Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов
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The coexistence of distinct structural units has important consequences because it has been invoked to account for the unusual properties of SiO2 glass such as a room‐temperature density maximum for glass quenched from temperatures near 1505 °C. Besides, a density minimum is observed near 950 °C for structurally relaxed glass. The anomalous pressure‐ and temperature‐dependence of SiO2 glass compressibility, with maxima near 3 GPa and 100 K, respectively, can also be modeled with two coexisting three‐dimensional structures in SiO2 glass.
2.2 Al2O3
The second most important network‐forming component in complex aluminosilicate glasses and melts is Al2O3 (Table 1). Its concentration range in most natural magma and commercial applications (5–20 wt % Al2O3) can have profound influence on glass and melt properties compared with pure SiO2. These include better glass‐forming ability of melts, improved durability, lower viscosity, and lower thermal expansion.
The type of metal cations serving to charge‐balance tetrahedrally coordinated Al3+ is central to understanding the structural roles of Al3+ in silicate melts and glasses and, therefore, their physicochemical properties. Charge‐balance commonly is achieved with alkali metals and alkaline earths (as in feldspar structures, for example). With an alkali metal, M+, one Al3+ can be charge‐balanced provided that XM+ ≥ XAl3+, whereas for alkaline earths, the requirement is 0.5 XM2+ ≥ XAl3+, where XM+, XAl3+, and XM2+ are atomic fractions of the respective cations.
The structural environment near alkalis and alkaline earths depends on whether these ions play a charge‐balancing or a network‐modifying role [5]. The type and proportion of charge‐balancing cations also are important because of their different effect on the energetics of the O─Al bonds and, therefore, on glass and melt properties. This is seen, for example, in enthalpy of solution (Figure 3), viscosity, and also in melt and glass density, compressibility, and thermal expansion.
Figure 2 Distribution of intertetrahedral angle, ∠(Si–O–Si)o, in SiO2 glass from fitting of 29Si MAS NMR spectra to an angle distribution function. Note that the maximum corresponds to that of cristobalite at its liquidus temperature (1723 °C), and is also similar to that obtained from X‐ray diffraction of SiO2 glass. A recent 17O NMR two‐dimensional dynamic angle study resulted in 147° [3]. These angle distributions are consistent with a SiO2 glass structure comprising predominantly six‐membered rings of three‐dimensionally interconnected SiO4 tetrahedra.
The glass structure along SiO2–MAlO2 joins (M = alkali metal as charge‐balancing cation – meta‐aluminosilicate; see Figure 1) is a continuous evolution of the SiO2 glass structure with substitution of Al3+ for Si4+ in tetrahedral coordination and with only a very small percentage or fraction of a percent of Al3+ in different structural roles. There is marginally more Al3+ in such roles in glasses along the SiO2–CaAl2O4 join [7].
The K+, Na+, and Ca2+ are the dominant charge‐balancers in natural magmatic liquids (Figure 4). For melts and glasses with multiple potential cations for Al3+ charge‐balance, thermodynamic data can be used to establish relative stability of aluminate complexes. There is near equal stability of (KAl)4+ and (NaAl)4+ charge‐balance followed by (Ca0.5Al)4+. In natural rocks, the proportion of Ca2+ relative to (Na+ + K+) decreases with increasing SiO2 concentration so that in rhyolite melt, for example, alkalis dominate over Ca+ for charge‐balancing of Al3+. For less silica‐rich melts, the main charge‐balancing cation is Ca2+. For