Entropies of kyanite, andalusite, and sillimanite: additional constraints on the pressure and temperature of the Al₂SiO₅ triple point

The low-temperature heat capacities of kyanite (Minas Gerais, Brazil), andalusite (Espirito Santo, Brazil), and sillimanite (Reinbolt Hills, Antarctica) were measured with an automatic, adiabatically shielded calorimeter between approximately 10 and 380 K. At 298.15 K the entropies are 82.30±0.13, 91.39±0.14, and 95.79±0.14 J/(mol · K) for kyanite, andalusite, and sillimanite, respectively. Our values are 1.8, 2.0 and 0.4% smaller than those of Todd (1950). Our calculated slope for the andalusite–sillimanite phase boundary is in significantly better agreement with the phase boundaries of Holdaway (1971), than with those of Richardson, Gilbert and Bell (1969) and strongly suggest that Al and Si are ordered in sillimanite to at least 1100 K.

The thermal Debye temperatures, $θDc$⁠, calculated from our heat capacity data in the range T < 18 K are 1100, 855, and 730 K for kyanite, andalusite, and sillimanite respectively. For andalusite and sillimanite our values for $θDc$ are in moderate agreement with those calculated from the room temperature adiabatic elastic stiffness constants of Vaughan and Weidner (1978), 838 and 800 K, respectively.