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

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-8(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-, S-4 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 S-4-bearing hauyne under oxidizing and reducing conditions have been studied using the multimethodical approach. The SO42- and S2- anions and the S-3(center dot-) radical anion are the most stable S-bearing species under high-temperature conditions (in the range of 700-800 degrees C); their ratio in the heated samples is determined by the redox conditions and charge-balance requirement. The HS- and S-5(2-) anions are stable only under highly reducing conditions.