Sillimanite-disordering enthalpy by calorimetry

Heat of solution measurements in an oxide-melt were performed on samples of natural sillimanite heat-treated in the range 1200-1700°C at pressures of 16-23 kbar. A distinct enthalpy of solution decrement relative to the unheated sillimanite of about 1.3 kc/mole is shown by samples run at 1400-1550°C. Pressure variations in the range 16-23 kbar cause little change in the heat of solution in this temperature range. This pressure-independent 'plateau' in heat of solution is interpreted to be due to Al-Si disordering on tetrahedral sites in the sillimanite structure. A simple temperature-dependent disordering model developed by navrotsky and Kleppa (1967) for spinels leads to an Al-Si interchange enthalpy, Δ_Hnt, of16 ± 1 kcal/mole, in good agreement with the value derived by Holdaway(1971) on entirely different grounds. Above 1550°C, larger heat of solution decrements were observed. Microprobe analyses of quenched samples indicate that the sillimanite has not deviated significantly from the ideal formula. Some unknown profound disordering process may account for the heat effects in the very high temperature range. Unit cell volumes of quenched samples also describe a 'plateau' region in the temperature range 1400-1550°C. This plateau consists of an increase of the 6 crystallographic axis beginning at 1350°C without much change in the other axes in the range 1350-1550°C. A sudden expansion of the α-axis occurs between 1550 and 1630°C. We conclude that Al-Si disorder of the type postulated by Beger et al. (1970), and Holdaway (1971) has been confirmed calorimetrically for samples heated under pressure in the temperature range 1400-1550°C.