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In order to characterize the structure of depolymerized, chemically complex aluminosilicate glasses and melts (Figure 1), it is first necessary to describe the structure of simple binary metal oxide–silica compositions. With this information, one can then consider multicomponent metal oxide silicate and aluminosilicate glasses and melts.

The metal oxides in multicomponent metal oxide–silica systems usually are K₂O, Na₂O, CaO, and MgO. In Al‐free silicate glasses such as window and container glass, for example, these oxides serve only as network‐modifiers. Glass used in television and computer monitors and in optical fibers comprises additional network‐modifying cations including rare earths, large alkaline earths (Sr²⁺ and Ba²⁺) and, sometimes, transition metals. In natural magmatic liquids, these cations can serve both as network‐modifiers and to charge‐balance Al³⁺ in fourfold oxygen coordination as described in Section 2.2. Similar compositions and structural environments can be found in glass and rock wool, E glass, and Vycor.

Figure 4 Summary of distribution of charge‐balancing cations (Na⁺ + K⁺ and Ca²⁺) 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. This web site contains a compilation of analyses of rocks in the published literature, where the individual rock names are those given in the source of the database. As seen in Figure 5, for each of these types of rocks, the NBO/T of their melts comprises a wide range. See Table 1 for average compositions of basalt and rhyolite.

The properties and behavior of SiO₂ in metal oxide silicate melts and glasses differ somewhat from those of pure silica glass and melt. The partial molar volume of this component is slightly smaller (26.8 cm³/mol) than the volume of pure SiO₂ (27.3 cm³/mol) because some of the oxygen in these glasses and melts are nonbridging (NBO) and the partial molar volume of NBO is slightly less than that of bridging oxygen. In metal oxide silicate, the partial molar volume of SiO₂ is independent of composition, however, over wide composition range [8]. Systematic relations between metal/silicon ratio can also be seen in other physical and chemical properties such as viscosity, conductivity, thermal expansion, and compressibility of glasses and melts [1].

In ternary and more complex metal oxide silica melts, the values of most properties cannot be described as linear combinations of the endmembers (mixed alkali effect). For example, window glass, which is essentially a mixture of Ca‐ and Na‐silicate components, is in this category. This behavior is related to the steric effects that govern metal cation ordering among different NBO in ternary and more complex metal oxide–SiO₂ glasses and melts. Ordering affects configurational and mixing properties and, therefore, rheological and thermodynamic properties. The greater the contrast in electronic properties such as their electrical charge and ionic radius of the network‐modifying cations, the greater the effect of mixing on melt and glass properties. This ultimately leads to liquid immiscibility in SiO₂‐rich metal oxide–SiO₂ melts. In fact, at given temperature the width of the immiscibility gap is a positive function, Z/r ² (Z = formal electrical charge, r = ionic radius), of the metal cation.