Sodium-aluminum and calcium-aluminum silicates are important rock-forming minerals. While many of them are of only theoretical interest, several are of practical importance--as ceramic raw materials and as potential future sources of aluminum. Heretofore, thermodynamic data were difficult to estimate reliably. This has seriously hampered thermodynamic calculations pertaining both to the natural formations of these minerals and to their decomposition in sintering and fluxing reactions.
This paper reports the results of the Federal Bureau of Mines measurements of low-temperature heat capacities of four crystalline silicate minerals--dehydrated analcite ( NaAlSi2O2 ), hexagonal anorthite ( CaAl2Si2O8 ), lawsonite (CaAl2Si2O8.2H2O ), and leonhardite (Ca2Al4Si8O24.7H2O ). It augments previous similar experiments of related minerals, which were summarized recently.
The measured heat-capacity data were used to derive entropy values for four minerals at 298.15° K. Some relations between these entropy values and those of bother similar minerals were discussed. These relations illustrate the difficulty of estimating entropy values of tertiary (or higher) silicates.