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.
Viscoelasticity of the Earth’s mantle
Published:January 01, 2005
The study of the Earth’s interior is based upon the comparison of laboratory data on longitudinal and shear wave speeds of minerals with the seismic wave speeds from the Earth (see Fig. 1). This requires
laboratory measurements of the temperature- and pressure-dependence of single-crystal elastic moduli to be recast in terms of wave speeds and densities of polycrystalline materials of possible mantle compositions and mineralogies; together with
highly accurate information on seismic wave speeds as a function of depth in the mantle, together with
jumps in wave speeds due to phase transitions,
a temperature profile of the Earth,
a density profile of the Earth,
a pressure profile of the Earth, together with
a petrological model of the Earth as a function of depth.
While seismologists and petrologists have been acquiring their data, mineral physicists have been working on new, varied and imaginative methods of measuring wave speed first in single crystals, and more recently in polycrystalline materials at the temperature and pressure conditions of the Earth.
There are a range of different methods used to determine the speed at which stress waves travel through materials at high-pressure and temperature conditions (see Fig. 2). These methods include the following:
Shock wave measurements involve shooting a projectile at the sample of interest. The resulting collision creates high temperature, high pressure conditions within the sample, and the speed at which the shock wave travels through the sample is measured (e.g. Jackson & Ahrens, 1979; Watt & Ahrens, 1986; Luo et al., 2002; Panero et al., 2003; Langenhorst & Hornemann, 2005).