of sapozhnikovite in the range of 800–4500 cm⁻¹ is a superposition of luminescence from S₂^•− and Fe³⁺ (see Figure 11).

The wavenumbers of the strongest Raman bands of cis-S₄ (a red chromophore) calculated at the B3LYP/6-31G(2df) level are equal to 330, 373 and 674 cm⁻¹ [48]. Similar bands (at 327–328, 362–363, 394–396 and 667–668 cm⁻¹) are present in the Raman spectrum of red–orange sulfhydrylbystrite (Sample 12, Figure 10) and orange–yellow bystrite (Sample 10, Figure 12). However, these bands are not observed in the Raman spectrum of yellow bystrite (Sample 9, Figure 12).

The bands in the range of 845–858 cm⁻¹ correspond to the first overtone of the bands in the range of 442–455 cm⁻¹. All other bands in the Raman spectrum of sulfhydrylbystrite correspond to the stretching and bending vibrations of the framework. The very weak bands at 975 and 2565 cm⁻¹ in the Raman spectrum of orange–yellow bystrite (Sample 10) indicate the presence of trace amounts of the SO₄²⁻ and HS⁻ anions, respectively.

3.3. Thermal Transformations of S₄-Bearing Haüyne and S-Sodalite–Haüyne Miscibility

The Raman spectra of S₄-bearing haüyne (Sample 5) and products of its thermal conversions are given in Figure 13. Photos of the heating products are presented in Figure 14. All the Raman bands of the initial Sample 5 in the range of 400–2600 cm⁻¹, except the band of SO₄² at 983 cm⁻¹, correspond to S₃^•–. The Raman spectrum is very sensitive to the S₃^•– radical anion that is a strong blue chromophore. Therefore, despite a low content of S₃^•– in Sample 5, the bands of this species in its Raman spectrum are most strong. The wavenumbers (cm⁻¹) and assignment of the Raman bands of Sample 5 preheated at 700 °C under reducing conditions are as follows: 453–stretching vibrations of the [(HS)^–Na₄]³⁺ cluster; 547–S₃^•– symmetric stretching (ν₁) mode; 596–stretching vibrations of the [(S²⁻)Na₄]²⁺ cluster and/or S₂^•– stretching mode; 728–O–C–O bending vibrations of oxalate anions; 850–C–C stretching vibrations of oxalate anions; 989–framework stretching vibrations; 1064–CO₃²⁻ symmetric stretching vibrations; 1350–symmetric C–O stretching vibrations of oxalate anions, 1605–antisymmetric C–O stretching vibrations of oxalate anions; 1909–overtone or a combination mode; and 2556–HS⁻ stretching mode. All the observed Raman bands Sample 5 heated at 800 °C in the air (Figure 13) correspond to S₃^•– because the intensities of the Raman bands of other components are below noise level.

Based on Raman, the ESR and IR spectroscopy data, Sample 5 which preheated at 700 °C over the Fe-FeS buffer for three days, is sulfate-free and contains HS⁻ and S^2- anions as well as subordinate amounts of S₂^•– and S₄^•– radical anions [13]. The presence of S₂^•– is the cause of the yellow–brown color of the preheated sample. A subsequent annealing in the air results in a change in the color, first to green (Figure 14) and then to blue because of the restoration of the S₃^•– radical anion (a blue chromophore) [13].

The XPS spectrum of Sample 5 annealed with a buffer is a section of the general signal increase in the range of the binding energies between those of S²⁻ and SO₃²⁻ (160–167 eV), without a clear separation of the individual peaks. However, the XAS method, which is more sensitive to the composition of the sample volume than XPS, revealed the features of the XANES S K-edge spectrum, which were initially interpreted as S²⁻ in two nonequivalent structural positions [78]. However, Raman spectroscopy shows the presence of large amounts of HS⁻ anions in this sample [13]. After annealing in air at 800 °C for 1 day, the XPS spectrum detects only oxidized forms of sulfur, SO₄²⁻ and SO₃²⁻.

The results indicate that the Ca-bearing S-sodalite obtained from natural S₃^•–-bearing haüyne can be transformed into phases containing extra-framework radical anions, the composition of which is determined by the redox conditions, partial pressures of the volatile components (fS₂ and fSO₂) and the conditions of the subsequent thermal treatment.

The transformation of the sulfide form into the sulfate form was studied in air at 600 and 700 °C by varying the heating time of the sample. The composition of the formed phases was determined based on the value of the unit cell parameter. The unit cell parameters of the sulfate and sulfide end-members of the MX₂ isomorphic series, where M = [(Na,Ca)₈(Si,Al)₁₂O₂₄]⁴⁻, X = SO₄²⁻, S²⁻, were estimated as follows. For the sulfate end member, the value of 9.085 Å is accepted according to the data of the study of cubic S₃^•–-bearing haüyne with the highest SO₄²⁻/S ratios (where S is the total amount of sulfide sulfur) and the data of the long-time experiments on the annealing of lazurite in the range of 600–800 °C. The reproducibility of this value is no worse than ±0.003 Å. The situation with the sulfide end member is less clear. The value of the unit cell parameter for it varies from 8.876 to 8.944 Å [79,80,81].

The causes of the variations are the variable amount of the haüyne component of the solid solution and partial oxidation of sulfur by atmospheric air. In further calculations, we used the parameter a = 8.910 Å, the average between the two extreme values indicated above. Under the assumption that Vegard’s law is in force for this system, the dependence of the unit cell parameter on the fraction of the sulfate end member [MSO₄] can be described by the following formula:

a (Å) = 8.91 + 0.00175 · [MSO₄] (mol.%)

(1)

The data on the kinetics of the thermal transformations during the annealing (at 600 °C and 700 °C, in air) of Sample 5 preheated over the Fe-FeS buffer are presented in

Table 6. Experimental data on thermal transformations during annealing of Sample 5 preheated over the Fe-FeS buffer.

Annealing Time, h	Parameters of Cubic Subcells, Å *	Content of the MSO4 Component, Mol.% **
		Sulfide Phase	Sulfate-Rich Phase
T = 600 °C
1	8.945	-	20
2	8.949	-	22
4	8.950	-	23
8	8.953	-	25
16	8.961	-	29
32	8.969	-	34
64	8.980, 9.069	91	40
128	8.988, 9.080	97	45
256	8.992, 9.078	96	47
T = 700 °C
1	8.975, 9.032	70	37
2	8.982, 9.042	75	41
4	8.984, 9.059	85	42
8	9.076	95	-
16	9.083	99	-

* Two parameters are given for two-phase products. ** According to Equation (1).

. The initial sulfide sample had a parameter of 8.944 Å, which may be due to the presence of minor amounts of S₄^•– [13].

The theoretical analysis of the miscibility of the components in this system was carried out in the framework of the Urusov theory [82,83,84] in the approximation of asymmetric regular solutions and in modification for complex aluminosilicate groups [84]. The value of the mixing energy is determined as Q = 4338 cal/mole, which gives the maximum decomposition temperature T_m of 1085 K or 812 °C (in a strictly regular approximation, T_m = Q/4). Taking into account the dimension parameter ΔR/R₁ = 0.037, the solvus deformation and the critical decomposition temperature in the approximation of an asymmetric solution are 48 mol.% MSO₄ and 780 °C, respectively. The miscibility diagram plotted using these data is shown in Figure 15 along with the experimental data. It can be seen that the experimental solvus is strongly shifted relative to the calculated one towards SO₄, and the points on its “sulfide” branch are in a poor agreement with the theoretical parabola.

Two factors can significantly affect the experimental results: the time of the experiment and the conversion of sulfur into various forms. The experiments at 800 °C on the annealing of sulfide-bearing haüyne (Sample 5) and vladimirivanovite (Sample 7) (see Table 9 in [68]) show that the latter factor plays a significant role and that this system cannot be considered as a quasi-binary one. According to XPS data, sulfur is present not only in sulfate form, but also in sulfite, thiosulfate and polysulfide forms. Thus, this diagram should be built in three dimensions, and the third axis should be the activity (volatility) of SO₂ [80]. The failure to take this circumstance into account complicates the use of sodalite-group minerals containing both sulfide and sulfate sulfur for geothermometric purposes. This applies, in particular, to the sodalite–nosean solvus [85], which, despite the critical temperature close to that obtained by us (~770 °C), is strongly deformed. This fact can be explained by the presence of various forms of sulfur in both coexisting phases (S-bearing sodalite and nosean) rather than the nonideality of the solid solutions.

3.4. Extra-Framework Components in Tugtupite

Tugtupite (Sample 8), when irradiated with ultraviolet light with a wavelength shorter than 450 nm, acquires a purple color. The purple color is associated with the appearance of an intense absorption band with a maximum of about 520 nm (Figure 16). When tugtupite is illuminated with light and the wavelength is in the range of 500–600 nm, the absorption band responsible for the purple color disappears. This is the cause of the color fading of this mineral in sunlight.

Along with the appearance of a band in the region of 520 nm in the absorption spectrum, an intense ESR signal appears in the UV-irradiated tugtupite samples (in Figure 17, its components are marked with asterisks). As the color intensity increased, the intensity of these components also increased. After bleaching, the intensity of this signal significantly decreased. In addition, a signal with the g tensor components g₁ = 2.210; g₂ = 2.010; and g₃ = 1.995 related to the S₂^•– radical anions as well as a sextet with g = 2.005 corresponding to the Mn²⁺ cations, which was observed in the ESR spectra of tugtupite both before and after the irradiation. Earlier, similar signals were observed in the ESR spectra of other sodalite-group minerals [12,13].

The presence of S₂^•– radical anions in the tugtupite is also confirmed by the presence of luminescence upon excitation with a light having a wavelength of about 400 nm. The luminescence spectrum (Figure 18) shows a band in the range of 600–800 nm, which has a pronounced vibrational structure with a phonon repetition frequency of about 520 cm⁻¹ [86]. The luminescence excitation spectrum shows an intense band with a maximum at 390 nm. Such a luminescence is characteristic of the S₂^•– radical anions. For example, the luminescence maximum of sapozhnikovite, associated with the S₂^•– radical anions, was observed in the region of 625 nm with a phonon repetition frequency of 536 cm⁻¹ [7]. The luminescence maximum of haüyne is about 650 nm, with a phonon repetition frequency of 690 cm⁻¹ [5]. For hackmanite, the luminescence maximum is observed at 610 nm, and the phonon repetition frequency is 535 cm⁻¹ [87].

The phenomenon of photochromism (color change under the influence of light of different wavelengths) for the tugtupite has not been fully studied. There is no consensus on the nature of this phenomenon [88]. At first glance, the nature of the photochromism in the tugtupite is the same as in hackmanite, where the phototransfer of an electron from S₂²⁻ to a chlorine vacancy occurs upon UV irradiation. The resulting F_Cl⁻ centers (chlorine vacancy that captured an electron) have absorption bands in the vicinity of 550 nm [89]. A similar point of view is shared by the authors of other works [90,91]. However, the authors of [91] noted that when tugtupite is heated above 700 °C, photochromism disappears and the samples cease to turn purple, but they still exhibit the intense photoluminescence associated with S₂^•–. This does not happen with hackmanite. Moreover, in some cases, after heating hackmanite, photochromism becomes more pronounced [13,91]. In addition, in the absorption spectrum of hackmanite, as a result of the ultraviolet coloration, the intensity of the shoulder at 400 nm increases, which is associated with an increase in the amount of S₂^•– radical anions as a result of the reaction S₂²⁻ + ☐_Cl → S₂^•– + F_Cl⁻, where ☐ is a vacancy. Along with the appearance of color, the EPR signal associated with the S₂^•– radical anions increases [90]. This does not take place in the case of tugtupite.

Thus, the nature of photochromism in the case of tugtupite may differ from that of hackmanite. In the ESR spectrum of the irradiated tugtupite, which has a saturated purple color, an ESR signal with g₁ = 2.020 and g₂ = 2.001 is observed. This signal was previously observed in the ESR spectra of the minerals of the sodalite and cancrinite groups and was attributed to the S₄^•– radical anions [12,13]. Moreover, such radical anions have absorption bands in the region of 500–540 nm, and samples containing S₄^•– and S₂^•– radical anions have a color close to that of tugtupite [13]. During heating, the decomposition of the S₄^•– radical anions occurs with a simultaneous increase in the concentration of S₂^•– [13]. This explains the disappearance of photochromism in the heated samples of tugtupite.

3.5. Extra-Framework Components in Vladimirivanovite and their Thermal Transformations

In the absorption spectrum of vladimirivanovite (Sample 7, Figure 19), there is an intense broad table-like band in the range of 465–850 nm with a small step in the region of 505 nm and a weak band with a maximum at 930 nm. The edge absorption starts below 430 nm. The observed absorption spectrum is characteristic of the previously studied haüyne samples [5,12].

The ESR spectrum of vladimirivanovite (Figure 20) shows an intense signal c g₁ = 2.049, g₂ = 2.038 and g₃ = 2.011. This signal is associated with the presence of the S₃^•– radical anions. The same species cause an absorption in the region of 465–850 nm. Based on the values of the components of the g tensor, we can conclude that the geometry of the S₃^•– radical anion is close to S₃^•– in haüyne.

The ESR spectrum of vladimirivanovite also shows a band with g = 2.021 (Figure 20). This ESR signal is characteristic of the S₄^•– radical anion, and this is confirmed both by the presence of a step in the region of 505 nm and by a weak band at 930 nm [13].

When vladimirivanovite is heated above 600 °C, the intensity of the absorption band in the region of 480–850 nm increases, and the step in the absorption spectrum and the ESR signal with g = 2.021 disappears. This is due to the fact that the S₄^•– radical anions are unstable upon heating and transform into S₃^•– [13].

3.6. S-Bearing Groups in Biachellaite and SO₃^2—-Rich Analogue of Marinellite

The Raman spectra of biachellaite (Sample 14) and SO₃^2—-bearing analogue of marinellite (Sample 13a) are presented in Figure 21. Both samples show a strong luminescence under the laser beam. The bands in the ranges of 1090–1170, 989–992, 614–640 and 436–458 cm⁻¹ correspond to the vibrations of the SO₄²⁻ anions (the F₂(ν₃), A₁(ν₁), F₂(ν₄) and E(ν₂) modes, respectively). The additional bands at 1037, 965 and 678 cm⁻¹ observed in the Raman spectrum of the SO₃^2—-bearing analogue of marinellite as well as the splitting of the band in the range of 436–458 cm⁻¹ are due to the presence of the SO₃^2— anions. Analogous bands in the Raman spectrum of hannebachite, Ca(SO₃)·0.5H₂O, observed at 1005, 969 and 655 cm⁻¹ and in the range of 444–520 cm⁻¹ have been assigned to the A₁(ν₁), E(ν₃), A₁(ν₂) and E(ν₄) modes, respectively [71]. However, some components of a triplet observed in the Raman spectrum of hannebachite in the range of 444–520 cm⁻¹ may correspond to resonance modes involving the H₂O liberation. Similarly, the doublet 436 + 458 cm⁻¹ in the Raman spectrum of Sample 13a may be due to a resonance between the E(ν₂) and E(ν₄) modes and the SO₄^2— and SO₃^2— anions occurring in the liottite cage.

Marinelite has a bluish-violet color due to the presence of an absorption band with a maximum at ~580 nm (Figure 22). The absorption spectrum also shows an increase in the absorption towards the short wavelength, starting from 400 nm. When excited by radiation with a wavelength of about 400 nm, marinellite exhibits an orange–red luminescence with a maximum at 580 nm, which has a characteristic vibrational structure with phonon repetitions at a distance of 535 cm⁻¹. The ESR spectrum of marinellite (Figure 23) shows a signal with g₁ = 2.220 and g₂ = 2.007. This signal and luminescence are associated with the presence of S₂^•– radical anions in marinellite (see [92]). Additionally, an intense signal with g = 2.019 is observed in the ESR spectrum. The presence of this EPR signal and the absorption band at 580 nm may indicate the presence in marinellite of S₃^•– radical anions, the point symmetry group which is close to D_3h, i.e., the sulfur ions form a three-fold ring close to a regular one [92,93,94]. Usually, such a configuration of the S₃^•– radical anion is not realized in the minerals.

The sample of biachellaite (Sample 14), when excited by a radiation with a wavelength of 405 nm, also shows a pinkish luminescence with a maximum of about 620 nm and a characteristic vibrational structure, where the phonon repetition frequency is about 500 cm⁻¹ (Figure 24). The ESR spectrum of biachellaite (Figure 24) shows a signal with the g-tensor components of 2.20, 2.02 and 1.99. This ESR signal and the observed pinkish luminescence can be attributed to the S₃^•– radical anions [94].

3.7. S-Bearing Scapolites

The assignment of the Raman bands of the SO₄^2—-rich meionite (Sample 15, Figure 25) is as follows. The peaks at 1102 and 1500 cm⁻¹ correspond to the symmetric and asymmetric stretching modes of the CO₃^2— anions, respectively. The bands at 986 and 1038 cm⁻¹ are due to the symmetric and asymmetric stretching modes of the SO₄^2— anions, respectively. SO₄^2— bending modes correspond to the peaks at 416, 584 and 648 cm⁻¹. Other bands in the range of 400–800 cm⁻¹ are due to the vibrations of the aluminosilicate framework. The bands of the lattice modes involving translational and rotational vibrations of the extra-framework components are observed below 400 cm⁻¹.

Weak bands above 1700 cm⁻¹ could not be assigned to any overtones or combination modes. Presumably, these bands correspond to extremely strong hydrogen bonds in local situations with H₃O⁺ at the Ca²⁺-dominant site and H₂O at the CO₃²⁻-dominant site, which would result in the formation of Zundel cation, H₅O₂⁺, by analogy with the nosean [12].

In the Raman spectrum of the SO₄²⁻-poor meionite (Sample 16, Figure 25) bands, the SO₄^2— groups are relatively week and the bands of presumed strong hydrogen bonds corresponding to a hydrated proton complex (in the range of 1190–3300 cm⁻¹ [76]) are stronger than the analogous bands of Sample 15.

As a rule, the Raman band of the symmetric stretching vibrations of the carbonate anion is observed in the range of 1040–1100 cm⁻¹ (usually from 1060 to 1090 cm⁻¹). Among several hundred carbonate minerals, few exceptions are several magnesium carbonates, two uranyl carbonates (kamotoite and rutherfordine) and tunisite, for which this band is shifted to higher frequencies [95]. Thus, the band at 1102 cm⁻¹ in the Raman band of Sample 15 can be considered as an anomaly, which may be caused by steric hindrances, which may increase with the enhancement of the content of the sulfate groups in the mineral.

3.8. Synthesis and Thermal Conversions of S-Bearing Sodalite- and Cancribite-Type Materials

Figure 26 illustrates the morphological features of the aggregates of the products of synthesis. The chemical data (Table 5), unit cell parameters (Table 4) and Raman spectrum (curve 1 in Figure 27) of the powder obtained in Experiment 1, correspond to sapozhnikovite Na₈[Al₆Si₆O₂₄](HS)₂, a newly described member of the sodalite group [7]. In particular, the Raman bands at 260, 455, 996 and 2666 cm⁻¹ correspond to the bending vibrations of the [(HS)Na₄]³⁺ cluster, stretching the vibrations of the [(HS)Na₄]³⁺ cluster, a stretching mode of the aluminosilicate framework and stretching vibrations of the HS⁻ anion. The broad Raman bands of this sample may correspond to an admixed X-ray amorphous phase. This is the first example of the synthesis of a HS-bearing sodalite-type compound. The synthetic analogue of sapozhnikovite occurs as tiny rhombic dodecahedral crystals up to 9 μm across (Figure 26a).

The heating of the synthetic analogue of sapozhnikovite under oxidizing conditions (Experiment 1a) resulted in the disappearance of the HS⁻ anion and the formation of the SO₄²⁻ group. In particular, the Raman bands at 433, 616, 980 and 1130 cm⁻¹ belong to the bending and stretching modes of SO₄ tetrahedron [5,8,12,13]. The bands at 311–336 and 1341 cm⁻¹ can be interpreted as mixed and antisymmetric 2ν₃ modes of the cis-S₄ molecule, respectively. The band at 2159 corresponds to an acid group, presumably HSO₄⁻.

The Raman spectrum of the synthetic sapozhnikovite analogue treated under reducing conditions (Experiment 1b) demonstrates the presence of SO₄²⁻ and S₃^•– that are the most stable S-bearing groups at temperatures above 800 °C. The band at 976 cm⁻¹ is due to the symmetric vibrations of the sulfate anions, whereas the bands at 254 cm⁻¹ (bending ν₂ mode), 544 cm⁻¹ (symmetric stretching ν₁ mode), 802 cm⁻¹ (combination mode ν₁ + ν₂), 1091 cm⁻¹ (overtone 2’ν₁), 1640 cm⁻¹ (overtone 3 × ν₁) and multiple bands above 1500 cm⁻¹ (different combination modes and overtones) correspond to S₃^•– [5,8,12,13]. The weak bands at 673 and 1337 cm⁻¹ correspond to the trans-S₄ symmetric stretching ν₃ mode and its first overtone, respectively. The band at 589 cm⁻¹ may be due to the stretching vibrations of the [(S^2-)Na₄]²⁺ cluster. Thus, the product of Experiment 1b is close to the minerals belonging to the lazurite-haüyne solid-solution series described by us earlier [5,8,12,13]. The presence of significant amounts of the S₃^•– radical anions in the product of Experiment 1b determines the deep blue color of its powder.

The transformation of the HS⁻ anion in the β-cage in the crystal structure of the synthetic analogue of sapozhnikovite during heating and the formation of the large sulfate group results in a phase transition accompanied by a significant expansion of the framework and, respectively, in the enhancement of the unit cell parameter (Table 4). The powder X-ray diffraction patterns of the products of Experiments 1, 1a and 1b are shown in Figure 28. The crystal structure of the initial sample corresponds to the space group P-43n. The strong reflections [100] and [300] which appeared in the XRD patterns of the products of Experiments 1a and 1b and indicate the space group P23, correspond to a sodalite-type structure with ordered [Na₄X]²⁺⁻³⁺ and [Na₄(H₂O)]⁴⁺ clusters, where X is an anion [96,97]. The heating of the studied compounds can result in both a water release and the transformation of the S-bearing groups. This causes changes in the composition of the extra-framework components and their redistribution, which may result in the commensurate and incommensurate modulations of the crystal structure [5,6].

The product of Experiment 2 with the Na₂SO₃·5H₂O salt contains both sodalite- and cancrinite-type compounds (Figure 28 and Figure 29, Table 4). Presumably, the cancrinite-type phase occurs as tiny elongate crystals in a matrix of a fine-grained aggregate containing a sodalite-type compound. The strongest band at 1077 cm⁻¹ observed in the Raman spectrum of this material corresponds to the symmetric stretching vibrations of the CO₃²⁻ anion that does not belong to a cancrinite-type phase or a sodium carbonate. Thus, the carbonate group occurs in the sodalite-type phase. The bands at 991 and 968 cm⁻¹ as well as 674 and (partly) 450 cm⁻¹ may be due to the stretching and bending vibrations of a minor admixture of the SO₃²⁻ anions. The band at 1331 cm⁻¹ may correspond to a Fermi resonance mode of the CO₂ molecules involved in strong dipole–dipole interactions. The remaining (weak) bands in the ranges of 100–400, 600–900 and 1200–3000 cm⁻¹ correspond to soft lattice modes, the framework vibrations and the O–H stretching vibrations of the Zundel cation, H₅O₂⁺, respectively. Both the chemical composition and the Raman spectrum of the product of Experiment 2a correspond to the Na-deficient nosean (Table 4, Figure 29). In this reference, it is to be noted that the partial substitution of Na⁺, belonging to the Na₄·H₂O cluster with the Na···O distance of 2.6 Å in the nosean structure, with hydronium results in the formation of the Zundel cation [12].

A very small a parameter of the sodalite-type phase (8.8867 Å) indicates that a major part of the sulfur belongs to a very small anion (presumably, S²⁻) rather than SO₃²⁻ or SO₄²⁻. According to the local microanalysis of the chemical composition, sulfur occurs only in the cancrinite-type phase, whereas S-bearing species are absent in the fine-grained aggregates containing a sodalite-type phase.

The initial sample is a mixture of the sulfite analogue of Ca-free cancrinite (space group P6₃) with a subordinate S-free sodalite-type phase. The powder X-ray diffraction data show that a thermal treatment results in the transition of the cancrinite-type phase to a compound with the sodalite-type structure (Figure 30). The heating of this sample under oxidizing conditions results in the decomposition of the carbonate groups and appearance of a full set of bands corresponding to fundamentals of the SO₄²⁻ anion (the peaks at 427, 615, 978, 1079 and 1130 cm⁻¹). The sulfate groups remain stable during heating under reducing conditions at 700 °C (Experiment 2b; see Figure 29). In the annealed samples (Experiments 2a and 2b), the main phase belongs to the sodalite type with space group P23 as in Experiments 1a and 1b. This transformation is accompanied by the enhancement of the unit cell parameter of the sodalite-type phase from 8.8867 Å to 9.0928 Å (Table 4). The structural transition can be due to the stabilizing effect of SO₄^2- and, to a lesser extent, SO₃^2- on the sodalite-type crystal structure at high temperatures. The impurity phase detected in Experiments 2a and 2b is a synthetic analogue of nepheline (Figure 30).

According to the chemical, powder X-ray diffraction and Raman spectroscopy data (Figure 31 and Figure 32, Table 4 and Table 5), the main phase in the product of Experiment 3 is a cancrinite-type compound with the thiosulfate group as the dominating extra-framework anion. In the Raman spectrum (Figure 31), a set of characteristic bands of the S₂O₃²⁻ anion is observed: the strongest band at 449 cm⁻¹ corresponds to the S–S stretching A1 mode; the bands at 995, 1135 and 1079 cm⁻¹ are due to the symmetric, asymmetric and mixed S–O stretching modes, respectively. The bands at 543 and 664 cm⁻¹ correspond to the O–S–O bending modes. The assignment of the Raman bands of S₂O₃²⁻ was made using data from [98,99,100,101,102,103].

4. Discussion

4.1. General Remarks on the Isomorphism of S-bearing Species in Feldspathoids

All available data on the crystal structures of S-bearing feldspathoids show that among acyclic polysulfide groups, S_n, S_n²⁻ and S_n^•–, only species with an n < 5 can occur in sodalite cages. The presence of the cyclic S₆ molecule in slyudyankaite, a mineral with the sodalite framework topology, results in the symmetry lowering to the triclinic and regular alternation of S₆- and SO₄²⁻-bearing sodalite cages. The acyclic S₅²⁻ anion is known only in the Losod cages of minerals belonging to the bystrite–sulfhydrylbystrite solid-solution series, for which the presence of variable amounts of the HS⁻ anion is typical. Except bystrite-type minerals, the HS⁻ anion is known in sapozhnikovite [7] and the products of the heating of other S-bearing sodalite-group minerals under strongly reducing conditions (over the Fe-FeS buffer) [13]. Based on these facts and taking into account the occurrence of sapozhnihovite in a close association with the oxalate member of the cancrinite group kianoxalite, one can conclude that the HS⁻ and S₅²⁻ anions can be considered as the markers of extremely reducing conditions. Moderately reducing conditions contribute to the formation of the sulfite analogues of cancrinite-group minerals, including the SO₃²⁻-bearing analogue of marinellite described in this paper and sulfite minerals with the afghanite-type framework described elsewhere [9,103,104].

4.2. Structural Relationships between S-Bearing Sodalite-Group Minerals

Some chemical and structural data for S-bearing minerals with the sodalite-type framework topology are given in

Table 7. Data on S-bearing sodalite-group minerals.

Mineral	SO3, wt.% S *, wt.%	Kinds of Sulfide Groups	Symmetry	Unit Cell Parameters	References
Nosean	10.27 0	-	Cubic	a = 9.084(2) Å	[12,105]
Haüyne	14.2 0	-	Cubic	a = 9.1164(5) Å	[106]
SO42−-deficient haüyne	12.43 0.11	S2•–, S3•–	Cubic **	a = 9.065(1) Å	[5]
S4-bearing haüyne (Sample 5)	13.89 1.31	S4, S3•–	Cubic **	a = 9.071(2) Å	[5], this work
Slyudyankaite	10.95 1.75	S6, S4, S3•–	Triclinic **	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)°	[107]
“Monoclinic lazurite”	10.89 1.94	S3•–, S4	Mono- clinic **	a = 9.069(6) Å b = 12.868(1) Å c = 25.744(1) Å γ = 90.19(1) (1)°	[108,109], this work
Vladimirivanovite	11.37 1.94	S3•–, S2•–	Ortho- rhombic **	a = 9.066(2) Å b = 12.851(3) Å c = 38.558(8) Å	[64], this work
Sapozhnikovite	0 4.20	HS−, S2•–	Cubic	a = 8.91462(7) Å	[7]
Lazurite	8.08 5.07	S3•–, S2−	Cubic **	a = 9.087(2) Å	[6]

* Total content of sulfide sulfur. ** Structures with commensurate and/or incommensurate modulation. The main sulfide groups are indicated in bold type.

. As one can see from these data, minerals with the lowest (<1.4 wt.%) and highest (>4 wt.%) contents of sulfide sulfur are cubic, whereas minerals with intermediate contents of sulfide sulfur have a lower symmetry. The simultaneous presence of high amounts of sulfate anion, trisulfide radical anion or cyclic hexasulfide molecule in a sodalite-group mineral results in a commensurate and/or incommensurate modulation of the crystal structure that may be due to differences in the sizes of sodalite cages hosting large SO₄²⁻, S₃^•– or S₆ groups. Significant amounts of these groups are present in the structures of all S-bearing minerals with the sodalite-type framework topology, except sapozhnikovite. As a result, sapozhnnikovite has a low unit cell a parameter (<8.92 Å), whereas the a parameter of other S-bearing sodalite-group minerals is in the ranges of 9.05–9.12 Å.

The symmetry lowering in the minerals with intermediate contents of sulfide sulfur may be due to the different sizes of the sodalite cages hosting sulfate and sulfide groups that results in strains and distortions of the framework. Hypothetically, in lazurite (a mineral with a cubic unit cell and the largest a parameter among the sulfide-bearing members of the sodalite group), the straining of the framework is small because of a partial ordering of SO₄²⁻ and S₃^•–.

4.3. Thiosulfate Anion as an Extra-Framework Constituent of Cancrinite-Type Compounds

The thiosulfate analogues of cancrinite and sodalite are unknown in nature. The data on the first synthesis of the phase with the empirical formula Na_6.9[Al_5.6Si_6.4O₂₄](S₂O₃)·nH₂O shows that there is only one S₂O₃²⁻ group per unit cell and the crystal structure of this phase has the space group P3 with the unit cell parameters a = 12.624(2) and c = 5.170(1) Å [102]. Our data on the thiosulfate cancrinite-type phase do not contradict the previous results on the crystal structure.

There are no data on the Raman spectroscopy of the thiosulfate anion as a constituent of sodalite- or cancrinite-type compounds. It is noteworthy that the frequencies of the S₂O₃²⁻ modes determined for the synthetic thiosulfate cancrinite-type phase (Experiment 3) are quite close to those inherent to the free S₂O₃²⁻ anion [100].

4.4. Thermal Transformations of S-Bearing Species in Feldspathoids

Preliminary experiments with the heating of S₄- and CO₂-bearing haüyne (Sample 5) have been described by us earlier [13]. It was shown that the main channel of the transformations of the extra-framework components at 700 °C under reducing conditions (in the presence of Fe-FeS buffer) is SO₄²⁻ → S²⁻ + 2O₂(gas), and the subordinate processes are CO₂ + 2SO₄²⁻ + H₂O → 2HS⁻ + CO₃²⁻ + 4O₂(gas), 3SO₄²⁻ → S₃^•– +5e +6O₂(gas), 2S₃^•– → S₂^•– + S₄^•– and 2CO₂ + 2e → C₂O₄²⁻ (e = electron). The 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. These conclusions are in agreement with the Raman spectra shown in Figure 13.

Based on the Raman spectroscopy data, one can suppose that the main channel of the high-temperature transformations of the HS⁻ anion in the synthetic analogue of sapozhnikovite is 2HS⁻ (solid) + 2.5O₂ (gas) → SO₄²⁻ (solid) + 0.25S₄ (solid) + H₂O (gas). Hypothetically, the subordinate channels of the transformations of the HS⁻ anion during the heating of this sample under oxidizing and moderately reducing conditions can be described by the schemes 2HS⁻ (solid) + 3.5O₂ (gas) → SO₄²⁻ (solid) + SO₂ (gas) + H₂O (gas) and 6HS⁻ (solid) → S₃^•– (solid) + 3H₂S (gas), respectively.

4.5. Color Centers in Feldspathoids and Related Minerals

The only strong red chromophore among the tetrasulfur species is the planar non-cyclic C2_v isomer of the neutral S₄ molecule [47]. As noted above, all strong bands of this molecule are present in the Raman spectrum of orange sulfhydrylbystrite, unlike yellow bystrite. Thus, it can be concluded that the color of the bystrite and sulfhydrylbystrite is due to the presence of S₅²⁻ (yellow chromophore) and the pair S₅²⁻ + S₄, respectively.

Undistorted S₃^•– radical anions are the cause of the blue color of kyanoxalite and different sodalite-group minerals, including lazurite, haüyne and vladimirivanovite. The bluish-violet color of the sulfite analogue of marinellite is unusual for cancrinite-group minerals. The ESR spectrum of this mineral shows the presence of distorted S₃^•– radical anions (with a lowered S–S–S angle), which are the most probable chromophore.

The colored and uncolored samples of tugtupite differ only in the presence of an absorption band with a maximum in the region of 520 nm. Otherwise, the absorption spectra in the range of 300–2000 nm coincide. Thus, we can assume the following mechanism of photochromism in tugtupite: ${\mathrm{S}}_4^{2-}+{\mathrm{S}}_2^{•-}\stackrel{h\nu =400 nm}{\to }{\mathrm{S}}_4^{•-}+{\mathrm{S}}_2^{2-}\stackrel{h\nu =500 nm}{\to }{\mathrm{S}}_4^{2-}+{\mathrm{S}}_2^{•-}$. No significant differences are found in the absorption spectra of colored and uncolored samples, since the absorption bands of S₄^•– are in the same spectral region as those of S₂^•– [93].

The bluish-violet color of meionite (Sample 16) is related with an absorption band with a maximum at 600 nm and vibrational repetitions at a distance of about 1100 cm⁻¹. A similar color was observed for (CO₃)²⁻-bearing marialite after the irradiation by short-wave ultraviolet light [89]. According to our preliminary data, the ESR spectra of the bluish samples of the scapolite-group minerals contain a signal with g = 2.014. Earlier, similar spectra were observed for other blue minerals containing carbonate anion, namely, cancrinite [110,111] and CO₃²⁻-bearing kyanoxalite [12]. In these minerals, the (CO₃)^•– radical anion is the main color center.

5. Conclusions

The application of a multimethodical approach involving various spectroscopic methods, as well as chemical and structural data, to feldspathoids belonging to the sodalite, cancrinite and scapolite groups made it possible to identify various S-bearing extra-framework anions, radical anions and neutral molecules. The complex mutual transformations of these components are observed during heating at high temperatures.

Based on these data and data on the synthesis of S-bearing feldspathoids under reducing and oxidizing conditions, it was confirmed that the composition of an extra-framework species in these minerals can be considered as a marker of mineral-forming media, including the fugacities of the volatile components (H₂O, O₂, CO₂, HF, SO₂ and polysulfide compounds).

The SO₄²⁻ anion and the S₃^•– radical anion are the most stable species at high temperatures under oxidizing conditions. Under reducing conditions, depending on the temperature and charge-balance requirement, these species transform into S²⁻, HS⁻, S₄^•–, S₂^•– and/or S_4.

The S-bearing species S₂^•–, S₃^•–, S₄^•–, S₄ and S₅²⁻, as well as S₆ (in slyudyankaite), are the most common color centers in feldspathoids. The luminescence of these minerals under UV light is mainly due to the presence of S₂^•– and/or Fe³⁺.