Book Review: Pegmatites and Their Gem Minerals
This new publication describes the special igneous rocks classified as pegmatites and the gems and other minerals recovered from them. Pegmatites often occur as dikes, sills, or pods in igneous and metamorphic rocks. Most have a composition similar to granite; they may consist of simple minerals such as quartz, feldspar, and mica or, in addition, a more complex set of minerals containing uncommon elements. They can display either no internal zonation or an internal structure with zones of distinct mineral assemblages. Some contain open cavities or pockets into which high-quality crystals can grow unobstructed by surrounding minerals. Pegmatites are major sources of gem tourmaline, beryl, topaz, spodumene, and garnet, along with a variety of other minerals.
The book represents a valuable reference work that will be indispensable to anyone seeking information on these uncommon rocks that produce spectacular crystals of so many colored gemstones as well as various industrial minerals. It focuses principally on pegmatites of granitic composition, since they have been the most prolific gem producers. It is divided into seven chapters, plus an extensive reference list and two appendices. Contained within are nearly 650 photographs along with well-prepared locality maps and educational line drawings. The photographs include images taken in the field as well as those of world-class mineral and gem specimens. The authors’ goal is to provide a broad description of gem-bearing pegmatites based on fundamental geological concepts, combined with field observations and magnificent photographs, written in a less technical and very readable style. The emphasis is on those localities that have produced gem-quality crystals and aesthetic mineral specimens. It is well organized, clearly written, and attractively laid out.
Following the introduction, Chapter 2 presents a historical summary of pegmatite studies and some of the researchers who have made significant contributions to the scientific understanding of these rocks. Chapter 3 contains a brief discussion of important geological concepts and terminology related to pegmatites. Chapters 4 and 7 introduce the current understanding of the occurrence and formation of pegmatites and of the conditions that allow for the creation of open spaces or pockets. Granitic pegmatites represent the results of the latter stages of magma crystallization. Uncommon elements not incorporated in the formation of earlier minerals can become concentrated in these residual magmas to produce minerals that are otherwise rare or absent in most igneous rocks.
Chapters 5 and 6 represent the most important sections of the book. Chapter 5 presents an extended review of the major gem-producing pegmatite districts and localities throughout the world. Localities described include those in the United States, Afghanistan, Brazil, Canada, Italy (Island of Elba), Madagascar, Myanmar, Namibia, Pakistan, Poland, Russia, Ukraine, and elsewhere. Each section includes a historical background, a summary of the geological setting, and information on the mines and the gem minerals found in each deposit. Chapter 6 provides a similar review of the most important gems and minerals that occur in pegmatites, along with a brief discussion of the causes of color in pegmatite minerals.
Pegmatites and Their Gem Minerals is a comprehensive reference for geoscientists, gemologists and gem dealers, mineral collectors, and anyone interested in the occurrences that have supplied some of the world’s finest mineral and gem crystals, as well as the geological conditions that allowed for the formation of these fascinating igneous rocks. It is a magnificent publication.