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.
Diamonds as optical windows to extreme conditions
Published:January 01, 2005
Reinhard Boehler, 2005. "Diamonds as optical windows to extreme conditions", Mineral behaviour at extreme conditions, Ronald Miletich
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There are two experimental techniques capable of generating the very high pressure and temperature conditions of the deep interior of planets: laser-heated diamond cells and shock compression. In laser-heated diamond cells temperatures of over 4000 K have been achieved at pressures up to 200 GPa (2 Mbar) (Boehler, 1993). The main advantage in using diamond cell over shock experiments is that P-T conditions can be kept constant for long periods of time (hours), and this allows a large variety of visual, spectroscopic and X-ray diffraction measurements. The principal drawback to the diamond cell are small sample size, temperature gradients, and in some cases chemical reaction of the sample with the diamond or the pressure medium. The installation of high-pressure beam lines at synchrotron facilities, along with recent developments in X-ray diffraction techniques, have significantly improved our ability to measure the phase behaviour of many materials at extreme pressure and temperature conditions.
In comparison to the wide range of temperatures and pressures accessible to the diamond cell method, shock experiments using guns or lasers provide measurements of densities and sound velocities only along a material-specific, nearly adiabatic P-T path (Hugoniot). Another drawback to the shock method is the short experimental time scale. However, the maximum pressures attainable by shock methods are virtually unlimited.
In this paper the experimental technique for obtaining reliable data at simultaneously high pressure and high temperature employing the laser-heated diamond cell is described. A schematic cross-section of a laser-heated diamond cell is shown in Figure.
The principal components of a diamond cell are two diamond anvils compressing a gasket.