Al environment in tectosilicate and peraluminous glasses: A Al MQ-MAS NMR, Raman, and XANES investigation

Tecto-aluminosilicate and peraluminous glasses have been prepared by conventional and laser heating techniques, respectively, in the CaO-Al₂O₃-SiO₂ system. The structure of these glasses were studied using Raman spectroscopy, X-ray absorption at the Al K-edge and ²⁷Al NMR spectroscopy with two different high fields (400 and 750 MHz). Raman spectroscopy and X-ray absorption are techniques sensitive to the network polymerization and, in particular, show different signal as a function of silica content. However, these two techniques are less sensitive than NMR to describe the local aluminium environment. For tectosilicate glasses, aluminium in five-fold coordination, ^[5]Al, was found and a careful quantification allows the determination of a significant amount of ^[5]Al (7% in the anorthite glass). The proportion of ^[5]Al increases for the peraluminous glasses with small amounts (<2%) of six-fold coordination, ^[6]Al. The presence of ^[5]Al agrees with previous observations of the existence of nonbridging oxygens (NBOs) in tectosilicate compositions. However, the proportion of ^[5]Al in the present study indicates that no major proportion of triclusters (oxygen coordinated to three (Si,Al)O₄ tetrahedra) is required to explain these NBOS.