Al environment in tectosilicate and peraluminous glasses:: A 27Al 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 Al-27 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, Al-[5]. was found and a careful quantification allows the determination of a significant amount of Al-[5] (7% in the anorthite glass). The proportion of Al-[5] increases for the peraluminous glasses with small amounts (<2%) of six-fold coordination, Al-[6]. The presence of Al-[5] agrees with previous observations of the existence of nonbridging oxygens (NTOs) in tectosilicate compositions. However. the proportion of Al-[5] in the present study indicates that no major proportion of triclusters (oxygen coordinated to three (SiAl)O-4 tetrahedra) is required to explain these NBOS. Copyright (C) 2004 Elsevier Ltd.