Evolution of the modulated superstructure of nepheline at high-pressure: in-sity single-crystal X-ray diffraction study

The pressure-induced structural evolution and the elastic behavior of a natural nepheline were investigated by in-situ single-crystal X-ray diffraction up to 7.5 GPa under hydrostatic conditions. The reflection conditions confirm that the space group of the average structure of nepheline remains as P63 throughout the pressure range investigated. At room pressure the diffraction pattern of nepheline includes satellite reflections while the structure refinement to the Bragg reflections confirms that the O1 site is displaced from the triad at (2/3, 1/3, z). At P~1.8 GPa no significant intensity of the satellites was detected. Over the same pressure range the O1 site moved towards the triad and thus the tilts of the T1 and T2 tetrahedra decreased. When the satellites disappear at pressures above 2 GPa, the O1 site is on the triad at (2/3, 1/3, z), corresponding to a straight T1-O1-T2 bond. The presence of the subsidiary non-Bragg reflections is therefore related to the split of the O1 site and tilts of T1 and T2. Below 2 GPa the structure responds to increased pressure by tilting of all four tetrahedra, and above 2 GPa by tilting of the T3 and T4 tetrahedra alone. The change in compression mechanism arising from the changes in the O1 position is associated with changes in the compression of the unit-cell axes and the unit-cell volume. The elastic behavior was described with a 4th-order Birch-Murnaghan Equation-of-State. Fitting the P-V data, the refined parameters are: V0=723.57(4)Å3, KT0= 47.32(26) GPa, K’=2.77(24) and K’’=0.758(79) GPa-1. The pressure-induced structural evolution in nepheline up to 7.5 GPa appears to be completely reversible. The recovery of the modulation upon complete pressure release points to the framework of nepheline having an instability corresponding to a rigid-unit mode with a wave vector corresponding to the observed positions of the satellite reflections.