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The enthalpy of formation and internally consistent thermodynamic data of Mg-staurolite

  • Klaus-D. Grevel EMAIL logo , Alexandra Navrotsky , Thomas Fockenberg and Juraj Majzlan
Published/Copyright: March 28, 2015
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Abstract

The enthalpies of drop solution in lead borate (2 PbO·B2O3) of four Mg-staurolite samples, synthesized at 720 °C and pressures between 2 and 5 GPa, were measured by high-temperature oxidemelt calorimetry at 702 °C. Staurolite compositions, determined by electron microprobe analysis, Karl-Fischer titration, and thermogravimetry, are: Mg3.71Al18.17Si7.60O44.31(OH)3.69, Mg3.87Al17.65Si7.75O43.68(OH)4.32, Mg3.66Al17.76Si7.68O43.31(OH)4.69, and Mg3.58Al18.05Si7.43O43.01(OH)4.99. The enthalpy of drop solution of the bulk samples (as well as the calculated values for the enthalpy of formation from the elements of Mg-staurolite) are strongly correlated to the H content of the samples. The enthalpy of formation from the elements is best described by the linear relation ΔfH0298 (Mgstaurolite) = (-25357.58 + 87.35 N) kJ/mol, where N = number of H atoms per formula unit in Mgstaurolite. The enthalpy of drop solution of two partially dehydrated Mg-staurolite samples is in a good agreement with the linear relation. Phase-equilibrium data for Mg-staurolite (Fockenberg 1998) were recalculated using the stoichiometric formula Mg3.5Al18Si7.75O44(OH)4. Based on these mineral equilibria and the internally consistent data set of Berman (1988), a mathematical programming analysis of the thermodynamic data of Mg-staurolite gave ΔfH0298 [Mg3.5Al18Si7.75O44(OH)4] = -25005.14 kJ/mol, and S0298 [Mg3.5Al18Si7.75O44(OH)4] = 937.94 J/(K·mol). Thus, for the first time, reliable thermodynamic data for Mg-staurolite, based on experimental constraints, are provided.

Received: 2001-5-30
Accepted: 2001-10-27
Published Online: 2015-3-28
Published in Print: 2002-4-1

© 2015 by Walter de Gruyter Berlin/Boston

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