Abstract

The anionic structure of aluminosilicate melts of intermediate degree of polymerization (NBO/T=0.5) and with Al/(Al+Si)=0-0.45 along the composition join Li2Si4O9-Li2(LiAl)4O9 (LS4-LA4) has been examined in-situ to ~1480°C, and compared with recent data for melts along the analog composition join K2Si4O9-K2(KAl)4O9 and with less polymerized melts along the join Li2Si2O5 - Li2(LiAl)2O5 and Na2Si2O5 - Na2(NaAl)2O5.

With Al/(Al+Si)<0.25, the anionic equilibrium, (1) 2Q3 <=> Q2+Q4, adequately describes the structure. With Al/(Al+Si)>0.25, a second expression, (2) 2Q2 <=> Q3+Q1, is required because an additional structural unit, Q1, is stabilized in the melts. The enthalpy, DH1, of reaction (1) increases from -36±4 kJ/mol in the absence to aluminum to 34±5 kJ/mol at Al/(Al+Si)=0.25 and 64±4 kJ/mol at Al/(Al+Si)=0.45. Similar trends are reported for other alkali aluminosilicate melts.

Least-squares fitting of abundance of structural units as a function of temperature and bulk composition has been conducted. The unit abundance is dominantly a function of temperature, Al/(Al+Si), and bulk melt polymerization. Configurational entropy and heat capacity of mixing of melts above their glass transition temperatures have been calculated with the aid of the least-squares fitted equations. The values of these parameters indicate that as the ionization potential of the metal cations increase, configurational heat capacity of alkali aluminosilicate melts becomes temperature-dependent. As a result, transport properties (viscosity, diffusivity, and conductivity) of such melts will be no show Arrhenian behavior even in the high-temperature range. Further, discontinuous changes in entropy and heat capacity of mixing results from temperature-inducted changed in types of structural units in the melts. Such discontinuous would also be reflected in discontinuous changes of temperature-dependent transport properties.