ALUMINOSILICATE MELTS - STRUCTURE, COMPOSITION AND TEMPERATURE

The anionic structure of aluminosilicate melts of intermediate degree of polymerization (NBO/T = 0.5) and with Al/(Al + Si) = 0-0.45 along t he composition join Li2Si4O9-Li-2(LiAl)(4)O-9 (LS4-LA4) has been exami ned in-situ to similar to 1480 degrees C, and compared with recent dat a for melts along the analog composition join K2Si4O9-K-2(KAl)(4)O-9 a nd with less polymerized melts along the join Li2Si2O5-Li-2(LiAl)(2)O- 5 and Na2Si2O5-Na-2(NaAl)(2) O-5. With Al/(Al + Si) < 0.25, the anioni c equilibrium, (1) 2Q(3) double left right arrow Q(2) + Q(4), adequate ly describes the structure. With Al/(Al + Si) greater than or equal to 0.25, a second expression, (2) 2Q(2) double left right arrow Q(3) + Q (1), is required because an additional structural unit, Q(1), is stabi lized in the melts. The enthalpy, Delta H-1, of reaction (1) increases from -36 +/- 4 kJ/mol in the absence of aluminum to 34 +/- 5 kJ/mol a t Al/(Al + Si) = 0.25 and 64 +/- 4 kJ/mol at Al/(Al + Si) = 0.45. Simi lar trends are reported for other alkali aluminosilicate melts. Least- squares fitting of abundance of structural units as a function of temp erature and bulk composition has been conducted. The unit abundance is dominantly a function of temperature, Al/(Al + Si), and bulk melt pol ymerization. Configurational entropy and heat capacity of mixing of me lts above their glass transition temperatures have been calculated wit h the aid of the least-squares fitted equations. The values of these p arameters indicate that as the ionization potential of the metal catio ns increases, configurational heat capacity of alkali aluminosilicate melts becomes temperature dependent. As a result, transport properties (viscosity, diffusivity; and conductivity) of such melts will not sho w Arrhenian behavior even in the high-temperature range. Further, disc ontinuous changes in entropy and heat capacity of mixing results from temperature-induced changes in types of structural units in the melts. Such discontinuous changes would also be reflected in discontinuous c hanges of temperature-dependent transport properties.