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Low-temperature heat capacity of synthetic Fe- and Mg-cordierite: thermodynamic properties and phase relations in the system FeO-Al₂O₃-SiO₂-(H₂O)

¹ Fachbereich Materialforschung & Physik, Abteilung Mineralogie, Universität Salzburg, Hellbrunnerstrasse 34, 5020 Salzburg, Austria
² Institut für Geowissenschaften, Abteilung Mineralogie, Christian-Albrechts-Universität zu Kiel, Olshausenstrasse 40, 24098 Kiel, Germany

^* Corresponding author, e-mail: edgar.dachs{at}sbg.ac.at

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 C_p’s of anhydrous low Mg-cordierite, Mg₂Al₄Si₅O₁₈, and a hydrous low Mg-cordierite of composition Mg_1.97Al_3.94Si_5.06O₁₈·0.625H₂O, both previously studied by adiabatic calorimetry (Paukov et al., 2006, 2007), were also determined. At low temperatures around 10 K the C_p data for Fe-cordierite 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, C_p polynomials for both phases were calculated for use at T > 270 K. They are given by:

and

respectively. The standard calorimetric entropy values at 298.15 K, S ^o, 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 C_p values for Fe-cordierite were separated by applying the Komada-Westrum model and the values S_vib^o= 447.7 J/(mol·K) and S_el^o = 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 C_p polynomial for hydrous Fe-cordierite, Fe₂Al₄Si₅O₁₈·H₂O, was derived as:

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 +3n H₂O. For anhydrous Fe-cordierite, _f H^o = –8448.26 kJ/mol was obtained and for hydrous Fe-cordierite _f H^o = –8750.23 kJ/mol and S ^o = 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.

Key-words: low-temperature heat capacity, thermodynamic properties, standard entropy, Fe-cordierite, Mg-cordierite.