STRUCTURE OF MINERAL GLASSES .2. SIO2-NAALSIO4 JOIN

The distribution of short range interatomic distances in NaAlSi2O6 and NaAlSiO4 glass has been determined by X-ray radial distribution analysis. The resulting radial distribution functions (RDF's) are interpreted by comparison with RDF's calculated for the Jadeite (NaAlSi2O6) and nepheline (NaAlSiO4) crystal structures and with previously determined RDF's for SiO2 and NaAlSi3O8 glass. The RDF of NaAlSi2O6 glass is considerably different from that calculated for the Jadeite structure and suggests that the NaAlSi2O6 melt at one atmosphere pressure has a stuffed tridymite-like structure with Al in tetrahedral coordination rather than a pyroxene-like structure with Al in octahedral coordination. The RDF of NaAlSiO4 glass is also consistent with a stuffed tridymite-like structure similar to that of crystalline nepheline. Replacement of SiO2 by NaAlO2 across the SiO2-NaAlSiO4 join causes 1. (1) a progressive expansion of the stuffed tridymite structure, 2. (2) an increase in the first T-O distance due to substitution of Al for Si in the tetrahedral sites, 3. (3) a decrease in the inferred Si, AlOSi, Al angle and 4. (4) a loss of detail in the RDF suggesting a progressive loss of structural regularity. The stuffed tridymite model is consistent with the observed densities of glasses along the SiO2NaAlSiO4 join and offers a molecular interpretation of the high viscosity of melts in the Na2OAl 2O3SiO2 system at Na Al ratios of 1.0. © 1979.