AbstractThe 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 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 with T, 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 measured T, were calculated. The magnitude and the orientation of all the three unit-strain coefficients are almost maintained constant with T. At T-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 AlO6 octahedra.
Zeolites at high pressure: A review