Mineral structures, defects and their evolution with pressure and temperature
Published:January 01, 2005
The classic picture of mineral structures is dominated by the periodic repetition of the asymmetric unit over infinite distances. This picture is governed by the principles of symmetry and it has its merits for the description of bulk structures of minerals and their associated equilibrium properties.
However, apart from bulk structure, the properties of the actual mineral assemblies found in nature in the form of rocks are equally determined by mineral surfaces, grain boundaries and sub-grain microstructures. These diverse features require a description of minerals at very different length scales, ranging from the properties of atoms at the sub-ångström range (1 Å = 10−10 m), to the now fashionable nanometre range that would encompass a relatively small and still enumerable number of atoms in a world governed by the forces of quantum mechanics (Fig. 1).
Going on to the micrometre length scales of mineral microstructure and further to the length scales directly accessible to the human eye, i.e. millimetre grain sizes or rock deformations ranging from the metre to the kilometre range, the properties of these structures are more and more dictated by classical mechanics.
Any consideration of the composition, structure and properties of matter leads to the existence of atoms as its basic building units. Atoms may be approximated as spherical, with a diameter between 1 and 5 × 10−10 m. However, they are not indivisible (“ατομοσ”) as stated by Democritus, and modern physics of the past 100 years revealed the three fundamental particles protons, electrons, and neutrons.
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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.