Mineral surfaces – Part I: Surface-sensitive techniques
Published:January 01, 2005
Over the decades that Earth scientists have been studying the solids and fluids of the Earth, our traditional mineralogical and geochemical methods have taught us a great deal about the composition and properties of the materials that make up our surroundings in nature and also about the materials of the deep Earth and the Universe. Through this information, we have been able to form conceptual models about the reactions that take place at mineral surfaces, at the interface between solid and fluid – be it a gas, a solution, or a melt. An interface is a boundary between phases. It is at the interface, the growing or dissolving front of the solid, where composition of both solid and fluid are defined and mineral structure and morphology are determined. Even processes such as solid-state diffusion and mineral transformation under heat and pressure, begin at a point or a line that may be a physical defect or a chemical inhomogeneity, and continue along a front that separates properties that are slightly different than in the rest of the solid.
Earth scientists have many bulk analytical methods at their disposal. Typically, we have been able to choose among: X-ray diffraction (XRD), electron microprobe (EPMA), scanning electron microscopy (SEM), inductively coupled plasma mass or atomic emission spectroscopy (ICP-MS or ICP-AES), atomic absorption spectroscopy (AAS), optical and other forms of microscopy, infrared, Mössbauer and other forms of spectroscopy, potentiometry, chromatography and other wet-chemistry methods. These techniques give us information about the morphology, composition and structure of minerals
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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.