Structure and properties of alkali silicate melts at magmatic temperatures

Abstract

The abundance of the structural units in alkali silicate melts, supercooled liquids, and glasses determined in-situ from 25 to 1651°C has been used to estimate the enthalpy (ΔH) for the disproportionation reaction, Si 2 O 5 (2Q 3 ) ↔ SiO 3 (Q 2 ) + SiO 2 (Q 4 ). The enthalpies are in the range of -34 to 30 kJ/mol with systematic compositional correlation. In potassium and sodium systems, where the reaction shifts to the right with increasing temperature, the ΔH increases with increasing polymerization (M/Si decreases, M = Na, K). For Li 2 O . 2SiO 2 (LS2) melts and supercooled liquids the ΔH is indistinguishable from 0. By decreasing the Li/Si to that of LS3 (Li 2 O . 3SiO 2 ) and LS4 (Li 2 O . 4SiO 2 ), the ΔH is negative (∼ −30 kJ/mol). For a given ratio M/Si (M=K, Na, Li) there is a negative correlation between the ΔH and the Z/r 2 of the metal cation. The slope of the regression lines increases as the system becomes more polymerized (i.e., M/Si is decreased). Activity-composition relations among structural units in melt on the liquidus of these three systems were derived from the abundance data for the structural units and the liquidus phase relations in these three systems. The activity coefficients of the individual units, γ 1 , are often linear functions of the mol fraction of an individual unit, X i , of the form lnγ i = αlnX i , where α is a constant and X i is the mol fraction of the unit