STRUCTURE AND PROPERTIES OF ALKALI SILICATE MELTS AT MAGMATIC TEMPERATURES

The abundance of the structural units in alkali silicate melts, superc ooled liquids, and glasses determined in-situ from 25 to 1651-degrees- C has been used to estimate the enthalpy (DELTAH) for the disproportio nation Si2O5(2Q3) double-line arrow pointing left and right SiO3(Q2) SiO2(Q4). The enthalpies are in the range of - 34 to 30 kJ/mol with s ystematic compositional correlation. In potassium and sodium systems, where the reaction shifts to the right with increasing temperature, th e DELTAH increases with increasing polymerization (M/Si decreases, M = Na, K). For Li2O . 2SiO2(LS2) melts and supercooled liquids the DELTA H is indistinguishable from 0. By decreasing the Li/Si to that of LS3 (Li2O . 3SiO2) and LS4 (Li2O . 4SiO2), the DELTAH is negative (approxi mately - 30 kJ/mol). For a given ratio M/Si (M = K, Na, Li) there is a negative correlation between the DELTAH and the Z/r2 of the metal cat ion. The slope of the regression lines increases as the system becomes more polymerized (i.e., M/Si is decreased). Activity-composition rela tions among structural units in melt on the liquidus of these three sy stems were derived from the abundance data for the structural units an d the liquidus phase relations in these three systems. The activity co efficients of the individual units, gamma1, are often linear functions of the mol fraction of an individual unit, X(i), of the form lngamma( i) = alphalnX1, where alpha is a constant and X(i) is the mol fraction of the unit.