Forms and origin of structure modulation in lazurites

Abstract

The crystal structure of lazurite, ideally Na6Ca2[Al6Si6O24](SO4,S)2, has a flexible framework formed with four- and six-membered rings of the (Al,Si)O4 tetrahedra shared by vertices. The framework cavities are occupied with Na,Ca cations and S-containing anionic groups. Six modulated lazurite polymorphs have been studied using X-ray diffraction techniques. General and special features of the structure modulation of lazurites are discussed for several different structures. A mechanism of commensurate modulation is revealed using the triclinic structure as an example. The 3D incommensurate modulation of the cubic framework is shown. The commensurate 1D-modulation of the triclinic structure is compared with the incommensurate 1D-modulation of the monoclinic structure. The S-containing groups are believed to play an important role in the formation of the structure modulations.