Silicate melts at magmatic temperatures: in-situ structure determination to 1651�C and effect of temperature and bulk composition on the mixing behavior of structural units

Abstract

The abundance of coexisting structural units in K-, Na-, and Li-silicate melts and glasses from 25° to 1654°C has been determined with in-situ micro-Raman spectroscopy. From these data an equilibrium constant, Kx, for the disproportionation reaction among the structural units coexisting in the melts, Si2O5(2Q3)⇔SiO3(Q2)+SiO2(Q4), was calculated (Kx is the equilibrium constant derived by using mol fractions rather than activities of the structural units). From ln Kx vs l/T relationships the enthalpy (ΔHx) 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 ΔHx increases with silica content (M/Si decreases, M=Na, K). For melts and supercooled liquids of composition Li2O·2SiO2 (Li/Si=1), the ΔHx 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 ΔHx and the Z/r2 of the metal cation. The slope of the ΔHx 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, γi, were calculated from the structural data combined with liquidus phase relations. These coefficients are linear functions of their mol fraction of the form γi=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.