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 mu(o2) 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 mu(o2) 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 Delta H-f,ox(298K)o = -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 (S-298K(o) = 93.6 J/mol.K) is similar to that for zircon, but the enthalpy of formation is slightly more exothermic (Delta H-f,ox(298K)o = -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 alpha-gamma 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 alpha-gamma transition. The redetermination of the mu o(2) 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 S-298K(o) for Fe2SiO4 than the currently accepted calorimetric datum (i.e., 153.5 vs. 151.0 +/- 0.2 J/K.mol).
