Abstract

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 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, Cp polynomials for both phases were calculated for use at T > 270 K. They are given by:

 
\[\mathit{C}_{p}^{Fe{-}Cd}\ =\ 911.1({\pm}9.7)\ {-}\ 5829.2({\pm}363)\ {\times}\mathit{T}^{{-}0.5}\ {-}\ 13.9424({\pm}2.522)\ {\times}\ 10^{6}\ {\times}\mathit{T}\ ^{{-}2}\ +\ 1470.4({\pm}454.84)\ {\times}\ 10^{6}\ {\times}\ \mathit{T}\ ^{{-}3}\]

and

 
\[\mathit{C}_{p}^{Mg{-}Cd}\ =\ 882.0({\pm}4.9)\ {-}\ 5155.8({\pm}167)\ {\times}\ \mathit{T}\ ^{{-}0.5}\ {-}\ 20.7584({\pm}0.806)\ {\times}\ 10^{6}\ {\times}\ \mathit{T}^{{-}2}\ +\ 2736.0({\pm}112.73)\ {\times}\ 10^{6}\ {\times}\ \mathit{T}^{{-}3},\]

respectively. The standard calorimetric entropy values at 298.15 K, So, 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 Svibo= 447.7 J/(mol·K) and Selo = 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:

 
\[\mathit{C}_{p}^{hFe{-}Cd}\ =\ 967.3({\pm}9.7)\ {-}\ 6070.4({\pm}363)\ {\times}\ \mathit{T}^{{-}0.5}\ {-}\ 13.9389({\pm}2.522)\ {\times}\ 10^{6}\ {\times}\ \mathit{T}\ ^{{-}2}\ +\ 1470.4({\pm}454.84)\ {\times}\ 10^{6}\ {\times}\ \mathit{T}^{{-}3}.\]

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 +3n H2O. For anhydrous Fe-cordierite, ΔfHo = −8448.26 kJ/mol was obtained and for hydrous Fe-cordierite ΔfHo = −8750.23 kJ/mol and So = 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.

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