Fivefold-coordinated aluminum in tectosilicate glasses observed by triple quantum MAS NMR

Eight glasses with molar Mg/2Al approximate to 1 in the system MgO-Al2O3-Si O2 have been studied by magic angle spinning (MAS) NMR spectroscopy. Using triple quantum (3Q) NMR techniques we find evidence for significant concent rations of Al coordinated to five O atoms in all glasses, the proportion in creasing with decreasing Mg/Al and decreasing silica content. In glasses wi th Mg/2Al = 1, up to 6% of the Al is estimated to be coordinated to five ra ther than four O atoms. Calculations of the polymerization state of these l iquids made assuming that all aluminum is in tetrahedral coordination charg e balanced by magnesium are thus seriously in error. Such errors may be of even greater importance at the high temperatures and pressures relevant to the Earth and materials sciences.