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Free energy of formation of zircon and hafnon

Abstract 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 defined 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,oxH0 298K = -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 (S0 298K = 93.6 J/mol·K) is similar to that for zircon, but the enthalpy of formation is slightly more exothermic (Δf,oxH0 298K = -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 S0 298K for Fe2SiO4 than the currently accepted calorimetric datum (i.e., 153.5 vs. 151.0 ± 0.2 J/K·mol).