SILICATE MELTS AT MAGMATIC TEMPERATURES - IN-SITU STRUCTURE DETERMINATION TO 1651-DEGREES-C AND EFFECT OF TEMPERATURE AND BULK COMPOSITION ON THE MIXING BEHAVIOR OF STRUCTURAL UNITS

The abundance of coexisting structural units in K-, Na-, and Li-silicate melts and glasses from 25-degrees to 1651-degrees-C has been determined with in-situ micro-Raman spectroscopy. From these data an equilibrium constant, K(x), for the disproportionation reaction among the structural units coexisting in the melts, Si2O5(2Q3) double line arrow pointing left and right SiO3(Q2) + SiO2(Q4), was calculated (K(x) is the equilibrium constant derived by using mol fractions rather than activities of the structural units). From ln K(x) vs 1/T relationships the enthalpy (DELTAHx) for the disproportionation reaction is in the range of -30 to 30 kJ/mol with systematic compositional dependence. In the potassium and sodium systems, where the disproportionation reaction shifts to the right with increasing temperature, the DELTAH(x) increases with silica content (M/Si decreases, M = Na, K). For melts and supercooled liquids of composition Li2O . 2SiO2 (Li/Si = 1), the DELTAH(x) is indistinguishable from 0. By decreasing the Li/Si to 0.667 (composition LS3) and beyond (e.g., LS4), the disproportionation reaction shifts to the left as the temperature is increased. For a given ratio of M/Si (M = K, Na, Li), there is a positive, near linear correlation between the DELTAH(x) and the Z/r2 of the metal cation. The slope of the DELTAH(x) vs Z/r2 regression lines increases as the system becomes more silica rich (i.e., M/Si is decreased). Activity coefficients for the individual structural units, gamma(i), were calculated from the structural data combined with liquidus phase relations. These coefficients are linear functions of their mol fraction of the form lngamma(i) = a lnX(i) + b, where a is between 0.6 and 0.87, and X(i) is the mol fraction of the unit. The value of the intercept, b, is near 0. The relationship between activity coefficients and abundance of individual structural units is not affected by temperature or the electronic properties of the alkali metal. The activity of the structural units, however, depend on their concentration, type of metal cation, and on temperature.