X-ray powder diffraction and 23Na, 27Al, and 29Si MAS-NMR investigation of nepheline-kalsilite crystalline solutions

Specimens of both natural (Monte Somma) and synthetic Na-rich nepheline have been used to synthesize two ion-exchange series, ranging in composition to pure kalsilite. Excess Si in the natural and synthetic series was 5.2 and 1.7 mol%, respectively. The associated vacancies (VAC) in the alkali sites were found to play an important role in the crystal chemical behavior of these minerals. Room temperature X-ray powder diffraction measurements revealed the presence of three phases in each series: nepheline from sodic compositions to 0.64 (± 0.01) mole fraction K + VAC, tetrakalsilite at 0.72 (± 0.02) K + VAC, and kalsilite above 0.80 K + VAC, with a narrow miscibility gap between K-rich nepheline and tetrakalsilite. Increases of a, c, and volume were linear with K content within each structural region. However, for nepheline the increases accelerated beyond compositions at which K (or K + VAC) was forced to occupy Na structural positions. For the more vacancy-rich natural series this occurred at lower K contents, suggesting preference of vacancies for the larger alkali position. The overall effect of ☐Si exchange in nepheline, in fact, was found to resemble that of large-ion substitution in the alkali site. Both the nepheline to tetrakalsilite and tetrakalsilite to kalsilite transitions were accompanied by apparent discontinuities in the c unit-cell dimension. Unit-cell volume was discontinuous for a hypothetical nepheline to kalsilite transition. Magic-angle-spinning (MAS) NMR data of ²³Na, ²⁷Al, and ²⁹Si for part of the natural series suggest that the nepheline structure is distorted on both a local (unit-cell) and long-range scale with the addition of K. Similarly, the kalsilite structure is distorted with increasing Na content. Stoichiometric nepheline (K + VAC = 0.25) is observed to have the least distorted tetrahedral sites as calculated from the ²⁷Al quadrupolar coupling constant. Lastly, the ²⁹Si and ²⁷Al NMR suggest complete ordering between the Si and Al tetrahedral sites, thus preserving the Al avoidance rule.