The heat capacity of synthetic anhydrous Mg and Fe cordierite

The heat capacity of synthetic anhydrous Mg and Fe cordierite was measured by differential scanning calorimetry (DSC). ME cordierite was synthesized from a glass at high temperatures and shows an ordered structural state. Fe cordierite was synthesized hydrothermally and dehydrated under reducing conditions to obtain H2O-free material. IR spectroscopy was used to ascertain the lack of H2O in both phases. The molar volume of both phases was measured using powder X-ray diffraction giving 23.316 (7) J/bar for Mg cordierite and 23.672 (6) J/bar for Fe cordierite. DSC measurements between 300 and 950 K were made following the procedure of Bosenick et al. (1996). The data show a precision of about 1% in the case of Mg cordierite and 1.5% for Fe cordierite. Fitted C-p polynomials in J/mol/K are: C-p = 825.98 (+/-8.6) - 3111.4 (+/-328.4) . T-0.5 - 34.938(+/-2.537) x 10(6) . T-2 + 5435.2 (+/-513.82) x 10(6) . T-3 and C-p = 788.1 (+/-9.0) - 1362.4 (+/-340.9) . T-0.5 - 44.817 (+/-2.635) x 10(6) . T-2 + 7351.7 (+/-533.47) x 10(6) . T-3 for Mg and Fe cordierite, respectively. The C-p values determined for Mg cordierite are slightly larger (1-3%) than those measured by drop calorimetry by Pankratz and Kelley (1963). The C-p data for Fe cordierite permit the calculation of its standard third-law entropy, S298 degrees K from the reaction 3Fe cordierite = 2 almandine + 4 sillimanite + 5 quartz giving 465 J/mol/K, which is in good agreement with recent model estimates.