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 degrees C. Within the investigated T range, no phase transition was observed. The axial and volume thermal expansion coefficient (alpha(j) = 1(j)(-1)(partial derivative l(j)/partial derivative T), alpha(v) = V-1 (partial derivative V/partial derivative T)), calculated by weighted linear regression through the data points, are: alpha(a) = 5.5(2) x 10(-5), alpha(b) = 5.9(2) x 10(-5), alpha(c) = 5.18(8) x 10(-5), alpha(v) = 7.4(1) x 10(-3) degrees C-1, with alpha(a):alpha(b):alpha(c) = 1.06:1.14: 1. All three angles of the kyanite lattice show a slight decrease with T, with partial derivative alpha/partial derivative T = -2(2) x 10(-5), partial derivative beta/partial derivative T = -4(1) x 10(-5), partial derivative gamma/partial derivative T = -10(2) x 10(-5)degrees/degrees C. The magnitudes of the principal Lagrangian unit-strain coefficients (epsilon(1), epsilon(2), epsilon(3)) 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-T-o = 1177 degrees C, epsilon(boolean AND)(1)a = 76(2)degrees, epsilon(boolean AND)(1)b = 70(2)degrees, epsilon(boolean AND)(1)c 38(3)degrees, epsilon(boolean AND)(2)a = 49(3)degrees, epsilon(boolean AND)(2)b = 66(3)degrees, epsilon(boolean AND)(2)c = 127(4)degrees, epsilon(boolean AND)(3)a = 135(3)degrees, epsilon(boolean AND)(3)b = 31(3)degrees, epsilon(boolean AND)(3)c = 91(2)degrees with epsilon(1):epsilon(2):epsilon(3) = 1.57:1.29:1. The structural refinements, performed at 23, 600, 650, 700, 750, 800, 900, 950, 1050 and 1200 degrees 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.