Heat capacity and thermodynamic properties of andradite garnet, Ca3Fe2Si3O12, between 10 and 1000 K and revised values for ΔfGom (298.15 K) of hedenbergite and wollastonite
Geochimica et Cosmochimica Acta
- Richard A. Robie , Zhao Bin , Bruce S. Hemingway , and Mark D. Barton
The heat capacity of synthetic andradite garnet (Ca3Fe2Si3O12) was measured between 9.6 and 365.5 K by cryogenic adiabatic calorimetry and from 340 to 990 K by differential scanning calorimetry. At 298.15 KCop,m and Som are 351.9 ± 0.7 and 316.4 ± 2.0 J/(mol·K), respectively.
Andradite has a λ-peak in Cop,m with a maximum at 11.7 ± 0.2 K which is presumably associated with the antiferromagnetic ordering of the magnetic moments of the Fe3+ ions. The Gibbs free energy of formation,ΔfGom (298.15 K) of andradite is −5414.8 ± 5.5 kJ/mol and was obtained by combining our entropy and heat capacity data with the known breakdown of andradite to pseudowollastonite and hematite at ≈ 1410 to 1438 K. From a reexamination of the calcite + quartz = wollastonite equilibrium data we obtained ΔfHom(298.15 K) = − 1634.5 ± 1.8 kJ/mol for wollastonite.
Between 300 and 1000 K the molar heat capacity of andradite can be represented by the equation Cop,m = 809.24 - 7.025 × 10−2T− 7.403 × 103T−0.5 − 6.789 × 105T−2. We have also used our thermochemical data for andradite to estimate the Gibbs free energy of formation of hedenbergite (CaFeSi2O6) for which we obtained ΔfGom (298.15 K) = −2674.3 ± 5.8 kJ/mol.
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- Heat capacity and thermodynamic properties of andradite garnet, Ca3Fe2Si3O12, between 10 and 1000 K and revised values for ΔfGom (298.15 K) of hedenbergite and wollastonite
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- Geochimica et Cosmochimica Acta
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