Heat capacity of synthetic hydrous Mg-cordierite at low temperatures: Thermodynamic properties and the behavior of the H2O molecule in selected hydrous micro and nanoporous silicates

摘要

The heat capacity, C_P, of a synthetic hydrous cordierite of composition Mg_1.97Al_3.94Si_5.06O₁₈·0.625H₂O was measured for the first time using precise adiabatic calorimetry in the temperature range from 6 to 300 K. Hydrous Mg-cordierite was obtained by hydrothermal treatment of anhydrous Mg-cordierite (Paukov et al. 2006) at 4 kbar and 600 °C for 24 hours. The synthetic product was characterized using X-ray diffraction and powder IR spectroscopy. Rietveld refinement gives a = 17.060(2) Å, b = 9.721(1) Å, and c = 9.338(1) Å with V = 1548.7(3) Å3 and = 0.25, and the IR spectrum shows only the presence of Class I-Type I H₂O in the channel cavities. Small C_P anomalies were observed at 272.98 ±0.03 K and 239.43 ±0.13 K, which are thought to be related to very small amounts of H₂O occurring in tiny fluid inclusions and to surface H₂O, respectively. From the heat-capacity data on hydrous Mg-cordierite, various thermodynamic functions were calculated and are presented in table form. The calculated partial molar entropy for one mole of H₂O in hydrous Mg-cordierite at 298.15 K and 1 bar is 80.5 J/(mol·K). The partial molar volume for H₂O in hydrous Mg-cordierite at 298 K and 1 bar is zero. The C_P results, together with published heat-capacity data on three different zeolites, permit a comparison and analysis of their heat-capacity behavior. The heat-capacity behavior of H₂O molecules in zeolites is more similar to that of ice at T < 300 K and not to gaseous H₂O, which can be attributed to the presence of hydrogen-bonded H₂O molecules. In contrast, the heat-capacity behavior for the "quasi-free" H₂O molecule in cordierite is more similar to that of a free H₂O molecule in the gaseous state between approximately 100 and 300 K. At T 300 K, the heat capacity for H₂O is the smallest in steam with values increasing in hydrous beryl and cordierite, to H₂O in various zeolites, and finally to liquid H₂O. This behavior may reflect the nature of the hydrogen bonding and the energies of internal H₂O stretching modes, which decrease in energy with increasing hydrogen-bonding strength in the various systems.