The standard Gibbs energy of formation of zircon was constrained by measuring the solubility of silica in H2O in equilibrium with zircon and baddeleyite at 800 °C, 12 kbar, by a sensitive weight-change method. Dissolution occurs incongruently according to the reaction:
where SiO2(t) is total dissolved silica. Blank runs demonstrated that the effects of ZrO2 solubility and/or capsule-Pt transfer were near the weighing detection limit, so weight losses or gains could be ascribed quantitatively to SiO2 solubility. Precise SiO2(t) concentrations were ensured by use of three types of starting material, by approaching equilibrium from zircon-undersaturation and oversaturation, and by demonstrating time-independence of the measurements. The results yielded a SiO2 concentration of 0.069 ± 0.002 (1 se) moles per kg H2O (ms), or a mole fraction (Xs) of 1.23 × 10−3 ± 3.3 × 10−5. Two runs on zircon solubility in NaCl-H2O solutions at 800 °C and 10 kbar showed silica solubility to decrease by nearly 1% per mol% NaCl.
The standard molar Gibbs free energy of formation of zircon from the oxides at a constant P and T is given by:
where ZB and Q refer, respectively, to equilibrium with zircon-baddeleyite and quartz, γs is the activity coefficient of total silica, and the relationship between γs and Xs accounts for aqueous silica activity. Our results yield ΔG°f,ox,zr = −18.5 ± 0.7 kJ/mol at 800 °C, 12 kbar (95% confidence), or a standard apparent Gibbs free energy of formation from the elements of −1918.3 ± 0.7 kJ/mol at 25 °C, 1 bar. Our value is consistent with determinations based on phase equilibrium studies, within reported error limits, but is more precise than most previous values. However, it is less negative than high-temperature determinations by calorimetry and electrochemistry. Our results indicate that solubility measurements at high T and P may be a superior method of free energy determination of other refractory silicate minerals.