An O K-edge XANES study of glasses and crystals in the CaO–Al2O3–SiO2 (CAS) system

We have measured the O K-edge XANES spectra of a series of standard minerals and glasses in the CaO–Al2O3–SiO2 (CAS) system. Changes observed in the spectra of simple compounds like the calcium aluminates, SiO2, AlPO4, and Al2O3, where the O atoms are surrounded by network former neighbours in four-, five-, and six-fold coordination can be used to interpret structural changes occurring in CAS glasses. In particular, changes in the coordination of Ca, as well as, changes in the Al–NBO and Al–BO distribution can be resolved. For glasses along the CaO–Al2O3 join some Ca is found to be in 9-fold coordination. However, along the (xCaO:Al2O3)–SiO2 joins (where x = 1, 1.57 and 3) the average Ca coordination decreases as the proportion of CaSiO3 increases. Furthermore, along the CaSiO3–Al2O3 join the O K-edge XANES exhibits changes consistent with AlO4Q2 and Q3 species being converted to Q4 species with the addition of Al2O3.