Free energy of formation of zircon and hafnon

Hugh St C. O'Neill

doi:10.2138/am.2006.2071

Free energy of formation of zircon and hafnon

Hugh St C. O'Neill

Research output: Contribution to journal › Article › Research › peer-review

29 Citations (Scopus)

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 μ₀₂ 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 μ₀₂ 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 Δ_f,oxH_298K⁰ = -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_298K⁰ = 93.6 J/mol·K) is similar to that for zircon, but the enthalpy of formation is slightly more exothermic (Δ_f,ox, H_298K⁰ = -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 α-γtransition 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 (α-γ transition. The redetermination of the μ₀₂ 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_298K⁰ for Fe₂SiO₄ than the currently accepted calorimetric datum (i.e., 153.5 vs. 151.0 ± 0.2 J/K·mol).

Original language	English
Pages (from-to)	1134-1141
Number of pages	8
Journal	American Mineralogist
Volume	91
Issue number	7
DOIs	https://doi.org/10.2138/am.2006.2071
Publication status	Published - Jul 2006
Externally published	Yes

Keywords

Fayalite
Free energy of formation
Hafnon
Thermodynamic data
Zircon

Access to Document

10.2138/am.2006.2071

Cite this

@article{a3baa32382ed453bbaab214e9f77ccdf,

title = "Free energy of formation of zircon and hafnon",

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 μ02 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 μ02 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 Δf,oxH298K0 = -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 (S298K0 = 93.6 J/mol·K) is similar to that for zircon, but the enthalpy of formation is slightly more exothermic (Δf,ox, H298K0 = -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 α-γtransition 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 (α-γ transition. The redetermination of the μ02 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 S298K0 for Fe2SiO4 than the currently accepted calorimetric datum (i.e., 153.5 vs. 151.0 ± 0.2 J/K·mol).",

keywords = "Fayalite, Free energy of formation, Hafnon, Thermodynamic data, Zircon",

author = "O'Neill, {Hugh St C.}",

year = "2006",

month = jul,

doi = "10.2138/am.2006.2071",

language = "English",

volume = "91",

pages = "1134--1141",

journal = "American Mineralogist",

issn = "0003-004X",

publisher = "Mineralogical Society of America",

number = "7",

}

TY - JOUR

T1 - Free energy of formation of zircon and hafnon

AU - O'Neill, Hugh St C.

PY - 2006/7

Y1 - 2006/7

N2 - 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 μ02 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 μ02 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 Δf,oxH298K0 = -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 (S298K0 = 93.6 J/mol·K) is similar to that for zircon, but the enthalpy of formation is slightly more exothermic (Δf,ox, H298K0 = -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 α-γtransition 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 (α-γ transition. The redetermination of the μ02 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 S298K0 for Fe2SiO4 than the currently accepted calorimetric datum (i.e., 153.5 vs. 151.0 ± 0.2 J/K·mol).

AB - 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 μ02 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 μ02 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 Δf,oxH298K0 = -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 (S298K0 = 93.6 J/mol·K) is similar to that for zircon, but the enthalpy of formation is slightly more exothermic (Δf,ox, H298K0 = -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 α-γtransition 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 (α-γ transition. The redetermination of the μ02 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 S298K0 for Fe2SiO4 than the currently accepted calorimetric datum (i.e., 153.5 vs. 151.0 ± 0.2 J/K·mol).

KW - Fayalite

KW - Free energy of formation

KW - Hafnon

KW - Thermodynamic data

KW - Zircon

UR - http://www.scopus.com/inward/record.url?scp=33746996454&partnerID=8YFLogxK

U2 - 10.2138/am.2006.2071

DO - 10.2138/am.2006.2071

M3 - Article

AN - SCOPUS:33746996454

SN - 0003-004X

VL - 91

SP - 1134

EP - 1141

JO - American Mineralogist

JF - American Mineralogist

IS - 7

ER -

Free energy of formation of zircon and hafnon

Abstract

Keywords

Access to Document

Other files and links

Cite this