Shock experiments on minerals: Basic physics and techniques
Published:January 01, 2005
About half a century ago the first experimental shock techniques and the basic laws governing the propagation of shock waves have been developed. During these early post-war years Russian and American pioneers were already able to experimentally compress solids to half of their specific volumes (see Trunin, 1998, or Zel’dovich & Raizer, 2002, for a review), i.e. pressures prevailing in the Earth’ s core were reproducible in laboratory shock experiments long before static compression techniques such as the diamond anvil cell approached this limit. The strength of shock experiments particularly lies in the fact that a combination of high pressures and high temperatures can be achieved, while the attainment of high temperatures is still problematic in diamond anvil cell experiments.
In Earth and planetary sciences there are numerous basic interests in employing shock techniques. On one hand, shock experiments are devoted to the measurement of the shock wave equation of state of minerals and rocks at extreme conditions (Wackerle, 1962; Grady, 1977; Marsh, 1980; Ahrens, 1987, 1993; Boslough & Ahrens, 1984; Ahrens & Johnson, 1995a, 1995b). Virtually, the entire range of pressures and temperatures prevailing in the Earth’ s and planetary interiors can be reproduced in the laboratory. In this context, important applications of shock wave data are the correlation of the pressure–density function with the inner structure of planets and the assessment of the melting temperatures at which planetary magma oceans can be produced.
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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.