The structure of lazurite, ideally Na6Ca2(Al6Si6O24)S2, a member of the sodalite group.

Crystal structure determinations are reported for two specimens of lazurite, from Sary-Sang, Afghanistan, and from Baffin Island. Al and Si are completely ordered. There is positional disorder of: 1) the framework O atoms over two sets of sites, 2) the inter-framework cations over three sets of sites and 3) the SO4 groups in two orientations in the Baffin Island specimen only (ordered in one orientation in the Afghanistan specimen). The S^2- ions occupy different size cages from 031SO4^2- and it is this which leads to positional disorder of the framework O atoms. The observed superstructure reflections appear to be caused by positional and substitutional ordering of all the atoms and not just the channel ions.-J.E.C.