Isothermal equation of state and structural evolution of the modulated super-structure of nepheline at high-pressure

Nepheline is a feldspathoid, with ideal formula KNa3Al4Si4O16, and occurs in a wide variety of rocks including phonolites, aplites, nepheline-pegmatites and potassic lavas. The crystal structure of nepheline was first solved by means of single-crystal X-ray diffraction, in space group P63. Nepheline has a tetrahedral framework structure that can be considered as a stuffed derivative of that of tridymite. The Si/Al-framework of nepheline consists of 6-membered rings parallel to (001): one-quarter are nearly hexagonal regular rings (hereafter 6mR[001]-1), whereas the other three-quarters are flattened hexagonal rings (hereafter 6mR[001]-2). The 6mR[001]-1 and 6mR[001]-2 rings form two systems of channels running parallel to [001]. The (001)-layers of tetrahedra formed by the 6mR rings are stacked along [001] to build up a 3-dimensional framework. The Si/Al- distribution within the tetrahedral framework of nepheline appears to be highly ordered and a recent study suggests that the degree of Si/Al-order is independent of the temperature of formation. The extra-framework content in the nepheline structure is represented by two independent sites, labeled as “K” and “Na”, which lie in the 6mR[001]-1 and 6mR[001]-2 channels respectively. In natural nephelines the K site is ~60% occupied by potassium with ~20-25% sodium or calcium which leaves about 1/5 of the sites vacant. The coordination number of the K site is 9. The Na site is usually fully occupied by sodium (and minor calcium) and lies in the 6mR[001]-2 channels. Previous studies of nepheline with both single-crystal X-ray and electron-diffraction techniques found additional non-Bragg reflections with respect to the Hahn and Buerger (1955) unit-cell (with a~9.99Å and c~8.37Å). The positions of these subsidiary reflections can be defined in reciprocal space in terms of the normal reciprocal unit-cell for nepheline by coordinates ±(1/3, 1/3, ± z*), with z* ~0.2. The exact value of z* varies with the composition of the nepheline and is irrational. 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.