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Physics and Chemistry of Minerals

, Volume 28, Issue 7, pp 475–487 | Cite as

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
ORIGINAL PAPER

Abstract

 Calorimetric and PVT 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 PVT 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)×10−8T dT], (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 ΔfH0298 (Mg-sursassite) = −13901.33 kJ mol−1, and S0298 (Mg-sursassite) = 614.61 J K−1 mol−1.

Key words Thermodynamics Calorimetry High-pressure Mg-sursassite 

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Copyright information

© Springer-Verlag Berlin Heidelberg 2001

Authors and Affiliations

  • K.-D. Grevel
    • 1
  • A. Navrotsky
    • 2
  • W. A. Kahl
    • 1
  • D. W. Fasshauer
    • 1
  • J. Majzlan
    • 2
  1. 1.Ruhr-Universität Bochum, Institut für Geologie, Mineralogie und Geophysik, 44780 Bochum, Germany Tel.: ++49 (0) 234 32 2 3517; Fax: ++49 (0) 234 32 1 4433 e-mail: Klaus-Dieter.Grevel@ruhr-uni-bochum.de DE
  2. 2.Thermochemistry Facility, Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, CA 95616-8779, USAUS

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