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Structural characterization of simple and complex metal oxide silicate glasses and melts can be expressed in terms of nonbridging oxygen, NBO, per tetrahedrally coordinated cation, T (Chapter 2.4). The NBO/T‐values of commercial glasses range from about 0.2–0.3 (for Pyrex glass, for example) to values greater than 3.0 for some slags (Chapter 7.4). The NBO/T of typical window glass is about 0.8, which is similar to those of rock wool. In nature, the NBO/T‐values of melts from individual rock types fall within relatively broad ranges (Figure 5). In general, there is a negative correlation between the NBO/T‐value and the SiO₂ concentration.

The distribution of network‐modifying cations in complex systems is linked to both their alkali metal/alkaline earth ratio and the types of metal cations available for charge‐balance of tetrahedrally coordinated Al³⁺. For the most part, the network‐modifying cations in natural magma are alkaline earths because their Na + K components charge‐balance tetrahedrally coordinated Al³⁺. Among the network‐modifying cations, Mg²⁺ is exclusively a network‐modifier, whereas Ca²⁺ is used both to charge‐balance Al³⁺ and to serve as a network‐modifier (Figure 6).

Figure 5 Calculated distribution of NBO/T‐values of major groups of natural magma compositions derived from the database, http://Earthchem.org. Also shown (arrows) are approximate NBO/T‐values for Pyrex glass and glass wool. Average basalt and rhyolite compositions are shown in Figure 4.

Figure 6 Distribution of network‐modifying cations (Na⁺, Ca²⁺, and Mg²⁺) in natural magmatic liquids of basalt and rhyolite melt compositions as a function of the NBO/T of the melts. The summary was developed from chemical data in http://Earthchem.org.