Local Al site distribution in aluminosilicate glasses by 27Al MQMAS NMR

Coordination and local environment around Al in glasses and liquids is a long-standing question, which has been a controversial issue in geochemistry and glass science. Nuclear magnetic resonance at high-field (750 MHz) was employed to investigate the geochemically and industrially important CaO–Al2O3–SiO2 system. We show that Al remains mainly in tetrahedral position in glasses, however, the presence of five-fold coordinated aluminum is the general rule throughout the ternary CaO–SiO2–Al2O3 system, except for the low silica percalcic region. The proportion of five-fold (AlV) and six-fold (AlVI) coordinated Al was quantified to determine the effects of composition overall the peralkaline part. Moreover our results indicate that Al may occupy more polymerized positions than Si. These two findings do put new questions to the modeling/understanding of these glasses and their parent liquids.