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-Tpowder X-ray diffraction up to 1000°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 (αT=a0+a1T+a2T-2). yieldinga0=2.55(2) × 10–5K–1,al=0 anda2=0 for andalusite, anda0=1.40(4) × 10–5K–1a1=7.1(8) × 10–9K–2anda2=0 for sillimanite. Using the new thermal expansion data determined in the present study and compressional data from the literature, theP-Tphase relations of the kyanite-andalusite-sillimanite system were calculated thermodynamically, with the invariant point located at ∼523°C and 3.93 kbar.