| Summary: | 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 Na<sub>8</sub>(Al<sub>6</sub>Si<sub>6</sub>O<sub>24</sub>)(HS)<sub>2</sub> 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 SO<sub>4</sub><sup>2−</sup>, S<sub>4</sub> and SO<sub>3</sub><sup>2−</sup> 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 S<sub>4</sub>-bearing haüyne under oxidizing and reducing conditions have been studied using the multimethodical approach. The SO<sub>4</sub><sup>2−</sup> and S<sup>2−</sup> anions and the S<sub>3</sub><sup>•–</sup> 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<sup>−</sup> and S<sub>5</sub><sup>2−</sup> anions are stable only under highly reducing conditions.
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