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

Dachs, E. and Geiger, C. A. (2008) Low-temperature heat capacity of synthetic Fe- and Mg-cordierite: thermodynamic properties and phase relations in the system FeO-Al2O3-SiO2-(H2O) European Journal of Mineralogy, 20 (1). pp. 47-62. DOI 10.1127/0935-1221/2008/0020-1785.

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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 C-p's of anhydrous low Mg-cordierite, Mg2Al4Si5O18, and a hydrous low Mg-cordierite of composition Mg1.97Al3.94Si5.06O18.0.625H(2)O, 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, Cp polynomials for both phases were calculated for use at T > 270 K. They are given by: C-p(Fe-Cd) = 91 1.1(+/- 9.7) - 5829.2(+/- 363) x T-0.5 - 13.9424(+/- 2.522) x 10(6) x T-2 + 1470.4(+/- 454.84) x 10(6) x T-3 and C-p(Mg-Cd) = 882.0(+/- 4.9) - 5155.8(+/- 167) x T-0.5 -20.7584(+/- 0.806) x T-2 + 2736.0(+/- 112.73) x 10(6) x 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 C-p values for Fe-cordierite were separated by applying the Komada-Westrum model and the values S degrees(vib) = 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 center dot H2O, was derived as: C-p(hFe-Cd) = 967.3(+/- 9.7) - 6070.4(+/- 363) x T-0.5 - 13.9389(+/- 2.522) x 10(6) x T-2 + 1470.4(+/- 454.84) x 10(6) x 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 center dot nH(2)O = 2 almandine + 4 sillimanite + 5 quartz + 3n H2O. For anhydrous Fe-cordierite Delta H-f(o) = -8448.26 kJ/mol was obtained and for hydrous Fe-cordierite, Delta H-f(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.

Document Type: Article
Keywords: low-temperature heat capacity thermodynamic properties standard entropy Fe-cordierite Mg-cordierite solid-solutions anhydrous mg hydrous cordierite iron cordierite minerals h2o calibration stability garnet water
Research affiliation: Kiel University
Refereed: No
DOI etc.: 10.1127/0935-1221/2008/0020-1785
ISSN: 0935-1221
Date Deposited: 12 Jan 2012 05:49
Last Modified: 08 Oct 2012 10:14
URI: http://eprints.uni-kiel.de/id/eprint/15831

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