Grant Henderson

Structure of glasses, melts and amorphous materials.

Magmas are generally considered to be mixtures of silicate melt, crystalline products and evolved fluids and gases. Studies of the structure of the silicate melt phase is an area of geological research that has been ongoing since the 1930s and continues to be an area of intense interest for several large research groups in Japan, USA, England, France, Germany, Italy, and Australia. This melt phase is widely acknowledged to play an important role in igneous processes because the physical and chemical behaviour of natural magmas is related to melt structure (cf., October issue of Elements by Henderson et al., 2006). For example, volcanic eruptive styles are dependent upon the viscosity of the eruptive magma and the viscosity is in turn directly related to the structure of the melt within the magma. While in-situ studies at temperature and pressure are most desirable, it is easier, for technical reasons, to use quenched melts or glasses as analogues. This is because glasses are considered to have structures that resemble the liquid state, and as glasses are solid, experimental data can be extracted more easily (e.g., at room temperature) than from molten analogues. A benefit from this approach is that not only has our understanding of melt structure improved, but so has our understanding of the structure of glasses; the latter having numerous technological applications.

The long term objective of my research is to develop a comprehensive understanding of melt structure, its response to changes in temperature and pressure (e.g., Poe et al, 2004), and be able to use this information to predict the behaviour of natural melts and magmas. Simple compositions (Na₂O-SiO₂ for example) are prepared and melted at high temperature then rapidly quenched to glass. The glasses produced have properties that are closely analogous to the corresponding melt. These glasses are then interrogated with sophisticated instrumentation (Raman spectroscopy, Synchrotron based techniques such as EXAFS/XANES), and analysis of the spectroscopic data provides key information with which to interpret the speciation, structure and coordination of the silicate glasses. By establishing how composition affects the structural properties of the synthetic melts it is possible to establish a basis upon which one can develop structural models of more complex natural melt systems.