Polymerization of silicate and aluminate tetrahedra in glasses,melts, and aqueous solutions—IV. Aluminum coordination in glasses and aqueous solutions and comments on the aluminum avoidance principle

The coordination of aluminum with oxygen in crystalline and amorphous alumina, aluminates, and aluminosilicates has been determined with magic angle spinning ²⁷Al nuclear magnetic resonance. The ²⁷Al NMR spectra of crystalline materials show that ^VIAl and ^IVAl can readily be distinguished. The same is not the case for amorphous aluminosilicates due to the superposition of a narrow peak, characteristic of ^IVAl, on a broad band. Our spectroscopic results indicate that Al coordination is not the determining factor in explaining differences in devitrification behavior of albite and anorthite glasses. The coordination of Al in aqueous solutions seems to prevent precipitation of the three common Al(OH)₃ polymorphs (^VIAl) at pH above 10. There is clear evidence to suggest that, in solutions containing Si as well as Al, aluminum coordination is related to the type of precipitate formed in acid (clays, ^VIAl, ^IVAl), and basic (zeolites, ^IVAl) environments. Zeolites can be precipitated in near neutral pH environments at higher temperatures, reflecting an increase in ^IVAl under these conditions. The Al avoidance principle for aluminosilicates does not seem to be a hard principle. It is likely that the validity of this principle depends on the type of modifying cation present in the aluminosilicate framework.