Barrie Clarke, The Canadian Mineralogist, 1998, 36. 933
Petrographic rendering has evolved considerably over the decade The epitome of black-and-white line drawings in Petrology of the Igneous Rocks by Hatch, Wells and Wells, dates back more than a century. Then came an era of fuzzy black-and-white photomicrographic atlases (typified by the various Augustithis atlases) which eventually evolved into atlases of sharp color images ( e.g. the MacKenzie, Donaldson and Guilford atlas of igneous rocks ) Mitchell's atlas is the epitome of color. It uses uniformly sharp, large (10.3 x 15.2 cm), true color images in plane polarized light (Ppl) and crossed nicols (xn), and false-color back-scattered electron (bse) images. Many subjects have been photographed at several different magnifications to permit an overview of the whole rock as well as a detailed examination of the groundmass. The result is eye-strain-free petrography at its best.
Roger Mitchell 's petrographic atlas of an important subset of alkaline rocks is an outstanding and indispensable reference for libraries, and essential for anyone doing pure or applied research with these types of rocks.
Scott Smith, Economic Geology, 1999, 94, 144
The rock types discussed are of interest to both academic and exploration geologists. These magmas derive from metasomatised lithospheric and asthenospheric mantle sources, and some of the rocks represent parental magmas. They also commonly carry xenoliths and/or xenocrysts of the mantle, including diamond. Most of the plates are of kimberlite, the main primary source of diamond. Lamproites and orangeite, the other primary sources of diamond, are also well represented. Many of the samples come from mines or prospects. The remaining two rock types, minettes and melilitites, although considered uneconomic, are frequently encountered during exploration. Distinction of the different rock types is a vital part of both exploration programs and petrogenetic studies. The distinction, however, is not always straightforward, given that contrasting rocks may belong to the same petrological clan (rock type) and petrographically similar rocks may belong to different clans.
The rock types covered in this atlas are diverse and complex. All studies of them, whatever the purpose, must be based on petrographic investigations. In recent years a number of publications, including Mitchell's trilogy of books, have dealt with different aspects of the nature of these rock types, and have advanced the nomenclature of them. The major drawback of most of the publications is that they lack illustrations of the unusual mineralogies and textures. When used with the previous specialist publications, the atlas will be of great value to both the novice and specialist in the identification, classification and understanding of the rocks.
Summaries of the main terminology and classification schemes are succinctly presented in eight pages at the beginning of the book. The summary includes the recognition of different petrological clans (or rock types) that has been achieved by establishing petrographic definitions based upon petrographic criteria. The definitions use typomorphic mineral assemblages, most of which are clearly illustrated in the atlas. The other aspect of nomenclature deals with textures and their classification. Textures reflect the mode of emplacment of any rock and are used in ore evaluation and volcanological investigations. Some of the magmas forming these rocks, in particular kimberlites, have unique characteristics that result in different styles of emplacment which produce textures not found in other "more normal" rocks. A good cross-section of the characteristic textures is covered in this book. Mitchell's interpretations of some of the more complex textures produced during different processes of magma disruption in kimberlites may be debated by other petrographers.
The photomicrographs successfully illustrate the main petrographic features of these rock types as well as their contrasting and diverse mineralogies and textures. Samples have been collected from around the world, including many type areas. While the plates include only thin sections, a variety of views are used depending upon what best highlights the different features (transmitted light and cross-polarized light are used, and false-colored backscatter images are shown at selected magnifications). Each plate is accompanied by a description. captions adjacent to the plates are kept to a minimum to allow a large format for the photograph (15 x 10 cm), a feature that definitely offsets what some might see as a disadvantage - the inconvenience of having the more detailed descriptions given at the back of the book.
Mitchell's atlas provides an authoritative pictorial guide to these unusual and important rocks. It will be an invaluable reference book for anyone interested in learning more about these rocks, and is essential for those investigating these or similar rock types, and should be useful to those teaching petrography.
Petrographers, it is said, are only as good as the number of rocks they have examined, and this atlas provides an opportunity to see many.