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Structural chemistry

Oxygen bridges in molten glass

Nature volume 390, pages 14–15 (1997)Cite this article

Understanding the behaviour of silicate and aluminosilicate liquids is vital to understanding the formation of igneous rocks, the explosive nature of volcanic eruptions, the viscosity of molten glass and the rate of mixing of melt batches inside industrial glass-melters. Their behaviour depends on their structure, which was thought to be well understood at a local level. But on page 60of this issue1, Stebbins and Xu report that a significant number of ‘non-bridging’ oxygen atoms are present in a glass quenched from an aluminosilicate melt — a glass that according to the conventional theory should not have them.

Models of the structure of aluminosilicate liquids derive from the continuous network theory of glasses proposed by Zachariasen2 and from analogy with feldspars3 (the crystalline aluminosilicate analogues of quartz). In this picture [SiO4]4- tetrahedra are linked at the corners by oxygen atoms to form a three-dimensional connected structure, with all oxygens bridging between tetrahedra. A fully connected network structure can be maintained if aluminium substitutes for silicon, with the charge deficit (Si4+-Al3+) being made up by ‘charge balancing’ cations such as Na+ and Ca2+ located close to Al3+. Any excess Na+ or Ca2+ will acquire non-bridging oxygens between themselves and a tetrahedral cation (Al, Si), thus interrupting the connectivity of the network or reducing the amount of cross-linking between tetrahedra. This concept has been termed ‘depolymerization’ and can be used to explain in general terms the very large variations in viscosity of silicate and aluminosilicate liquids with composition4.

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Figure 1: 2D representation of the structure of an aluminosilicate network.
Figure 2: Cartoon of the atomic-scale flow step in a modified silicate melt.
Figure 3: Molten glass being snipped.

References

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  1. Ian Farnan is in the Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK

    Ian Farnan

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  1. Ian Farnan
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Farnan, I. Oxygen bridges in molten glass. Nature 390, 14–15 (1997). https://doi.org/10.1038/36199

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