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.

Acknowledgements

The author thanks A.N. Sapozhnikov and A.A. Kashaev for the screening of suitable crystals; A. Schönleber and G. Chapuis for the fine data set obtained from the cubic lazurite and V.N. Molchanov for the assistance with X-ray measurements. We appreciate the financial support of the Russian Foundation of Basic Research, grant 04-05-64192, as well as the Scientific School NSh-4964.2006.5.