The quantification of the intrinsic disorder in archetypal noncrystalline magnesium aluminosilicates remains unsolved. This lack of knowledge is because of the increased structural perturbation caused by Mg2+, a high field strength cation, resulting in substantial broadening in both spectral and scattering responses. Most progress regarding amorphous aluminosilicate has thus been made with relatively low field strength cations (e.g., Na+ and Ca2+). Here, we quantified the nature of structural disorder in Mg-aluminosilicate glasses in the enstatite (MgSiO3)-pyrope (Mg3Al2Si3O12) join using 17O and 27Al NMR. While Mg-aluminosilicate glasses show a much larger topological and configurational disorder around Al than those of Na- and Ca-analogues, the fraction of [5,6]Al (∼8–10%) and the magnitude of topological disorder do not vary significantly with composition. This implies spatial proximity between Mg2+ and the under-bonded bridging oxygens, such as Al-O-Al and Si-O-Al, while Mg2+ preferentially forms Mg-O-Si over Mg-O-Al. The estimated degree of Al avoidance (Q) of ∼0.65 for Mg-aluminosilicates based on 17O NMR is close to a random distribution of Si/Al (Q = 0) and is thus much smaller than those estimated for Na- and Ca-aluminosilicate glasses (from ∼0.95 to ∼0.85) that often show evidence for Si/Al ordering (Q = 1, complete Al avoidance). The results also revealed that degree of Al avoidance decreases linearly with increasing cation field strength of non-network-forming cations, highlighting the first simple predictive relationship between the nature of chemical disorder and the types of non-network forming cation. This established correlation can be utilized to explain and predict the diverse properties of the Mg-bearing multicomponent glasses and melts with complex composition-dependence.

Knowledge of the extent of chemical and topological disorder in topological disordered oxide glasses and melts is essential for understanding the atomistic origins of their macroscopic properties. Here, we report the high-resolution B-11 and O-17 triple quantum magic angle spinning (3QMAS) NMR spectra for binary borogermanate glasses. The NMR results, together with quantum chemical calculations of cluster energy difference, allow us to estimate the extent of chemical disorder and topology variation with composition. The B-11 NMR result shows that the boroxol ring fraction decreases nonlinearly with increasing mole fraction of Ge and is smaller than that in binary borosilicate glasses, suggesting that the Ge/Si content influences the topological changes. Whereas oxygen clusters are not well resolved in O-17 NMR spectra, the Ge-O-Ge fraction apparently increases with increasing GeO(2) content. The estimated degree of framework disorder (Q) in borogermanate glasses is approximately 0.4, according to quantum chemical calculations based on density functional theory. This is halfway between chemical order (Q = 1) and a random distribution (Q = 0). In contrast, Q is approximately -0.6 for borosilicate glasses, indicating a moderate tendency toward complete phase separation (Q = -1). This result confirms that the degree of framework disorder shows a strong dependence on the type of framework cations (Si or Ge). The predicted configurational enthalpy of borogermanate glasses, explicitly considering both chemical and topological disorder, shows a negative deviation as predicted from the positive Q value. The results demonstrate that the macroscopic properties of topologically disordered noncrystalline solids can be established from the detailed quantification of topological and chemical disorder.

We measured the effect of pressure on the compressional and shear wave velocity (VP, VS) as well as density of Fe‐bearing bridgmanite, Mg0.96(1)Fe2+0.036(5)Fe3+0.014(5)Si0.99(1)O3, using impulsive stimulated light scattering, Brillouin light scattering, and X‐ray diffraction, respectively, in diamond anvil cells up to 70 GPa at 300 K. A drastic softening of VP by ~6(±1)% is observed between 42.6 and 58 GPa, while VS increases continuously with increasing pressure. A significant reduction in Poisson's ratio from 0.24 to 0.16 occurs at ~42.6–58 GPa, while VS increases by ~3(±1)% above ~40 GPa compared to MgSiO3‐bridgmanite. Thermoelastic modeling of the experimental results shows that the observed elastic anomaly of Fe‐bearing bridgmanite is consistent with a spin transition of octahedrally coordinated Fe3+ in bridgmanite. These results challenge traditional views that Fe enrichment will reduce seismic velocities, suggesting that seismic heterogeneities in the mid‐lower mantle may be due to a spin transition of Fe in Fe‐bearing bridgmanite.

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