The lattice distortion and possible causes of the lazurite incommensurate and commensurate modulation structure

By methods of X-ray structure analysis commensurate (one dimensional) and incommensurate (three-dimensional) modulations of the lazurite structure from Baikal deposits are considered. The analysis of the X-ray diffraction powder and single crystal data showed that the one-dimensional (anisotropic) modulation deforms the lazurite cubic structure and is manifested in a broadening and splitting of sublattice lines on the powder diffraction pattern of the mineral. At a three-dimensional modulation, the cubic structure is maintained. It is concluded that a density modulation is a cause of the incommensurate modulation of the lazurite structure. Due to this arrangement, a crystallographic equivalency of subcells is maintained. The cause of the commensurate modulation is an ordered distribution of intraframework units and the displacement modulation where all atoms of the mineral structure participate.