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A thermodynamic model for polymerized sodium aluminosilicate melts

¹ Institute of Experimental Mineralogy, Russian Academy of Sciences, 142432 Chernogolovka, Russia
² Institute of Physiologically Active Compounds RAS, 142432 Chernogolovka, Russia
³ Institut für Mineralogie, Universität Hannover, Callinstr. 3, D-30167 Hannover, Germany
⁴ Faculty of Geology, Moscow University, Vorobjovy Gory, Moscow, 119992, Russia

^* Present address: Department of Geology and Engineering Geology, PO Box 8358, Grand Forks, ND, 58202, U.S.A. E-mail: alexander_simakin{at}mail.und.nodak.edu or simakin{at}hotmail.com

A simplified thermodynamic model is proposed assuming nepheline-like and trydimite-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^L = Ne^L + Qz^L. Static ²³Na NMR spectra recorded on glasses along the binary join Ab-Qz are consistent with the proposed structural units. The NMR spectra can be decomposed 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 proportions of both sites estimated from static NMR of glasses of the dry Ab-Qz 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-Qz can be described well by assuming non-ideal mixing of structural units in the melt with small interaction parameters.

Key-words: aluminosilicate melts, thermodynamic model, ²³Na NMR spectroscopy, liquidus, melting relations.

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