Transmission electron microscopy, defects, and exsolution in rock-forming minerals
Published:January 01, 2001
Although the term defect may conjure up descriptions like “unusual” or “abnormal”, crystal defects are an ever-present and integral part of any crystal structure. In some cases, these structural and/or chemical heterogeneities are present at very low densities, but in many cases their abundance is large enough to cause profound deviations from nominal stoichiometry. Furthermore, many types of defects are not thermodynamically stable and thus affect the thermodynamic properties of the mineral in which they reside. Similarly, the process of unmixing (precipitation or exsolution) commonly produces both chemical and structural heterogeneity in solid solutions. It is therefore appropriate, perhaps essential, to consider crystal defects and mechanisms of exsolution in a volume devoted to rock-forming mineral solid solutions.
Crystal defects are commonly classified according to their dimensionality. Point defects are zero-dimensional: although obviously of finite size, they do not extend for appreciable distances in any direction. Their associated strain fields may, however, extend well beyond the unit cell in which they reside (Carpenter & Boffa Ballaran, 2001). Dislocations are one-dimensional, or line defects, which may traverse an entire crystal while being spatially restricted to the unit-cell scale in the other two dimensions. “Planar” defects are two-dimensional. In fact, the term is used loosely to describe not only defects that are truly planar, but also two-dimensional defects that curve and thus are not restricted to a single plane. Taken together, dislocations and planar defects are commonly called extended defects.
This chapter will provide an introduction to extended defects, although a brief
Figures & Tables
Solid Solutions in Silicate and Oxide Systems
The EMU book series or notes, as they are called, were introduced to provide university teachers with up-to-date reviews in important, rapidly evolving areas of mineralogy, petrology and geochemistry. They are also meant to introduce scientists into special and often interdisciplinary fields of research. In this regard, a volume on solid solutions is current and sorely needed. The solid Earth, as well as many meteorites and the other solid planets, consists for the most part of mineral solid solutions. Research on solid solutions is extremely broad encompassing work in physics and chemistry, metallurgy, materials science and, last but not least, mineralogy and petrology. Hence, because the theme is so strongly interdisciplinary in nature, the workshop was organised to include solid state physicists, physical chemists, crystallographers, mineralogists and petrologists. The various chapters reflect some of this diversity and show what mineralogy has become. Experimental investigations in mineralogy now routinely include different types of spectroscopies along with more traditional phase equilibrium, X-ray diffraction, calorimetry, and TEM methods. There have also been new and impressive developments in theory and computation. Many computational approaches relating to the study of solid solutions, for example, the Cluster Variation Method or Monte Carlo simulations, have been brought in from materials science, chemistry and physics. It can be concluded that the traditional or historical, and perhaps artificial, boundaries between the various disciplines are disappearing. Many current research efforts in mineralogy are similar to those in chemistry, materials science and physics.