Phase transitions and microstructures in natural kaliophilite

An investigation using TEM, high temperature X-ray powder diffraction and high temperature single-crystal X-ray diffraction revealed previously unreported microstructures and phase transitions in natural kaliophilite. Measurements of unit cell dimensions from room temperature to 895°C indicate the existence of a displacive phase transition at ~725°C between two hexagonal phases with the same unit cell. A symmetry change of P6₃22 ⇄ P6₃ is tentatively suggested with the concomitant development of merohedral twins. 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. -from Authors