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

Abstract

AbstractThe high-temperature (HT) behaviour of kyanite (Al2SiO5) was investigated byin situneutron powder diffraction up to 1200°C. Within the investigatedTrange, no phase transition was observed. The axial and volume thermal expansion coefficient (αj= lj-1(𝜕1j/𝜕T), αv= V-1(𝜕V/𝜕T)), calculated by weighted linear regression through the data points, are: αa= 5.5(2) x 10-5, αb= 5.9(2) x 10-5, αc= 5.18(8) xl0-5, αv= 7.4(1) x 10-3“C-1, with αa:αb:αc= 1.06:1.14:1. All three angles of the kyanite lattice show a slight decrease withT,with𝜕α/𝜕T =-2(2) x 10-5, 𝜕β/𝜕T = -4(1) x 10-5, 𝜕γ/𝜕T = –10(2) x 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 measuredT, were calculated. The magnitude and the orientation of all the three unit-strain coefficients are almost maintained constant withT. AtT-To=1177°C , ε^a= 76(2)°,ε^b =70(2)°, ε^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. 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 AlO6octahedra.