A low-temperature heat-capacity study of synthetic anhydrous Mg-cordierite (Mg2Al4Si5O18)

The heat capacity of synthetic anhydrous Mg-cordierite (Mg2Al4Si5O18) was measured using adiabatic calorimetry between 6 and 300 K. Mg-cordierite was synthesized from a glass of stoichiometric cordierite composition at 1250 degrees C for one week. The refined cell dimensions using powder X-ray diffraction give a = 17.0553(2) angstrom, b = 9.7167(l) angstrom, and c = 9.3375(l) angstrom with V = 1547.428(1) angstrom(3) and a distortion index of A = 0.25 indicating complete Al-Si ordering. The absence of channel H2O was confirmed by powder IR spectroscopy, which did not show the presence of any bands located around 3600 cm(-1). The C-p results values agree with those obtained previously by Weller and Kelley (1963) who measured the C-p of Mg-cordierite between 50 K and 298 K. The calculated entropy of Mg-cordierite at standard condition is S-298(0) (15)= 404.0 +/- 1.6 J/(mol(.)K) using the C-p data from this study. This value is in agreement with the value presented by Weller and Kelley [407.2 +/- 3.77 J/(mol(.)K)], but appears to be very slightly lower than estimates made from various modeling studies incorporating phase-equilibrium data on Mg-cordierite.