style="font-size:15px;"> 17 17 Greaves, G.N., Greer, A.L., Lakes, R.S., and Rouxel, T. (2011). Poisson's ratio and modern materials. Nat. Mater. 10: 823–837.18 18 Chumakov, A.I. et al. (2014). Role of disorder in the thermodynamics and atomic dynamics of glasses. Phys. Rev. Lett. 112: 025502.
19 19 Shintani, H. and Tanaka, H. (2008). Universal link between the boson peak and transverse phonons in glass. Nat. Mater. 7: 870–877.
20 20 Luo, P., Li, Y.Z., Bai, H.Y. et al. (2016). Memory effect manifested by a boson peak in metallic glass. Phys. Rev. Lett. 116: 175901.
21 21 Wondraczek, L. et al. (2018). Kinetics of decelerated melting. Adv. Sci. 5 (5): 1700850.
22 22 Huang, P.Y. et al. (2012). Direct imaging of a two‐dimensional silica glass. Nano Lett. 12: 1081–1086.
23 23 Hirata, A. et al. (2011). Direct observation of local atomic order in a metallic glass. Nat. Mater. 10: 28–33.
24 24 Frischat, G.H., Poggemann, J.‐F., and Heide, E. (2004). Nanostructure and atomic structure of glass seen by atomic force microscopy. J. Non‐Cryst. Solids 345–346: 197–202.
25 25 Bernal, J.D. (1960). Geometry of the structure of monatomic liquids. Nature 185: 68–70.
26 26 Stanley, H.E. (ed.) (2013). Liquid polyamorphism. Adv. Chem. Phys. 152: 1–611.
27 27 Le Losq, C. et al. (2017). Percolation channels: a universal idea to describe the atomic structure and dynamics of glasses and melts. Sci. Rep. 7: 16490.
28 28 Greaves, G.N. and Ngai, K.L. (1995). Reconciling ionic transport properties with atomic structure in oxide glasses. Phys. Rev., B 52: 6358–6380.
29 29 Adler, B.J. and Wainwright, T.E. (1959). Studies in molecular dynamics. 1. General method. J. Chem. Phys. 31: 459–466.
Note
1 Reviewers:J. F. Stebbins, Geological Sciences, Stanford University, Stanford, CA, USAA. Takada, Research Center, Asahi Glass Co. Ltd., Yokohama, Japan
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.
Figure 1 Compositional environment of complex silicate melts and glasses. Peralkaline denotes compositional range where there is excess metal cations (alkali metals + alkaline earths) over that necessary for charge‐balance of tetrahedrally coordinated Al3+. Meta‐aluminous compositions are those where the proportion of
