The low-temperature inversion in sub-potassic nephelines

Differential scanning calorimetrie (DSC) and high-temperature X-ray diffraction studies on seven nephelines belonging to the system NaAlSiO₄(Ne)−KAlSiO₄(Ks) show that samples with less than 2.5 mol percent Ks (sub-potassic nephelines) exhibit complex relations. A Type-C; nepheline containing 1.6 percent Ks has a single sharp DSC peak near 360 K; this temperature is close to that at which extra X-ray reflections characteristic of an orthorhombic 3c superstructure disappear (365 K) and marks an inversion to hexagonal symmetry. Type-B nephelines containing 0 to 0.36 percent Ks have two DSC peaks below 480 K which are separated by 20° to 40°. The two peaks confirm the existence of two phases in Type-В nephelines and the presence of two inversions. For each sample, the lower-temperature DSC peak coincides with the disappearance of the orthorhombic superstructure (cf. Type-С) and the higher-temperature peak with the disappearance of extra X-ray reflections typical of a low-symmetry second phase. Type-В and -C nephelines have DSC scans similar to that for tridymite which suggests that the low-temperature inversions in sub-potassic nephelines are caused by the collapse of the ‘tridymite-like’ framework about the relatively small Na⁺ in the larger cation sites. In contrast, nephelines with more than 2.5 percent Ks are hexagonal at room temperature and show no inversions. Thus a Type-H nepheline containing 27 percent Ks does not have extra X-ray reflections and does not show any peaks on DSC scans between 340 and 520 K.

The heat capacity $(Cp∘)$ data for A.407 (pure-Na, Type-В) above 500 К show excellent agreement with the $Cp∘$ equation fitted to the drop calorimetrie data of Kelley et al. (1953) on synthetic pure Na nepheline between 467 and 1180 К and can be used for thermochemical calculations on Ks-bearing nephelines above about 500 K.