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

Abstract

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 Mössbauer spectral changes at T>210 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 C p,m/ R, { S ° m( T′)& minus; S° m(10 K)}/ R, and −{Φ° m( T′)−Φ° m(10 K)}/ Rat T′=298.15 K for deerite are 94.55, 95.21, and −43.50, respectively, where C p,mdenotes molar heat capacity, S° mstandard molar entropy, H° mstandard molar enthalpy, and where Φ° m=Δ T 0 S° m−Δ T 0H° 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· Rfor deerite Fe II 6Fe III 3O 3(Si 6O 17)(OH) 5except for possible residual S mat T→0. That for grunerite Fe 7Si 8O 22(OH) 2is 90.01· R, except for possible magnetic excess entropy at T<10 K.