Thermodynamic data of the high-pressure phase Mg₅Al₅Si₆O₂₁(OH)₇ (Mg-sursassite)

Calorimetric and P-V-T data for the highpressure phase Mg₅Al₅Si₆O₂₁(OH)₇ (Mg-sursassite) have been obtained. The enthalphy 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 ^oC as sovent. The resulting values were used to calculated 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 ^oC to T = 500 ^oC by differential scanning calorimentry 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, (K¹ = 4), V_T0 = 446.49 A³ exp £ (0.33 ± 0.05) × 10 ^-4 (065 ± 0.85) × 10⁸ TdT), (∂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 △_fH⁰₂₉₈ (Mg-sursassite) = -13901.33 kJ mol^-1, and S⁰₂₉₈ (Mg-sursassite) = 614.61 J K^-1 mol^-1.