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2.6 Structure of Chemically Complex Silicate Systems

       Bjorn Mysen

       Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC, USA

1 Introduction

      Chemically complex silicate glasses and melts include natural magmatic liquids as well as many commercial glasses. Magmatic rocks sometimes also are used for commercial purposes with slight compositional adjustment made to optimize processes or material properties and reduce costs (e.g. rock wool, Chapter 9.3).

Glass families such as boro‐ and phosphosilicates are specifically dealt with in Chapters 7.6 and 7.9, respectively. The chemically complex glasses and melts considered here are mainly aluminosilicates with Si⁴⁺ and Al³⁺ as the main network‐formers and alkali metals, alkaline earths, and Fe²⁺ the dominant network‐modifiers. Their structure, properties, and structure/property relations can be described with the aid of information obtained with compositionally simpler unary, binary, and ternary compositions and composition joins (Figure 1). The principal composition variables are metal oxide/silica, alumina/silica, and types and proportions of metal cations with different electronic properties (see also [1]).

      The structural environment changes when pressure during melting is sufficiently high to cause oxygen coordination changes of Al³⁺ and Si⁴⁺ (≥6 GPa). High‐pressure data are so limited, however, that a survey will not be very informative and high‐pressure industrial processes are virtually nonexistent. Pressure will not, therefore, be discussed here.

2 Glass and Melt Polymerization

      The degree of polymerization of the aluminosilicate network affects most glass and melt properties. Melt polymerization can be expressed as the proportion of nonbridging oxygen (NBO) per tetrahedrally coordinated cations (T), NBO/T. The NBO/T can be calculated from the chemical composition of a glass and melt, provided that types and proportions of network‐forming cations are known. Then, NBO/T = (2·O–4·T)/T, where T and O are atomic proportions of tetrahedral cations and oxygen, respectively, and T is given a formal charge of 4 can be readily calculated.

      The principal network‐formers (tetrahedral cations) in complex glasses and melts are Si⁴⁺ and Al³⁺. These will be discussed first.

      2.1 SiO₂

In nature, the SiO₂ concentration in some cases can exceed 80 wt % although in the most common magma, basalt, the SiO₂ range is 45–55 wt %. By comparison, most commercial glasses have from 50 to 70 wt % SiO₂ contents (Table 1).

From the relatively small enthalpy, entropy, and volume of fusion (ΔH, ΔS, and ΔV) of crystalline SiO₂ polymorphs (see [2] for review of data), it may be inferred that silica melt and glass retain a three‐dimensional structure of interconnected SiO₄ tetrahedra that exist in its crystalline polymorphs (quartz, tridymite, and cristobalite). From vibrational, X‐ray, and NMR spectroscopic studies, one also concludes that the SiO₂ glass structure is essentially fully polymerized [3]. Vibrational spectroscopic spectra recorded at temperature above that of the glass transition of silica glass (1208 °C) do not reveal significant structural differences between glass and supercooled liquid. There is an asymmetric distribution of intertetrahedral angles, ranging from ~120 to 180° (Figure 2) with a maximum between 145 and 155° [4]. A 145–155° Si─O─Si angle is that expected in a three‐dimensionally interconnected SiO₂ glass structure consisting predominantly of six‐membered rings. The somewhat asymmetric Si─O─Si angle distribution (Figure 2) suggests that more than one exists in SiO₂ glass. Rings with three or four SiO₄ tetrahedra coexisting with six‐membered rings are those most commonly suggested.