- journal_title：American Mineralogist
- Contributor：Robert C. Newton ; Craig E. Manning ; John M. Hanchar ; Clinton V. Colasanti
- Publisher：Mineralogical Society of America
- Date：2010-
- Format：text/html
- Language：en
- Identifier：10.2138/am.2010.3213
- journal_abbrev：American Mineralogist
- issn：0003-004X
- volume：95
- issue：1
- firstpage：52
- section：Articles
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:
ex" alt="Formula" src="52/embed/tex-math-1.gif" />
where SiO2(<em>tem>) 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(<em>tem>) 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 (<em>mem><em>sem>), or a mole fraction (<em>Xem><em>sem>) 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 <em>Pem> and <em>Tem> is given by:
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where <em>ZBem> and <em>Qem> refer, respectively, to equilibrium with zircon-baddeleyite and quartz, γs is the activity coefficient of total silica, and the relationship between γs and <em>Xem><em>sem> accounts for aqueous silica activity. Our results yield Δ<em>Gem>°<em>fem>,<em>oxem>,<em>zrem> = −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 <em>Tem> and <em>Pem> may be a superior method of free energy determination of other refractory silicate minerals.