Plastic deformation of minerals at high pressure: Experimental techniques
Patrick Cordier, Hélène Couvy, Sébastien Merkel, Donald Weidner, 2005. "Plastic deformation of minerals at high pressure: Experimental techniques", Mineral behaviour at extreme conditions, Ronald Miletich
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The Earth is a hot, dynamically evolving planet as shown at the surface either by slow, progressive manifestations (plate motion, continental drift) or more violent ones (earthquakes, volcanoes). Mantle convection, which underlies most geological processes, involves large-scale flow of rocks at high pressure and high temperature. Studying the rheological properties of deep-mantle materials is thus one of the biggest issues in mineral physics. It is also one of the most challenging as most of the deep Earth’s minerals are stable only at high pressure. We now have a broad range of experimental techniques allowing us to cover most of the entire P-T conditions range of the inner Earth. However, the usual methods used for mechanical testing - creep at constant stress, deformation at constant strain rate and stress relaxation - can usually not be achieved under those conditions. Measuring strain and stress is in itself a challenging problem at high pressure. The primary aim of this chapter is to give a rapid overview of the recent advance in the field of experimental deformation of minerals at high pressure.
The most traditional experiment consists of applying a confining pressure to a cylindrical sample on to which an independent axial load is superimposed in order to generate differential stresses. The strain rate is usually held constant while the force on the piston is monitored with time and total deformation. Pressure can be applied by means of a gas as in the system designed by Paterson (Paterson, 1970). This apparatus has the great advantage of high accuracy with regard to axial stress
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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.