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-silica te melts and glasses from 25-degrees to 1651-degrees-C has been determ ined with in-situ micro-Raman spectroscopy. From these data an equilib rium constant, K(x), for the disproportionation reaction among the str uctural units coexisting in the melts, Si2O5(2Q3) double line arrow po inting left and right SiO3(Q2) + SiO2(Q4), was calculated (K(x) is the equilibrium constant derived by using mol fractions rather than activ ities 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 sh ifts to the right with increasing temperature, the DELTAH(x) increases with silica content (M/Si decreases, M = Na, K). For melts and superc ooled liquids of composition Li2O . 2SiO2 (Li/Si = 1), the DELTAH(x) i s indistinguishable from 0. By decreasing the Li/Si to 0.667 (composit ion LS3) and beyond (e.g., LS4), the disproportionation reaction shift s 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 betwe en the DELTAH(x) and the Z/r2 of the metal cation. The slope of the DE LTAH(x) vs Z/r2 regression lines increases as the system becomes more silica rich (i.e., M/Si is decreased). Activity coefficients for the i ndividual structural units, gamma(i), were calculated from the structu ral data combined with liquidus phase relations. These coefficients ar e linear functions of their mol fraction of the form lngamma(i) = a ln X(i) + b, where a is between 0.6 and 0.87, and X(i) is the mol fractio n of the unit. The value of the intercept, b, is near 0. The relations hip between activity coefficients and abundance of individual structur al units is not affected by temperature or the electronic properties o f the alkali metal. The activity of the structural units, however, dep end on their concentration, type of metal cation, and on temperature.