Thermal expansion and phase transitions in akermanite and gehlenite

Thermal expansion has been measured by laboratory and synchrotron X-ray powder diffraction for end-member (a) over circle kermanite (ak, Ca2MgSi2O7) and gehlenite (ge, Ca2Al2SiO7) in the range 20-1,500 K. In ak in the range 340-390 K, there is a negative linear thermal expansion in [001] direction. This is related to the phase transition from an incommensurate modulated structure (IC) to a normal one (N). The volumetric mean thermal expansion coefficients for ak and ge, obtained with a linear fit of the experimental data in the temperature range 298-1,400 K, are respectively 32.1x10(-6) and 28.3x10(-6) K-1. The variation of the c/a ratio with temperature, due to different thermal expansion along the crystallographic axes, can be related to the different behaviour of the tetrahedral layers in the N and IC phases. Analysis of the variation of the superstructure peaks intensity across the phase transition confirms the tricritical behaviour of the IC/N transition in ak.