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 oxide-melt calorimetry at 702 °C. Staurolite compositions, determined by electron microprobe analysis, Karl-Fischer titration, and thermogravimetry, are: Mg 3.71Al 18.17Si 7.60O 44.31(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 (Mg-staurolite) = (−25357.58 + 87.35 N) kJ/mol, where N = number of H atoms per formula unit in Mg-staurolite. 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.

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