viscosity of iron‐bearing silicates depends on both redox state of iron and on the coordination state of Fe2+ and Fe3+. For example, with Fe3+ in fourfold coordination and Fe2+ in sixfold coordination with oxygen, melt viscosity increases systematically with increasing Fe3+/SFe because silicate polymerization also increases with increasing Fe3+/SFe [12]. When both Fe3+ and Fe2+ are surrounded by octahedral oxygen ligands, this relationship is reversed. Given that the redox ratio in basaltic melts normally is considerably lower and the oxygen coordination number around Fe3+ higher than in more silicate rick melts (andesite and rhyolite, for example), decreasing Fe3+/Fe2+ in the former may result in increased melt viscosity, whereas the opposite trend obtains for the latter.
6 Minor Components in Silicate Glasses and Melts
Minor components such as TiO2 and P2O5 are important in natural and commercial glasses, including optical fibers and glass wool insulating materials. The structural behavior of P5+ in silicate glasses and melts is fairly well known, whereas that of Ti4+ remains more controversial, perhaps because the oxygen coordination environment surrounding Ti4+ may be a composition‐dependent variable.
6.1 Phosphorus Substitution for Silicon
In P2O5 glass, the P─O bridging bond distance (1.60 Å) is nearly identical to the Si─O distance in SiO2 glass (1.62 Å). Additionally, there is a second double‐bonded and shorter (1.43 Å) P=O bond. These structural features remain for glasses in the P2O5–SiO2 system. In the latter, Si–O–P bridges can also be detected.
Phosphorus in metal oxide silicate and aluminosilicate glasses and melts is dissolved by formation of phosphate (PO4) groups. Their degree of polymerization can be derived from 31P NMR spectra as a function of metal oxide/P2O5 ratio [13] in a way similar to Q n ‐species determinations in metal oxide–SiO2 glasses (see also Chapter 2.4 and Section 3.1). In addition, there are minor contributions from Si─O─P linkages. Mixed alumino‐silico phosphate complexes are more common (Chapter 2.4).
6.2 Multiple Roles of Ti4+
The ionic radius of Ti4+ is nearly twice that of Si4+. It is not surprising, therefore, that Ti4+ in crystalline materials commonly occupies sixfold coordination, whereas Si4+ is in tetrahedral coordination. In glasses and melts, on the other hand, the structural behavior of Ti4+ is more complex. From partial molar volume of TiO2,
Raman and XANES spectroscopic data of SiO2─TiO2 glasses suggest Ti4+ in five‐ and sixfold coordination with oxygen at low concentrations (<3 mol % TiO2), whereas in more concentrated solution, Ti4+ is surrounded by four oxygens. However, it also has been suggested from X‐ray and neutron diffraction data that fourfold coordination dominates at low TiO2 concentrations in alkali silicate glasses, whereas fivefold coordination is more important at higher concentrations. The latter conclusions are in qualitative agreement with inferences drawn from Raman spectra of alkali aluminosilicate glasses. In aluminosilicate glasses, the Al/(Al + Si) ratio also affects the oxygen coordination number around Ti4+.
7 Perspectives
We are poised to advance our understanding of silicate compositions from empirical modeling, based on current understanding of property–structure relationships and simple oxide‐based descriptions, to direct determination of the structure of complex systems. The anionic structural backbone is established. As indicated by the first steps taken in this direction, quantitative understanding of structural variations with composition and intrinsic variables is possible. Moreover, with increased miniaturization and micro‐processing, measurements of glass and melt properties with samples under the conditions of interest are now feasible. It will also be possible to track and characterize structurally the kinetics of transformations, to quantify glass‐forming processes, and, likely, to engineer properties of glasses on the basis of our rapidly advancing enhanced understanding of structure–property relations.
References
1 1 Mysen, B.O. and Richet, P. (2018). Silicate Glasses and Melts, 2e. New York: Elsevier.
2 2 Richet, P. and Bottinga, Y. (1986). Thermochemical properties of silicate glasses and liquids: a review. Rev. Geophys. 24: 1–25.
3 3 Randall, J.T., Rooksby, H.P., and Cooper, B.S. (1930). X‐ray diffraction and the structure of vitreous solids. Z. Kristall. 75: 196–214.
4 4 Clark, T.M., Grandinetti, P.J., Florian, P., and Stebbins, J.F. (2004). Correlated structural distributions in silica glass. Phys. Rev. 70: 1–8.
5 5 Lee, S.K. and Stebbins, J.F. (2003). The distribution of sodium ions in aluminosilicate glasses: a high‐field Na‐23 MAS and 3Q MAS NMR study. Geochim. Cosmochim. Acta 67: 1699–1710.
6 6 Whittaker, E.J.W. and Muntus, R. (1970). Ionic radii for use in geochemistry. Geochim. Cosmochim. Acta 34: 945–957.
7 7 Stebbins, J.F., Dubinsky, E.U., Kanehashi, K., and Kelsey, K. (2008). Temperature effects of non‐bridging oxygen and aluminum coordination number in calcium aluminosilicate glasses and melts. Geochim. Cosmochim. Acta 72: 910–925.
8 8 Bockris, J.O.M., Tomlinson, J.W., and White, J.L. (1956). Viscous flow in silica and binary liquid silicate. Trans. Faraday Soc. 52: 299–310.
9 9 Liebau, F. and Pallas, I. (1981). The influence of cation properties on the shape of silicate chains. Z. Kristall. 155: 139–153.
10 10 Mysen, B.O. (1987). Relations between bulk composition, structure and properties. In: Magmatic Silicate Melt (ed. B.O. Mysen), 375–400. Amsterdam: Elsevier.
11 11 Dingwell, D.B. and Brearley, M. (1988). Melt densities in the CaO‐FeO‐Fe2O3‐SiO2 system and the compositional dependence of the partial molar volume of ferric iron in silicate melts. Geochim. Cosmochim. Acta 52: 2815–2825.
12 12 Dingwell, D.B. and Virgo, D. (1988). Viscosities of melts in the Na2O‐FeO‐Fe2O3‐SiO2 systems and factors‐controlling relative viscosities in fully polymerized melts. Geochim. Cosmochim. Acta 52: 395–404.
13 13 Dupree, R., Holland, D., Mortuza, M.G. et al. (1989). Magic angle spinning NMR of alkali phospho‐alumino‐silicate glasses. J. Non Cryst. Solids 112: 111–119.
Note
1 Reviewers:J.F. Stebbins, Geological and Environmental Sciences, Stanford University, Stanford, CA, USAA. Takada, Research Center, Asahi Glass Co. Ltd., Yokohama, Japan
2.7
