Vladimirivanovite Revised: General Crystal Chemistry and Isomorphous Substitutions of Extra-Framework Species

Abstract

New data on the crystal structure, chemical composition, and nature of extra-framework components of the orthorhombic sodalite-group mineral vladimirivanovite were obtained using chemical and single-crystal X-ray diffraction data as well as infrared and Raman spectroscopy. The crystal structure of vladimirivanovite is based on the sodalite-type aluminosilicate framework with ordered Al and Si atoms. Sodalite-like cages are mainly occupied by Na+ and Ca2+ cations and (SO4)2− anions. It was shown that vladimirivanovite is characterized by significant variations in the content of extra-framework polysulfide groups (S3•−, S4), as well as other neutral molecules (H2O and CO2), the presence of which in the structure is the main cause of structural modulations and the orientation disordering of sulfate anions. Three samples with different S3•−:S4 ratios were studied. All of them are orthorhombic (space group Pnaa) with the unit-cell parameters a ≈ 9.1, b ≈ 12.9, and c ≈ 38.6 Å; Z = 6. The general crystal-chemical formula of vladimirivanovite is (Na+6.0–6.4Ca2+1.5–1.7)(Al6Si6O24)(SO42−,S3•−,S4)1.7–1.9(CO2)0–0.1·nH2O (n = 1–3), where the S4 molecule occurs in different conformation states.