Water speciation in hydrous silicate and aluminosilicate glasses: Direct evidence from ²⁹si-¹h and ²⁷al-¹h double-resonance NMR

Through a combination of ¹H MAS NMR, ¹H → ²⁹Si → ¹H double cross-polarization (CP) MAS NMR and ²⁷Al → ¹H CP MAS NMR, different OH species [SiOH, AlOH, and (Ca,Mg)OH (free OH)] have been unambiguously identified for hydrous Ca,Mg-(alumino)silicate glasses. This confirms my earlier speciation assignments made partially on the basis of ¹H chemical shift arguments. The dissolution mechanisms of water in both Al-free silicate and aluminosilicate glasses (quenched melts) are fundamentally similar. For relatively polymerized compositions, it involves dominantly the formation of TOH species (T: Si, Al) through the rupture of T-O-T linkages, in addition to molecular H₂O; for more depolymerized compositions containing network-modifying cations of large field strength (e.g., Ca, Mg), free OH species are also important.