THERMODYNAMIC DATA OF THE HIGH-PRESSURE PHASE MG5AL5SI6O21(OH)7 (MG-SURSASSITE)

Abstract

Calorimetric and P–V–T data for the high-pressure phase Mg5Al5Si6O21(OH)7 (Mg-sursassite) have been obtained. The enthalpy of drop solution of three different samples was measured by high-temperature oxide melt calorimetry in two laboratories (UC Davis, California, and Ruhr University Bochum, Germany) using lead borate (2PbO·B2O3) at T=700°C as solvent. The resulting values were used to calculate the enthalpy of formation from different thermodynamic datasets; they range from −221.1 to −259.4 kJ mol−1 (formation from the oxides) respectively −13892.2 to −13927.9 kJ mol−1 (formation from the elements). The heat capacity of Mg5Al5Si6O21(OH)7 has been measured from T=50°C to T=500°C by differential scanning calorimetry in step-scanning mode. A Berman and Brown (1985)-type four-term equation represents the heat capacity over the entire temperature range to within the experimental uncertainty: C P (Mg-sursassite) =(1571.104 −10560.89×T −0.5−26217890.0 ×T −2+1798861000.0×T −3) J K−1 mol−1 (T in K). The P V T behaviour of Mg-sursassite has been determined under high pressures and high temperatures up to 8 GPa and 800°C using a MAX 80 cubic anvil high-pressure apparatus. The samples were mixed with Vaseline to ensure hydrostatic pressure-transmitting conditions, NaCl served as an internal standard for pressure calibration. By fitting a Birch-Murnaghan EOS to the data, the bulk modulus was determined as 116.0±1.3 GPa, (K ′=4), V T,0 =446.49 3 exp[∫(0.33±0.05) × 10−4 + (0.65±0.85)×10−8 T dT], (K T/T) P = −0.011± 0.004 GPa K−1. The thermodynamic data obtained for Mg-sursassite are consistent with phase equilibrium data reported recently (Fockenberg 1998); the best agreement was obtained with Δf H 0 298 (Mg-sursassite) = −13901.33 kJ mol−1, and S 0 298 (Mg-sursassite) = 614.61 J K−1 mol−1.