Glasses and Melts: Linking Geochemistry and Materials Science

The structure and properties of both natural and industrial glasses are influenced by the presence of small amounts of specific elements. Selected examples illustrate the roles of various elements in controlling properties such as coloration, stability, and optical and thermal properties, as well as the effect of redox state of multivalent elements (Ni, Fe, Zn, Zr, Mo) on chemical properties.

An intimate physical and chemical interplay between crystals and melt in magmatic systems gives rise to a vast diversity of igneous rocks and the very structure of terrestrial planets. Yet the actual physical means by which this happens is unclear. The long-standing notion of crystals nucleating, growing, and settling ad infinitum from the interior of large pools of magma to eventually form continental rocks is foundering. Processes operating at the smallest scales within marginal solidification fronts and in mingling crystal slurries throughout highly integrated, vertically extended mush columns give rise to planetary-scale effects.

The transport of magmas in the Earth is a phenomenon of first-order importance to the physical, chemical, and climatological evolution of our planet. Volcanism, in particular, can have dramatic impact on human lives, not only as an immediate environmental hazard but also as a longer-term influence on climate. The transport properties of magma—that is, physical flow in response to stress and diffusion of dissolved components as a result of chemical gradients—have been the subject of intensive study in recent decades. Nevertheless, the complexity of these physical and chemical responses requires an even more generalized picture of magma transport than is currently available. The emerging view of magma transport incorporates melt dynamics, non-Newtonian flow, brittle failure, and the fundamental nature of the glassy and liquid states.

Dissolved volatiles can have tremendous effects on the physical and chemical properties of silicate melts. The most abundant volatile in terrestrial magmas is H2O. A few weight percent of added H2O can reduce melting temperatures of rocks by several hundred degrees and enhance the fluidity of magmas by orders of magnitude. Carbon dioxide and sulfur, although less abundant in natural magmas than H2O, often control the initial stage of magma degassing. The strong effect of volatiles on melt properties is related to the chemical bonding of the volatiles in the melt, which depends in particular on melt composition, temperature and oxygen fugacity. The oxygen fugacity, although very low at magmatic conditions, nevertheless has a large influence on the magma, determining the abundance and composition of minerals, fluid–melt partitioning and the physical properties of the melt.

Much progress has been made in elucidating the complex structures of silicate glasses and melts. X-ray and neutron scattering, spectroscopy, and theoretical calculations now provide a reasonably clear picture of many aspects of the short-range structure of glasses (which approximates the melt structure at the glass transition temperature). Critical effects of redox conditions and volatiles on structure have been clarified. Qualitatively, links between structure and properties such as molar volume, entropy, cation partitioning, and viscosity have been established, but quantitative connections remain challenging. Effects of temperature and pressure on structure have been the subject of much recent work.

Silicate melts are major components of magmatic activity and of its most spectacular expression, volcanic eruptions. The “hidden part” is even more fascinating, as silicate melts are directly involved in matter and heat transfer within the Earth and planets. Silicate glasses, often investigated as a frozen picture of their molten counterparts, are also materials of major importance in technology. Despite the difficulties in rationalizing physical and chemical properties of glasses and melts, due to an incomplete knowledge of their structure, major progress has been made recently in synthetic and natural systems. This issue of Elements reviews the properties of silicate glasses and melts from the molecular to the field scale. It includes insights into their technological applications and describes some recent advances this fast-evolving field.