Thermal expansion of kyanite at ambient pressure: An X-ray powder diffraction study up to 1000 °C

The thermal expansion coefficients of kyanite at ambient pressure have been investigated by an X-ray powder diffraction technique with temperatures up to 1000 °C. No phase transition was observed in the experimental temperature range. Data for the unit-cell parameters and temperatures were fitted empirically resulting in the following thermal expansion coefficients: αa = 5.8(3) ×× 10−5, αb = 5.8(1) ×× 10−5, αc = 5.2(1) ×× 10−5, and αV = 7.4(1) ×× 10−3 °C−1, in good agreement with a recent neutron powder diffraction study. On the other hand, the variation of the unit-cell angles α, β and γ of kyanite with increase in temperature is very complicated, and the agreement among all studies is poor. The thermal expansion data at ambient pressure reported here and the compression data at ambient temperature from the literature suggest that, for the kyanite lattice, the most and least thermally expandable directions correspond to the most and least compressible directions, respectively.