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Low-temperature heat capacity of synthetic Fe- and Mg-cordierite; thermodynamic properties and phase relations in the system FeO-Al (sub 2) O (sub 3) -SiO (sub 2) -(H (sub 2) O)

Edgar Dachs and Charles A. Geiger
Low-temperature heat capacity of synthetic Fe- and Mg-cordierite; thermodynamic properties and phase relations in the system FeO-Al (sub 2) O (sub 3) -SiO (sub 2) -(H (sub 2) O)
European Journal of Mineralogy (February 2008) 20 (1): 47-62

Abstract

The heat capacity of anhydrous low Fe-cordierite, Fe (sub 2) Al (sub 4) Si (sub 5) O (sub 18) , 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 (sub p) 's of anhydrous low Mg-cordierite, Mg (sub 2) Al (sub 4) Si (sub 5) O (sub 18) , and a hydrous low Mg-cordierite of composition Mg (sub 1.97) Al (sub 3.94) Si (sub 5.06) O (sub 18) . 0.625H (sub 2) O, both previously studied by adiabatic calorimetry (Paukov et al., 2006, 2007), were also determined. At low temperatures around 10 K the C (sub 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 (sub p) polynomials for both phases were calculated for use at T>270 K. They are given by: C (sub p) (super Fe-Cd) = 911.1(+ or -9.7) - 5829.2(+ or -363)XT (super -0.5) - 13.9424(+ or -2.522)X10 (super 6) XT (super -2) + 1470.4(+ or -454.84)X10 (super 6) XT (super -3) and C (sub p) (super Mg-Cd) + 882.0(+ or -4.9) - 5155.8(+ or -167)XT (super -0.5) - 20.7584(+ or -0.806)X10 (super 6) XT (super -2) + 2736.0(112.73)X10 (super 6) XT (super -3) , respectively. The standard calorimetric entropy values at 298.15 K, S (super o) , for anhydrous Fe-cordierite, anhydrous Mg-cordierite and hydrous Mg-cordierite are 460.5+ or -0.5, 406.1+ or -0.4 and 450.9+ or -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 (sub p) values for Fe-cordierite were separated by applying the Komada-Westrum model and the values S (sub vib) (super o) = 447.7 J/(mol.K) and S (sub el) (super 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-Al (sub 2) O (sub 3) -SiO (sub 2) with and without H (sub 2) O. A model C (sub p) polynomial for hydrous Fe-cordierite, Fe (sub 2) Al (sub 4) Si (sub 5) O (sub 18) .H (sub 2) O, was derived as: C (sub p) (super hFe-Cd) = 967.3(+ or -9.7) - 6070.4(+ or -363)XT (super -0.5) - 13.9389(+ or -2.522)X10 (super 6) XT (super -2) +1470.4(+ or -454.84)X10 (super 6) XT (super -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 H (sub 2) O pfu were derived using the experimental phase equilibrium results of Mukhopadhyay & Holdaway (1994) on the reaction 3 Fe-cordierite.nH (sub 2) O = 2 almandine +4 sillimanite +5 quartz +3n H (sub 2) O. For anhydrous Fe-cordierite, Delta (sub f) H (super o) = -8448.26 kJ/mol was obtained and for hydrous Fe-cordierite Delta (sub f) H (super o) = -8750.23 kJ/mol and S (super o) = 520.6 J/(mol.K). Phase relations in the FeO-Al (sub 2) O (sub 3) -SiO (sub 2) -(H (sub 2) O) systems at low pressures are analyzed and isohydrons for H (sub 2) O in hydrous Fe-cordierite are modelled. H (sub 2) O contents decrease with increasing temperature and increase with increasing pressure.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 20
Serial Issue: 1
Title: Low-temperature heat capacity of synthetic Fe- and Mg-cordierite; thermodynamic properties and phase relations in the system FeO-Al (sub 2) O (sub 3) -SiO (sub 2) -(H (sub 2) O)
Affiliation: Universitaet Salzburg, Abteilung Mineralogie, Salzburg, Austria
Pages: 47-62
Published: 200802
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 47
Accession Number: 2008-072088
Categories: General geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Annotation: Includes appendices
Illustration Description: illus. incl. 2 tables
Secondary Affiliation: Christian-Albrechts-Universitaet zu Kiel, DEU, Federal Republic of Germany
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200819
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