Average structure of incommensurately modulated monoclinic lazurite

The average structure of the monoclinic modification of lazurite Ca1.26Na6.63K0.04[Al6Si6O24](SO4)(1.53)S0.99Cl0.05 (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) angstrom, b = 12.868(1) angstrom, c = 12.872(1) angstrom, gamma = 90.19(1)degrees, sp. gr. Pa, R = 6.9%, 2057 reflections. The AlO4 and SiO4 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 SO4, S-2, S-3, and SO2 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.