Fluid-mineral interaction at high pressure
Published:January 01, 2005
The term “fluid” is used in different ways in the geologic literature. Sometimes “fluid” is used to denote any kind of mobile phase, including silicate melts. In this chapter, we will, for purely pragmatic reasons, define a fluid as a mobile phase which is not a silicate or carbonate melt. Sometimes the term is defined even more narrowly as a mobile phase in a regime of pressure and temperature where no distinction between “vapour” and “liquid” is possible anymore. We will not follow this use, i.e. a “fluid” in the sense as it will be used in this chapter can have either “vapour-like” or “liquid-like” or transitional properties, unless otherwise stated.
Evidence for the composition of fluids in the Earth’s interior comes essentially from three sources of evidence: (i) the analysis of volcanic gases, (ii) the investigation of fluid inclusions and (iii) considerations of phase equilibria. Gases from volcanoes with a non-explosive eruption style can sometimes be directly sampled, while direct sampling is impossible during major explosions. Naturally, this introduces some bias in the data on volcanic gas compositions, since gas analyses can be much more easily acquired from basaltic magmas than from the often highly explosive andesitic and rhyolitic ones. However, in recent years remote sensing of gas compositions by infrared spectroscopy has become possible and it is to be expected that the further development of these methods will ultimately allow a more representative sampling of volcanic gases from a variety of magma sources and tectonic environments. In any case, although there are quite significant variations, virtually all available analyses show that the predominant constituents
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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.