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

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, Mg_3.87Al_17.65Si_7.75O_43.68(OH)_4.32, Mg_3.66Al_17.76Si_7.68O_43.31(OH)_4.69, and Mg_3.58Al_18.05Si_7.43O_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_fH⁰₂₉₈ (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 Mg_3.5Al₁₈Si_7.75O₄₄(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 Δ_fH⁰₂₉₈ [Mg_3.5Al₁₈Si_7.75O₄₄(OH)₄] = −25005.14 kJ/mol, and S⁰₂₉₈ [Mg_3.5Al₁₈Si_7.75O₄₄(OH)₄] = 937.94 J/(K·mol). Thus, for the first time, reliable thermodynamic data for Mg-staurolite, based on experimental constraints, are provided.