Elastic and piezoelectric properties of minerals II. Structure-property relationships
Our knowledge of physical properties of crystals is rather limited compared to the number of solved crystal structures. Mainly the basic tensorial properties of numerous crystal species belonging to simple structure types like halite-, CsCl-, fluorite-, perovskite-, spinel- and garnet-type have been extensively studied over the years. On the basis of these data relations between chemical composition and certain physical properties could be established. For instance, the mean values of magnetic susceptibility of para- and diamagnetic crystals, dielectric constants, optical refractivity and the Faraday effect can be easily estimated by additivity rules as sums of quasi-persistent contributions of individual atoms, ions or molecules. A well-known example is the Clausius–Mosotti equation
which relates the mean dielectric constant ε of a crystal to the polarisabilities αj of its constituents. nj is the number of particles of type j per unit volume and ε0 denotes the permittivity of vacuum.
In contrast to such physical properties, elasticity exclusively arises from interactions between the constituents of a crystal. The mean elastic stiffness is therefore closely correlated to the lattice energy, and the elastic anisotropy directly reflects the anisotropy of the crystal’s bonding system. The modifications of carbon, SiO2 and Mg2SiO4 provide instructive examples (Fig. 1). Due to their higher tensorial rank, already the second-order elastic properties (Hooke’s law) behave anisotropically even in crystals possessing cubic symmetry (Fig. 2). Consequently, elasticity provides one of the most powerful probes for the investigation of structure-property relationships. Further, a series of rules on the qualitative interpretation of the structural dependence of many other physical properties can be derived from the elastic behaviour. Examples are listed in Table 1.
Figures & Tables
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.