Microstructures and high-temperature phase transitions in kalsilite

A diversity of twinning and exsolution textures has been observed by transmission electron microscopy in natural kalsilite from volcanic and metamorphic rocks (Mount Nyiragongo, Zaire; Alban Hills, Italy; Kerala, southern India). The phase transitions responsible for these microstructures were examined by high-temperature powder and singlecrystal X-ray diffraction. Pure kalsilite has P6₃mc symmetry at high temperatures but transforms to its P6₃ state through a two-phase field, between ∼870 and ∼920 °C. In this two-phase field, the low form coexists with a structure that has a sixfold repeat of its a unit-cell dimension. For ∼Ks₈₈, the high-temperature state is high tetrakalsilite with possible space group P6₃mc; a two-phase field between ∼890 and ∼930 °C involves the coexistence of this phase with P6₃ kalsilite. High tetrakalsilite with composition ∼Ks₇₄ reverts to low tetrakalsilite on quenching to room temperature from 950 °C. Lattice-parameter variations, and estimates of the Al-O-Si angles that can be derived from them, imply that the stability limit for the high structural states occurs when the angles for apical and basal O atoms converge. At lower temperatures, pure P6₃ kalsilite appears to have transformed during a metamorphic evolution in nature to an intergrowth of the P6₃ structure and a structure for which P31 c symmetry is proposed. The latter can be thought of as a polytype of kalsilite, with (001) layers stacked in an eclipsed array rather than in the staggered array of normal low kalsilite. In this regard, KAlSiO₄ seems to be remarkably similar to KLiSO₄. If ∼ 3-12 mol% NaAlSiO₄ is present in solid solution, a P6₃ structure with an a parameter √3 greater than normal low kalsilite develops. An anomaly in the lattice parameters of Ks88 suggests that the transition temperature below which this superstructure develops may be ∼500 °C. Natural nepheline exsolved from kalsilite displays merohedral twinning, which can be accounted for by a P6₃mc → P6₃ transition during cooling. Transformation behavior in the Ne-Ks system may be explained, at least qualitatively, in terms of Na-K ordering between cavity sites, ordering of basal O atoms between two sites on either side of the mirror plane parallel to the c axis of P6₃mc structures, ordering of apical O atoms between three sites, and coupling between all these processes.