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USGS1991

Melting and thermodynamic properties of pyrope (Mg3Al2Si3O12)

The heat capacity of Mg 3 Al 2 Si 3 O 12 glass has been measured from 10 to 1000 K by adiabatic and differential scanning calorimetry. The heat capacity of crystalline pyrope has been determined from drop-calorimetry measurements between 820 and 1300 K. From these and previously published results a consistent set of thermodynamic data is presented for pyrope and Mg 3 Al 2 Si 3 O 12 glass and liquid for the interval 0–2000 K. The enthalpy of fusion at 1570 ± 30 "> 1570 ± 30 K, the metastable congruent 1-bar melting point, is 241 ± 12 kJ/mol "> 241 ± 12kJ/mol .

Geochimica et Cosmochimica Acta↗

USGS1990

Thermodynamics of open networks: Ordering and entropy in NaAlSiO4 glass, liquid, and polymorphs

The thermodynamic properties of carnegieite and NaAlSiO4 glass and liquid have been investigated through Cp determinations from 10 to 1800 K and solution-calorimetry measurements. The relative entropies S298-S0 of carnegieite and NaAlSiO4 glass are 118.7 and 124.8 J/mol K, respectively. The low-high carnegieite transition has been observed at 966 K with an enthalpy of transition of 8.1??0.3 kJ/mol, and the enthalpy of fusion of carnegieite at the congruent melting point of 1799 K is 21.7??3 kJ/mol. These results are consistent with the reported temperature of the nepheline-carnegieite transition and available thermodynamic data for nepheline. The entropy of quenched NaAlSiO4 glass at 0 K is 9.7??2 J/mol K and indicates considerable ordering among AlO4 and SiO4 tetrahedra. In the liquid state, progressive, temperature-induced Si, Al disordering could account for the high configurational heat capacity. Finally, the differences between the entropies and heat capacities of nepheline and carnegieite do not seem to conform to current polyhedral modeling of these properties ?? 1990 Springer-Verlag.

Physics and Chemistry of Minerals↗