Crystal structure of vishnevite using X-ray-diffraction data. The final conventional residue R is 3.7% using space group P6 3 . The structure shows great similarity to that of the carbonate-rich end member of the solid-solution series, however, owing to space limitations, the number of SO 4 (super 2-) radicals is limited to one per unit cell. Although the crystal fragment used in the refinement shows no superstructure, other fragments of the same crystal do. The superstructure can be attributed to the orientation of SO 4 (super 2-) groups and also slight changes in chemistry on unit-cell scale. The ideal formula for vishnevite is Na 8 Al 6 Si 6 O 24 .SO 4 .2H 2 O.--Modified journal abstract.