Free energy of formation of zircon and hafnon

The free energy of formation of zircon (ZrSiO4) from its oxides was determined between 1100 and 1300 K by an electrochemical method, in which values of μO2 deÞ ned by the two assemblages Fe2SiO4-Fe-SiO2 (fayalite-iron-quartz) and Fe2SiO4-Fe-ZrO2-ZrSiO4 were each measured using oxygen concentration cells with calcia-stabilized zirconia solid electrolytes. The difference in μO2 between these two assemblages corresponds to the reaction ZrO2 + SiO2(qz) = ZrSiO4. The results, when analyzed using calorimetric data for the entropies and high-temperature heat capacities of ZrSiO4, ZrO2, and SiO2(quartz), yields Δf,oxH298K = –24.0 ± 0.2 kJ/mol for ZrSiO4, in good agreement with the calorimetric value of Ellison and Navrotsky (1992). ZrSiO4 is predicted to decompose to ZrO2 plus SiO2 (cristobalite) at 1938 K, assuming a temperature of 1430 K for the martensitic phase transition between the tetragonal and monoclinic forms of ZrO2 (baddeleyite), with an enthalpy of transition of 8.67 kJ/mol. The same experimental approach was used also to determine the free energy of formation of hafnon (HfSiO4). The entropy of hafnon (S298K = 93.6 J/mol·K) is similar to that for zircon, but the enthalpy of formation is slightly more exothermic (Δf,oxH298K = –25.0 ± 0.2 kJ/mol). The cells with either ZrSiO4 + ZrO2 or HfSiO4 + HfO2 produce an anomalous excursion in EMF when the temperature of the α–γ transition in Fe metal at 1184 K is traversed; this excursion takes >12 hours to decay back to the equilibrium value. This behavior is presumably related to strain caused by the volume change of the α–γ transition. The redetermination of the μO2 of the Fe2SiO4-Fe-SiO2(qz) equilibrium (the quartz-fayalite-iron or QFI oxygen buffer) carried out in the course of this study gave results in reasonable agreement with previous work, but with a different slope vs. temperature, implying a slightly higher value of S298K for Fe2SiO4 than the currently accepted calorimetric datum (i.e., 153.5 vs. 151.0 ± 0.2 J/K·mol).