Abstract

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.

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