Thermal expansion of andalusite and sillimanite at ambient pressure: a powder X-ray diffraction study up to 1000°C

The unit-cell parameters of andalusite and sillimanite have been measured by high-T powder X-ray diffraction up to 1000 degrees C at ambient pressure. Within the temperature range investigated, all the unit-cell parameters varied smoothly, indicating no phase transition. The volume-temperature data were fitted with a polynomial expression for the thermal expansion coefficient (alpha(T) = a(0) + a(1)T + a(2)T(-2)), yielding a(0) = 2.55(2) x 10(-5) K(-1), a(1) = 0 and a(2) = 0 for andalusite, and a(0) = 1.40(4) x 10(-5)K(-1), a(1) = 7.1(8) x 10(-9)K(-2) and a(2) = 0 for sillimanite. Using the new thermal expansion data determined in the present study and compressional data from the literature, the P-T phase relations of the kyanite-andalusite-sillimanite system were calculated thermodynamically, with the invariant point located at similar to 523 degrees C and 3.93 kbar.