On the thermo-elastic behaviour of kyanite: a neutron powder diffraction study up to 1200°C

The high-temperature (HT) behaviour of kyanite (Al2SiO5) was investigated by in-situ neutron powder diffraction up to 1200°C. Within the investigated T-range, no phase transition has been observed. The axial and volume thermal expansion coefficient (αj=lj-1 dlj/dT , αV=V-1 dV/dT), calculated by weighted linear regression through the data points, are: αa = 5.5(2)•10-5, αb = 5.9(2)•10-5, αc = 5.18(8)•10-5, αV = 7.4(1)•10-3 °C-1, with αa:αb:αc = 1.06 : 1.14 : 1.00. All the three angles of the kyanite lattice show a slight decrease with T, with dα/dT= -2(2)•10-5, dβ/dT= -4(1)•10-5, dγ/dT= -10(2)•10-5 °/°C. The magnitudes of the principal Lagrangian unit-strain coefficients (ε1, ε2, ε3) and the orientations of the thermal strain-ellipsoids, between the ambient temperature and each measured T, were calculated. The magnitude and the orientation of all the three unit-strain coefficients are almost maintained constant with T. At T-T0=1177 °C , ε1^a = 76(2)°, ε1^b=70(2)°, ε1^c= 38(3)°, ε2^a= 49(3)°, ε2^b= 66(3)°, ε2^c= 127(4)°, ε3^a= 135(3)°, ε3^b= 31(3)°, ε3^c= 91(2)° with ε1:ε2:ε3 =1.57:1.29:1.00. The structural refinements, performed at 23, 600, 650, 700, 750, 800, 900, 950, 1050 and 1200°C allowed the description of the structural evolution and the main T-induced deformation mechanisms, which are mainly represented by the polyhedral distortions of the AlO6-octahedra.