Heat capacity and thermodynamic properties of andradite garnet, Ca3FezSi3012, between 10 and 1000 K and revised values for AfGz (298.15 K) of hedenbergite and wollastonite
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چکیده
Ahshact-The heat capacity of synthetic andradite garnet (Ca~Fe&Oir) 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 KC&,, and Sz arc 351.9 + 0.7 and 316.4 rf: 2.0 J/(mol-K), respectively. Andradite has a X-peak in Ci, with a maximum at 11.7 t 0.2 K which is presumably associated with the antiferromagnetic ordering of the magnetic moments of the Fe-‘+ ions. The Gibbs free energy of formation, ArGz (298. I5 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 pseudowohastonite and hematite at = 1410 to 1438 K. From a reexamination of the calcite + quartz = wollastonite equilibrium data we obtained ArHk (298.15 K) = 1634.5 -I 1.8 kJ/mol for wollastonite. Between 300 and 1000 K the molar heat capacity of andradite can be represented by the equation
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