Phase transitions and microstructures in natural kaliophilite
Cellai, Daniela; Carpenter, Michael A.; Heaney, Peter J.
European Journal of Mineralogy Volume 4 Number 6 (1992), p. 1209 - 1220
published: Dec 15, 1992
manuscript accepted: Feb 11, 1992
manuscript received: Aug 28, 1991
ArtNo. ESP147050406005, Price: 29.00 €
Abstract An investigation using TEM, high temperature X-ray powder diffraction and high temperature singlecrystal 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. Spontaneous strain determinations suggest that the transition is close to being tricritical in character. Above ~ 750°C kaliophilite crystals undergo a first order epitaxial phase transition to a different hexagonal phase with a = 8.9 A and c = 8.4 A. Dark and bright field images in the TEM show the presence of twins and heterogeneously distributed linear defects. These 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.