Geophysical measurements, such as the lateral variations in seismic wave velocities that are imaged by seismic tomography, provide the strongest constraints on the structure of the Earth's deep interior. In order to interpret such measurements in terms of mineralogical/compositional models of the Earth's interior, data on the physical and chemical properties of minerals at high pressures and temperatures are essential. Knowledge of thermodynamics, phase equilibria, crystal chemistry, crystallography, rheology, diffusion and heat transport are required to characterize the structure and dynamics of the Earth's deep interior as well as the processes by which the Earth originally differentiated.

Many experimental studies have been made possible only by a range of technical developments in the quest to achieve high pressures and temperatures in the laboratory. At the same time, analytical methods, including X-ray diffraction, a variety of spectroscopic techniques, electron microscopy, ultrasonic interferometry, and methods for rheological investigations have been developed and greatly improved. In recent years, major progress has been made also in the field of computational mineralogy whereby "ab initio" simulations are used to investigate the structural and dynamical properties of condensed matter at an atomistic level.

This volume contains a broad range of contributions that typify and summarize recent progress in the areas of high-pressure mineral physics as well as associated technical developments.

The fourth volume in this series consists of eleven chapters. The first five deal with more theoretical aspects of the kinetics and mechanisms of meta morphic reactions, and the next six consider the interdependence of defor mation and metamorphism. All papers deal with natural processes that inter act on various time scales and with different degrees of mass and heat transfer. Consequently, many fundamental axioms of metamorphic petrol ogy and structural geology are questioned both for their accuracy and their usefulness. In raising such questions, most contributors have pointed to ways in which the answers could be forthcoming from appropriate experi mental studies or observations on natural materials. In their discussion of how order/disorder can influence mineral assem blages, Carpenter and Putnis emphasize that metastable crystal growth is common in metamorphic systems and state' 'there may be some reluctance (among many earth scientists) to accept that significant departures from equilibrium could occur." On the basis of presented evidence, they question whether reactions ever occur close to an equilibrium boundary. The neces sity for pressure or temperature overstepping is also required by nucleation rate theory. In any case, the degree of order is severely influenced by these kinetic effects in igneous, sedimentary, and metamorphic environments."