Water Speciation and Diffusion in Silicate Melts and Glasses

Water dissolved in silicate melts and glasses exists in two main forms—molecular H2O and hydroxyl groups (OH)—each altering the silicate network in distinct ways. Incorporation of water induces depolymerisation of the Si–O framework by breaking bridging oxygen bonds and forming Si–OH linkages, thereby reducing viscosity and modifying density and other transport properties. Diffusion of these species underpins volcanic processes such as bubble growth, degassing and eruptive dynamics, and also governs post-eruption hydration of natural glasses and engineered glass products. Recent advances combining in situ high-energy X-ray diffraction, vibrational spectroscopy and molecular dynamics simulations have clarified bond rearrangements at the atomic scale, while numerical transport models now integrate speciation kinetics with concentration- and temperature-dependent diffusivities. A deeper mechanistic understanding of water speciation and diffusion not only informs models of magmatic evolution and hazard assessment but also guides the design of hydrous glass materials with tailored properties.

Research from Nature Portfolio

One recent direct structural analysis of a water-saturated albite melt at 800 °C and 300 MPa employed high-energy X-ray diffraction coupled with molecular dynamics simulations to reveal that water reacts preferentially at Si–O–Si bridges, producing Si–OH species while leaving Al–O bonds intact. The study demonstrated that Na+ remains an active network modifier under hydrous conditions, shifting from bridging to non-bridging coordination and sustaining network depolymerisation. Furthermore, hydrous conditions were shown to expand Si–O and Al–O bond lengths by approximately 6 % compared with dry melts, emphasising the need to incorporate detailed structural changes into models of water dissolution in alkali aluminosilicate systems under high-pressure and high-temperature conditions.

Water Speciation and Diffusion in Silicate Melts and Glasses publication trend

The graph below shows the total number of articles in water speciation and diffusion in silicate melts and glasses across all publications each year (not limited to Nature Index journals).

Technical terms

Speciation: The distribution of dissolved water between molecular H2O and hydroxyl (OH) within the silicate network.

Diffusion: The transport of dissolved species through a melt or glass driven by concentration gradients.

Bridging oxygen (BO): An oxygen atom that links two tetrahedral cations (e.g. Si) in the silicate network, contributing to polymerisation.

Non-bridging oxygen (NBO): An oxygen atom bonded to only one tetrahedral cation, produced when the network is depolymerised by water or other modifiers.

Hydroxyl group (OH): A water species bonded to network-forming cations following dissociation of molecular H2O, which alters local connectivity.

Molecular water (H2Om): Undissociated water molecules dissolved interstitially within the silicate matrix, with distinct transport and bonding behaviour.

Melt polymerisation: The degree of connectivity of the silicate tetrahedral network, determined by the relative abundance of BO and NBO species.

References

1. Insights on the dissolution of water in an albite melt at high pressures and temperatures from a direct structural analysis. Scientific Reports (2023).
2. Intramolecular hydrogen isotope exchange inside silicate melts – The effect of deuterium concentration. Chemical Geology (2022).
3. An overview on the effect of dissolved water on the viscosity of soda lime silicate melts. Journal of Non-Crystalline Solids X (2023).
4. An experimentally-validated numerical model of diffusion and speciation of water in rhyolitic silicate melt. Geochimica et Cosmochimica Acta (2020).