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Free energy of formation of zircon, hafnon and fayalite (revisited)

H.StC. O'Neill

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(qz) and Fe2SiO4-Fe-ZrO2-ZrSiO4 were measured using oxygen concentration cells with calcia-stabilized zirconia solid electrolytes. The difference in µO2 corresponds to the reaction ZrO2+SiO2(qz)=ZrSiO4. The results, when analysed using calorimetric data for the entropies and high-temperature heat capacities of ZrSiO4, ZrO2 and SiO2(qz), yields Deltaf,ox 298K =24.0±0.2 kJ/mol for ZrSiO4, in good agreement with the calorimetric value of Ellison and Navrotsky (1992). Assuming a temperature of 1430 K for the martensitic phase transition between the monoclinic and tetragonal forms of ZrO2 (baddeleyite), with an enthalpy of transition of 8.67 kJ/mol, ZrSiO4 is predicted to decompose to ZrO2 plus SiO2 (cristobalite) at 1940 K. For hafnon (HfSiO4), the results show that its entropy of formation is similar to that for zircon, implying S°298K = 93.6 J/mol.K, but with Deltaf,ox 298K = -25.0±0.2 kJ/mol. 

Cells with both ZrSiO4 + ZrO2 and HfSiO4 + HfO2 show an anomalous excursion in EMF when the temperature of the a-g transition in Fe metal at 1184 K is traversed; this excursion takes >12 hours to decay back to the equilibrium value. This behaviour is presumably related to strain caused by the volume change of the a-g transition. 

Redetermination of the µO2 of the Fe2SiO4-Fe-SiO2(qz) equilibrium (the quartz-fayalite-iron or QFI oxygen buffer) gave results in reasonable agreement with previous work, but with a different slope versus temperature, implying a slightly higher value of S°298K  for Fe2SiO4 than the currently accepted calorimetric datum.