Average structure of incommensurately modulated monoclinic lazurite

The average structure of the monoclinic modification of lazurite Ca 1.26 Na 6.63 K 0.04 [Al 6 Si 6 O 24 ](SO 4 ) 1.53 S 0.99 Cl 0.05 (discovered in the Lake Baikal region) incommensurately modulated along the c axis is solved by the single-crystal X-ray diffraction method. The unit-cell parameters are a = 9.069(1) Å, b = 12.868(1) Å, c = 12.872(1) Å, γ = 90.19(1)°, sp. gr. Pa, R = 6.9%, 2057 reflections. The AlO 4 and SiO 4 tetrahedra form a partially ordered framework. The positions in the cavities of the framework are split and randomly occupied by Na and Ca atoms and the SO 4 , S 2 , S 3 , and SO 2 anionic groups. The structure of the mineral is compared with the superstructure of triclinic lazurite. Conclusions are drawn about the causes of the incommensurate modulation in monoclinic and other lazurites.