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

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)}/R at T′=298.15 K for deerite are 94.55, 95.21, and -43.50, respectively, where C_{p, m}denotes molar heat capacity, S°_mstandard molar entropy, H°_m standard molar enthalpy, and where Φ°_m=Δ^T₀S°_m-Δ^T₀H°_mT. 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^II₆Fe^III₃O₃(Si₆O₁₇)(OH)₅ except for possible residual S_m at T→0. That for grunerite Fe₇Si₈O₂₂(OH)₂ is 90.01·R, except for possible magnetic excess entropy at T<10 K.