Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов
cation for tetrahedral Al3+, whereas in rock wool with a composition more akin to natural basalt (Table 1; see also [1], chapter 18), alkali metals and Ca2+ serve to charge‐balance Al3+ together. Somewhat similar structural features can be found in the glass containment of incinerated household waste.
Figure 3 Energetics of Al,Si substitution along meta‐aluminosilicate joins as a function of ionization potential, Z/r 2, of metal cation that serves to charge‐balance Al3+ in tetrahedral coordination (ionic radius, r, assumed that of six‐coordinated metal cations – data from [6]). Heat of solution of glasses in molten lead borate solution is used as a measure of the substitution energetics. Simple and systematic relations with Z/r 2 are evident, but with distinct separation of relationships for cations with different charge‐balancing cations. This difference stems from different substitution mechanisms of Al3+ for Si4+ depending on whether the charge‐balance is accomplished with monovalent or divalent cations.
3 Metal Oxide–SiO2 Systems
3.1 General Remarks
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.
3.2 Structure
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,