Structural behavior of Al3+ in peralkaline, metaluminous, and peraluminous silicate melts and glasses at ambient pressure

Peralkaline and peraluminous glasses close to the metaluminous join in the systems Na2O-Al2O3-SiO2 (NAS), CaO-Al2O3-SiO2 (CAS), and MgO-Al2O3-SiO2 (MAS) have been examined with Raman spectroscopy. At least three different SiO2 contents in each of the systems (NAS, CAS, and MAS) have been studied. Each series of glasses spans the metaluminous join at constant silica content. The spectra of glasses in all three systems show changes consistent with a continuous decrease in abundance of depolymerized species and an increase in fully polymerized species as compositions change from peralkaline to peraluminous. There is no evidence for maxima or minima of these abundances across the metaluminous joins for any of the studied series. These observations confirm previous suggestions that non-bridging 0 atoms are a general feature of "fully polymerized" glasses, and that a population of Al exists in the melt structure that is not associated with a charge-balancing cation, even in peralkaline compositions.