A thermodynamic model for polymerized sodium aluminosilicate melts
A thermodynamic model for polymerized sodium aluminosilicate melts (in EMPG X; Experimental mineralogy, petrology and geochemistry, A. B. Woodland (editor) and S. F. Foley (editor))
European Journal of Mineralogy (April 2005) 17 (2): 243-250
A simplified thermodynamic model is proposed assuming nepheline-like and tridymite-like structural units being major constituents of polymerized aluminosilicate melts. In the frame of the model, dissolution of albite in the melt can be described by the dissociation reaction Ab (super L) = Ne (super L) + Q (super L) . Static (super 23) Na NMR spectra recorded for glasses along the binary join Ab-Q are consistent with the proposed structural units. The NMR spectra can be broken into two mixed (Lorentzian + Gaussian) components. The narrow line is comparable in chemical shift and width with the symmetric position of sodium in crystalline nepheline. The second much broader line is assigned to a more asymmetric albite-like environment. The proportion of both sites estimated fom static NMR for glasses of the dry Ab-Q binary are close to the calculated proportion in the melt at the glass transition. Using the new thermodynamic model, the liquidus of silica polymorphs in the system Ab-Q can be described well by assuming non-ideal mixing of structural units in the melt with small interaction parameters.