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

Abstract

The enthalpies of drop solution in lead borate (2 PbO·B 2 O 3 ) 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: Mg 3.71 Al 18.17 Si 7.60 O 44.31 (OH) 3.69 , Mg 3.87 Al 17.65 Si 7.75 O 43.68 (OH) 4.32 , Mg 3.66 Al 17.76 Si 7.68 O 43.31 (OH) 4.69 , and Mg 3.58 Al 18.05 Si 7.43 O 43.01 (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 Δ f H 0 298 (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 Mg 3.5 Al 18 Si 7.75 O 44 (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 Δ f H 0 298 [Mg 3.5 Al 18 Si 7.75 O 44 (OH) 4 ] = -25005.14 kJ/mol, and S 0 298 [Mg 3.5 Al 18 Si 7.75 O 44 (OH) 4 ] = 937.94 J/(K·mol). Thus, for the first time, reliable thermodynamic data for Mg-staurolite, based on experimental constraints, are provided.