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

K. D. Grevel, A. Navrotsky, W. A. Kahl, D. W. Fasshauer, J. Majzlan

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Abstract

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: Cp (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), VT0 = 446.49 A3 exp £ (0.33 ± 0.05) × 10 -4 (065 ± 0.85) × 108 TdT), (∂KT/∂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.

Original languageEnglish (US)
Pages (from-to)475-487
Number of pages13
JournalPhysics and Chemistry of Minerals
Volume28
Issue number7
DOIs
StatePublished - 2001
Externally publishedYes

Keywords

  • Calorimetry High-pressure Mg-sursassite
  • Thermodynamics

ASJC Scopus subject areas

  • Materials Science(all)
  • Geochemistry and Petrology

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