The study of the physical properties of silicate melts is now at an exciting point. Given that a large amount of data exists for average melts at average conditions, we can now build on this knowledge to investigate melts at extreme conditions, to observe the unusual behaviour of melts. The combination of the average data that already exist and newer observations from extreme conditions illustrates how challenging the understanding of silicate melts is. Melts at extreme conditions do not show the physical properties extrapolated from the measurements at average conditions.
There is a range of extreme conditions for silicate melts:
Structure: structure varies with temperature, pressure and composition (T, P, X) and controls the physical properties of melts.
Composition: both Si-rich and Si-poor melts are yet to be investigated thoroughly, as well as Al-rich and Al-poor melts.
Temperature: both high- and low-temperature conditions - this means low and high viscosities, respectively.
Pressure: viscosity will either increase or decrease with pressure depending upon composition.
Time: the investigation of the change in physical properties with time as the melt structure equilibrates with the change in applied stress or temperature.
Although the physical properties of melts at these different conditions will be discussed separately, they are inter-related. The physical properties are a function of structure, which in turn is a function of composition, temperature, pressure and time.
In studying silicate melts the properties density, viscosity, surface tension, compressibility, electrical conductivity, and their dependence on pressure, temperature and composition are determined.
Figures & Tables
Mineral behaviour at extreme conditions
Extreme conditions and their effects on matter and materials are currently fashionable topics in modern science. Perhaps the fascination derives from the unimaginable dimensions that grab our attention and push the boundaries of our imagination. Imagine the pressures in extremely dense neutron stars where electrons and protons are fused together and atoms collapse to the density of an atomic nucleus; imagine temperatures of thousands of degrees Kelvin at the solar surface, or multimegabar and terapascal pressures deep within the interior of our planets. But even a simple droplet of water represents an extreme environment when it comes into contact with an otherwise stable crystal of rock salt, causing the crystal to dissolve as external conditions are drastically changed. We have an inherent desire to understand these diverse kinds of phenomena in nature, the mechanisms of the material changes involved, as well as the extreme conditions which are becoming increasingly demanded to achieve the extraordinary performance of new engineering materials. This rapidly evolving area of science is necessarily interdisciplinary, as it combines fundamental physics, chemistry and biology with geoplanetary and materials science, in addition to increasingly becoming one of the keys to engineering and technology aimed at process optimisation. Current experimental methods permit materials to be studied at pressures of several megabars, temperatures of tens of thousands of degrees Kelvin, and to achieve magnetic fields of several thousand teslas. Moreover, the rapid surge in computer technology has, in turn, permitted the solution of many previously intractable problems, and now even allows the behaviour of matter to be predicted far beyond the range of conditions currently accessible to experimentation. Previously unknown phenomena such as the formation of new phases, new forms of electronic and magnetic order, melting, atomic and electronic excitation, ionisation or the formation of a plasma state might result from exposing matter to extreme conditions well beyond those which were characteristic of the equilibria at the time of formation. With this volume of EMU Notes in Mineralogy we have endeavoured to provide up-to-date reviews of our understanding of the behaviour of minerals and geomaterials at exterior conditions that are sufficiently extreme to induce changes. In total 18 chapters reflect the diversity of this theme, but also demonstrate how strongly interdisciplinary this domain of modern mineralogy has become, bringing together physicists, chemists and geologists as well as experimentalists and computer scientists. The present volume contains the contributions of the lectures presented at the 7th EMU School, held at the University of Heidelberg from June 19 to June 25, 2005.