Phase transitions and microstructures in natural kaliophilite

Measurements of unit cell dimensions from room T to 895 degrees C indicate the existence in kaliophilite of a displacive transition at approx 725 degrees C between two hexagonal phases with the same unit cell. A symmetry change of P6 (sub 3) 22 right and left arrows P6 (sub 3) is tentatively suggested, with the concomitant development of merohedral twins. Spontaneous strain determinations suggest that the transition is close to being tricritical in character. Above approx 750 degrees C kaliophilite crystals undergo a first order epitaxial phase transition to a different hexagonal phase with a 8.9, c 8.4 Angstrom . The presence of twins and heterogeneously distributed linear defects seen by TEM are interpreted respectively as transformation twins and stacking defects incorporated during crystal growth. The thermal evolution of kaliophilite is sufficiently different from that of kalsilite to indicate that it has a different framework topology and is not a true stuffed tridymite structure.