equal to that needed for charge‐balance of tetrahedrally coordinated Al3+. Peraluminous compositions are those where there is excess Al3+ over that which can be charge‐balanced with alkali metals + alkaline earths.
Table 1 Oxide composition (wt %) of common commercial glasses and glass of common magmatic rocks with additional data from http://Earthchem.org.
Source: Modified from [1]
| Window glass | Pyrex | Glass wool | Rockwool | Rhyolite | Dacite | Andesite | Basalt | Phonolite | |
|---|---|---|---|---|---|---|---|---|---|
| SiO2 | 72.6 | 81.1 | 65 | 46.6 | 72.18 | 65.13 | 57.51 | 50.29 | 56.56 |
| TiO2 | 2.4 | 0.39 | 0.64 | 0.93 | 2.06 | 0.87 | |||
| Al2O3 | 0.6 | 0.43 | 2.5 | 13.3 | 13.23 | 15.67 | 16.93 | 14.79 | 19.31 |
| B2O3 | 22 | 4.5 | |||||||
| FeO(T) | 0.8 | 0.2 | 10.6 | 2.90 | 4.73 | 7.08 | 10.94 | 4.02 | |
| MnO | 0.10 | 0.82 | 0.05 | 0.03 | 1.05 | ||||
| MgO | 3.6 | 0.3 | 2.5 | 9.1 | 0.48 | 1.03 | 1.82 | 2.5 | 1.86 |
| CaO | 8.7 | 1.1 | 8 | 10 | 1.53 | 1.47 | 1.85 | 1.38 | 2.28 |
| Na2O | 14.3 | 1.5 | 16.5 | 5.6 | 4.03 | 0.81 | 0.77 | 0.55 | 1.57 |
| K2O | 0.2 | 0.7 | 1.4 | 3.76 | 0.96 | 0.86 | 0.38 | 1.01 | |
| NBO/T | 0.79 | 0.00 | 0.62 | 0.99 | 0.08 | 0.18 | 0.36 | 0.72 | 0.22 |
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.
