Low-temperature heat capacity of synthetic Fe- and Mg-cordierite: thermodynamic properties and phase relations in the system FeO-Al2O3-SiO2-(H2O)
Dachs, Edgar; Geiger, Charles A.
published: Mar 7, 2008
ArtNo. ESP147052001003, Price: 29.00 €
The heat capacity of anhydrous low Fe-cordierite, Fe2Al4Si5O18, was measured for the first time between 5 and 300 K on a milligram-sized synthetic sample using low-temperature heat-pulse calorimetry. The Cp's of anhydrous low Mg-cordierite, Mg2Al4Si5O18, and a hydrous low Mg-cordierite of composition Mg1.97Al3.94Si5.06O18·0.625H2O, both previously studied by adiabatic calorimetry (Paukov et al., 2006, 2007), were also determined. At low temperatures around 10 K the Cp data for Fecordierite show a small feature that is interpreted as a Schottky anomaly. Using published DSC and adiabatic calorimetry results for anhydrous Fe-cordierite and Mg-cordierite and the results herein, Cp polynomials for both phases were calculated for use at T > 270 K. They are given by:CFe-Cdp = 911.1(±9.7) − 5829.2(±363)× T−0.5 − 13.9424(±2.522) × 106 × T−2 + 1470.4(±454.84) × 106 × T−3andCMg-Cdp = 882.0(±4.9) − 5155.8(±167) × T−0.5 − 20.7584(±0.806) × 106 × T−2 + 2736.0(±112.73) × 106 × T−3,respectively. The standard calorimetric entropy values at 298.15 K, S°, for anhydrous Fe-cordierite, anhydrous Mg-cordierite and hydrous Mg-cordierite are 460.5 ± 0.5, 406.1 ± 0.4 and 450.9 ± 0.5 J/(mol·K), respectively. The latter two values are in good agreement with those determined by adiabatic calorimetry. The lattice (vibrational) and non-lattice contributions to the experimental Cp values for Fe-cordierite were separated by applying the Komada-Westrum model and the values S°vib = 447.7 J/(mol·K) and S°el = 13.6 J/(mol·K) were obtained for the vibrational and electronic contributions to the standard third-law entropy. Thermodynamic calculations and analysis were carried out in the system FeO-Al2O3-SiO2 with and without H2O. A model Cp polynomial for hydrous Fe-cordierite, Fe2Al4Si5O18 · H2O, was derived as:ChFe−Cdp = 967.3(±9.7) − 6070.4(±363) × T−0.5 − 13.9389(±2.522) × 106 × T−2 + 1470.4(±454.84) × 106 × T−3.The enthalpy of formation from the elements for both hydrous and anhydrous Fe-cordierite and the standard entropy for hydrous Fe-cordierite with one mole of H2O pfu were derived using the experimental phase equilibrium results of Mukhopadhyay & Holdaway (1994) on the reaction 3 Fe-cordierite · nH2O = 2 almandine + 4 sillimanite + 5 quartz + 3nH2O. For anhydrous Fe-cordierite, ΔfH° = −8448.26 kJ/mol was obtained and for hydrous Fe-cordierite ΔfH° = −8750.23 kJ/mol and S° = 520.6 J/(mol·K). Phase relations in the FeO-Al2O3-SiO2-(H2O) systems at low pressures are analyzed and isohydrons for H2O in hydrous Fe-cordierite are modelled. H2O contents decrease with increasing temperature and increase with increasing pressure.