Abstract

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 was observed. The axial and volume thermal expansion coefficient (αj = lj−1(∂lj/∂T), αV = V−1 (∂V/∂T)), 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 αabc = 1.06:1.14:1. All three angles of the kyanite lattice show a slight decrease with T, with ∂α/∂T = −2(2)×10−5, ∂β/∂T = −4(1)×10−5, ∂γ/∂T = −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, ε1a = 76(2)°, ε1b = 70(2)°, ε1c = 38(3)°, ε2a = 49(3)°, ε2b = 66(3)°, ε2c = 127(4)°, ε3a = 135(3)°, ε3b = 31(3)°, ε3c = 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.

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