THERMODYNAMIC PROPERTIES OF 2 IRON SILICATES - HEAT-CAPACITIES OF DEERITE FROM THE TEMPERATURE 10K 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 an d 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 heal-ca pacity curve. The smaller than expected magnetic susceptibility and Mo ssbauer 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 resolvin g the small excess heat capacities by the Komada-Westrum approach. A s harp anomaly centered at T = 35.5 K was observed in the heat capacity of grunerite curve. Measured values of C-y,C-m/R, {S-m degrees(T') - S -m degrees(10 K)}/R, and -{Phi(m) degrees(T') - Phi(m) degrees(10 K)}/ R at T' = 298.15 K for deerite are 94.55, 95.21, and -43.50, respectiv ely, where C-y,C-m denotes molar heat capacity, S-m degrees standard m olar entropy, H-m degrees standard molar enthalpy, and where Phi(m) de grees = Delta(0)(T)S(m) degrees - Delta(0)(T)H(m) degrees/T. The corre sponding values for grunerite are 82.38, 83.98, and -39.69, respective ly. The standard molar entropy at T = 298.15 K, after adjusting to the end-member compositions, is 96.39 . R for deerite (Fe6Fe3O3)-Fe-II-O- III(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. (C) 1995 Academic Press Limited