Al coordination and speciation in calcium aluminosilicate glasses: Effects of composition determined by 27Al MQ-MAS NMR and Raman spectroscopy

The structure and properties of glasses and melts in the CaO–Al2O3–SiO2 (CAS) system play an important role in earth and material sciences. Aluminum has a crucial role in this ternary system, and its environment is still questioned. In this paper, we present new results using Raman spectroscopy and 27Al Nuclear Magnetic Resonance on CAS glasses obtained by classic and rapid quenching methods. We propose an Al/Si tetrahedral distribution in the glass network in different Qn species. In this system, we show that Al and Si are mainly in Q4 species along the join R = CaO/Al2O3 = 1, and in depolymerized Q2 and Q3 units at high CaO content for other joins (R = 1.57 and 3). Five- ([5]Al) and six-fold ([6]Al) coordinated aluminum can be detected in the peraluminous glasses (R < 1) in agreement with the deficit of charge compensators, Ca2+ions, near Al atoms. Unexpectedly, between 5% and 8% of [5]Al is also observed for percalcic glasses (R > 1), except for glasses with low silica and high CaO content. The presence of [5]Al is related to viscous flow mechanisms while, in highly depolymerized glasses, the absence of [5]Al may indicate different mechanisms for melts to flow. This systematic study on the CAS system modifies the simple picture of aluminosilicate glasses, and the existence of [5]Al should be included by geochemists, geophysicists and glass scientists to model appropriately the physical properties of aluminosilicate glasses and melts.