Thermodynamic properties of two iron silicates. Heat capacities of deerite from the temperature 10 K to 700 K and of grunerite from 10 K to 1000 K

✍ Scribed by Norikazu Komada; Edgar F. Westrum; Jr.; Bruce S. Hemingway; Lawrence M. Anovitz

• Publisher: Elsevier Science
• Year: 1995
• Tongue: English
• Weight: 643 KB
• Volume: 27
• Category: Article
• ISSN: 0021-9614
• DOI: 10.1006/jcht.1995.0115

✦ Synopsis

Heat-capacity measurements were made on deerite and on grunerite with an adiabatic calorimetric system from the temperature 10 K to 350 K and by d.s.c. in the super-ambient region. Slight Schottky-like and bell-shaped anomalies appear near T=16 K and 34 K in the deerite heat-capacity curve. The smaller than expected magnetic susceptibility and Mo¨ssbauer spectral changes at Tq210 K were interpreted as a thermally activated electron-delocalization mechanism in the deerite sample and yet another deerite sample by adjusting for anharmonicity and resolving the small excess heat capacities by the Komada-Westrum approach. A sharp anomaly centered at T=35.5 K was observed in the heat capacity of grunerite curve. Measured values of Cp, m/R, {S°m(T')-S°m(10 K)}/R, and -{F°m(T')-F°m(10 K)}/R at T'=298.15 K for deerite are 94.55, 95.21, and -43.50, respectively, where Cp, m denotes molar heat capacity, S°m standard molar entropy, H°m standard molar enthalpy, and where F°m=D T 0 S°m-D T 0 H°m/T. The corresponding values for grunerite are 82.38, 83.98, and -39.69, respectively. The standard molar entropy at T=298.15 K, after adjusting to the end-member compositions, is 96.39•R for deerite Fe II 6 Fe III 3 O3(Si6O17)(OH)5 except for possible residual Sm at T :0. That for grunerite Fe7Si8O22(OH)2 is 90.01•R, except for possible magnetic excess entropy at TQ10 K.