J. Am. Ceram. Soc., 88 [7] 1854–1858 (2005) DOI: 10.1111/j.1551-2916.2005.00348.x Journal Gibbs Free Energy of Formation of Zircon from Measurement of Solubility in H2O
Robert C. Newton and Craig E. Manningw Department of Earth and Space Sciences, University of California, Los Angeles California 90095-1567
John M. Hanchar Department of Earth and Environmental Sciences, The George Washington University, Washington, District of Columbia 20006
Robert J. Finch Argonne National Laboratory, Argonne, Illinois 60439
We exploited the large difference in the solubility of SiO2 and Co2SiO4 þ 2CO ¼ SiO2 þ 2Co þ 2CO2 (3) ZrO2 in H2O to constrain precisely the Gibbs energy of forma- tion of zircon (ZrSiO ). Solubility in H O was determined at 4 2 The DGf of zircon was derived from the CO/CO2 ratios of gases 8001C, 1.2 GPa, by weight loss of synthetic zircon crystals. The equilibrated with the solid reactants and products of the two experiments yielded fine-grained monoclinic ZrO as an incon- 2 reactions at high temperatures. Variation of DGf with temper- gruent solution product uniformly coating zircon crystals. Ex- ature is inconsistent with high-temperature heat capacity data, periments on the ZrO2-coated zircon crystals were also carried suggesting a systematic error, perhaps because of Co substitu- out with an initially slightly SiO -oversaturated fluid, causing 6 2 tion in zircon. Nevertheless, a value of DGf of zircon at 298 K, weight gain by zircon regrowth. The mean SiO concentration 5 2 10 Pa, can be derived by combining the derived midpoint DGT 7 6 for forward and reverse experiments was 0.069 0.010 mol/kg with refitted Cp functions (Table I). The apparently small un- H2O(2r). When combined with precise activity–composition certainty (1.16 kJ/mol, all uncertainties are 2s unless otherwise measurements for aqueous SiO2, the data constrain the Gibbs noted) is an artifact of the narrow temperature interval over 5 free energy of zircon from its oxides at 298 K, 10 Pa, to be which the measurements were made (11801–13661C). 19.3071.16 kJ/mol (2r). This determination is comparable Schuiling et al.4 used hydrothermal synthesis experiments at in precision to the best measurements obtainable by more con- 1000 K, 0.1 GPa to determine the directions of several possible ventional methods, which suggests that determination of the reactions of zircon with other minerals having reasonably well- thermochemical properties of other important ceramic materials defined thermodynamic properties. The equilibria investigated may also be amenable to this method. are metastable, but constraints on the Gibbs energy change of reaction 1 were obtained by determining whether the postulated reactions would or would not take place. Only broad limits on I. Introduction Gibbs energy change were derived, leading to a large uncertainty ECAUSE of its durability, stability, and resistance to thermal in DGf of zircon (4.5 kJ/mol). Bshock, zircon is potentially useful in a wide variety of ap- Determination of the position at high temperature (T)and plications. However, its refractory character hinders precise de- pressure (P) of the univariant equilibrium 1,2 termination of its Gibbs energy of formation, DGf ,whichis required for computation of phase equilibria involving ZrSiO . 4 ZrSiO þ 2MgCO ¼ Mg SiO þ ZrO þ CO The DG of zircon from the oxides 4 3 2 4 2 2 f zircon magnesite forsterite baddeleyite (4)
ZrO2 þ SiO2 ¼ ZrSiO4 (1) baddeleyite quartz zircon also permits derivation of DGf of zircon. Although dependent 3–5 on CO2 equation of state and other data for the minerals, has been derived from phase-equilibrium studies and high- 5 7 6 the results indicate DGf of zircon at 298 K, 10 Pa, of 18.44 temperature oxide-melt solution calorimetry, and it is listed in 1.71 kJ/mol.5,9 compilations of thermodynamic data;7–9 however, values differ 6 Heat of solution measurements in molten Pb2B2O5 yielded a by over 11 kJ/mol (Table I). 3 mean enthalpy of reaction 1 at 977 K of 27.90 kJ/mol. This Rose´n and Muan made use of the equilibria 10–12 result and refitted heat capacity data gave a value of DGf at 298 K, 105 Pa, of 23.5773.62 kJ/mol (Table I), where the un- Co2SiO4 þ ZrO2 þ 2CO ¼ ZrSiO4 þ 2Co þ 2CO2 (2) certainty is the 95% confidence interval. The phase-equilibrium and calorimetric results suggest that and the Gibbs energy change of reaction 1 is between 18 and 24 kJ/mol at 298 K, 105 Pa, neglecting uncertainties in indi- L. Q. Chen—contributing editor vidual constraints. This range only partly overlaps that given in compilations of thermodynamic data in current use for ceramic and geochemical applications7–9 (Table I), which rely on at least one of the above studies for free energy of formation values or Manuscript No. 10118. Received April 10, 2003; approved January 12, 2005. imprecise estimates of the equilibrium temperature of reaction 1 Supported by the Earth Sciences Division, National Science Foundation, under Grant 5 13 Nos. EAR-990583 & EAR-0337170. at 10 Pa. Sources of uncertainty in Gibbs energy of formation wAuthor to whom correspondence should be addressed. e-mail: [email protected] include experimental errors, the finite width of bracketing 1854 July 2005 Gibbs Free Energy of Formation of Zircon 1855