Grains: Accessory Minerals
2015. "Grains: Accessory Minerals", A Color Guide to the Petrography of Sandstones, Siltstones, Shales and Associated Rocks, Dana S. Ulmer-Scholle, Peter A. Scholle, Juergen Schieber, Robert J. Raine
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Accessory minerals include all the many detrital minerals that are found in clastic terrigenous rocks that do not contribute directly to rock classification (thus, primarily minerals other than quartz and feldspar). Although thousands of minerals could potentially fall under that definition; practically, a limited number are found with any great frequency. Accessory minerals as a whole typically make up less than 1% (rarely more than 2%) of most terrigenous sedimentary rocks. Quartz arenites commonly have the fewest accessory minerals (as little as 0.05% in some cases); arkoses are somewhat richer in accessories, and lithic arenites generally have the highest levels. This results from the fact that most accessory minerals, like some feldspars and lithic fragments, lack the abrasion resistance or chemical stability to survive erosion, transport and diagenesis. Accessory minerals can be examined in thin sections; alternatively, they can be concentrated by mechanical (shaker table) or flotation (heavy liquid) methods (see, for example, Munsterman and Kerstholt, 1996; Koroznikova et al., 2008) and can then be viewed with stereoscopic microscopes, SEM or other methods. Thin- section examination shows the grains in the context of rock fabric, but such minerals can be quite scarce in any single section. Disaggregation and concentration is much more effective for evaluating the full assemblage of such minerals in rock or sand samples and also allows identification by x-ray or geochemical methods. Because accessory minerals are so commonly studied as separates, they generally are divided into light and heavy minerals with a boundary drawn by various workers at specific gravities between 2.85 and 3 (they also are commonly divided into opaque and nonopaque minerals). The most commonly encountered detrital light accessory minerals are micas (mainly muscovite and also biotite). Heavy minerals are vastly more numerous, and can be grouped into ultrastable, intermediate stability, and unstable categories. The ultrastable minerals are the ultimate survivors, even more stable than quartz under most conditions — thus, they are found in most clastic terrigenous rocks. The intermediate group has varied levels of survivability, but most such minerals can be degraded or removed under specific conditions; minerals in the unstable group survive only under very favorable conditions (minimal mechanical and chemical stresses). There are so many detrital accessory minerals that occur in clastic terrigenous rocks that it is simply impossible to provide a brief yet usable summary of the mineralogical features and optical characteristics of all these minerals. We provide instead a chart of the relative stabilities of the most common accessory minerals (Table 4.1). In addition, characteristic mineral properties are described in the individual photo captions for each mineral illustrated. For additional information readers are encouraged to consult the references at the end of this chapter or the more general mineralogy texts listed in the bibliography in the introduction to this book.