Felsic/Mafic

Word to the Wise JOHN RAKOVAN Felsic Department of Geology Miami University & Oxford, Ohio 45056 Mafic [email protected] n the description of igneous rocks (see the article on pegmatites of Tanakamiyama, Japan, this issue), Itwo commonly used terms are felsic and mafic. These are both used to indicate the chemical composition of igneous rocks, the silicate minerals that comprise them, and the magmas from which they form (Best 1982; Le Maitre et al. 2002). Felsic is used to describe rocks containing greater than 66 weight percent silica (silicon concentration reported as a neutral oxide, SiO2). The term mafic is used to describe igneous rocks with 45–52 weight percent silica. Felsic rocks are usually also enriched in sodium and potassium and depleted in iron, magnesium, and calcium relative to mafic rocks. The mineralogy of an igneous rock depends largely on the chemistry of its parent magma but is also influenced by temperature and pressure conditions during crystallization. Because of such dif- ferences, rocks formed from felsic and mafic magmas have contrasting mineralogies. Key minerals in felsic rocks are sodium and potassium feldspars, quartz, feldspathoids, and muscovite. Indeed, the term felsic is a mnemonic, based on this mineralogy, formed from (fe) for feldspar, (l) for lenad (a.k.a. feldspathoid), and (s) for silica, plus (-ic) a suffix meaning “having the character of.” Likewise, mafic rocks are dominantly composed of iron- and magnesium-rich silicates, specifically olivine, pyroxenes, amphiboles, and biotite. The term mafic comes from (ma) for magnesium and (f) from ferrum, the Latin word for iron, plus (-ic). Calcium-rich plagioclase, although not an iron-magnesium silicate, is also a common constituent in mafic rocks because mafic magmas are enriched in calcium relative to potassium and sodium. Figure 2 Figure 1 summarizes the principal chemical and miner- alogical characteristics of the spectrum of common igne- Figure 1. Igneous rock diagram (modified from Grotzinger et ous rock types. Note that rock types that fall between felsic al. 2007). Any vertical line (e.g., the red dashed line) through the and mafic mineralogy are described as intermediate (52–66 diagram will indicate the minerals present (in relative amounts weight percent silica), and those with less than 45 weight proportional to the length of the line segment passing through percent silica are described as ultramafic. The most common each mineral field), the percent silica, and the relative amounts felsic rocks are granite and rhyolite, whereas the most com- of Na, K, Ca, Fe, and Mg in the rock type that the line intersects. mon mafic rocks are gabbro and basalt. Peridotite, a family Figure 2. Examples of felsic and mafic igneous rocks from Ant- of ultramafic rocks (including dunite, wehrlite, harzburgite, arctica. Left: basalt (Ross Island); right: granite (Taylor Valley, and lherzolite) that dominates the earth’s upper mantle, Transantarctic Mountains). consists primarily of olivine and pyroxenes. Although exceptions abound, there is a general relation- usually light-colored. Likewise, because iron-rich silicates ship between color intensity and the type of igneous rock are typically dark-colored, the mafic rocks that they com- (felsic, intermediate, or mafic). Because the minerals that prise are also dark-colored. A comparison of the most comprise felsic rocks are often light-colored, felsic rocks are common felsic and mafic rock types, granite and basalt, exemplify this color difference nicely (fig. 2). A striking illus- Dr. John Rakovan, an executive editor of Rocks & Minerals, tration of this color relationship is seen in an aerial image is a professor of mineralogy and geochemistry at Miami of the Harrat Khaybar volcanic field in Saudi Arabia, where University in Oxford, Ohio. both felsic and mafic rocks are juxtaposed (fig. 3). Volume 84, November/December 2009 559 posed of 90–100 percent calcium-rich plagioclase feldspar (i.e., labradorite, bytownite, or anorthite). The dark color is the result of small amounts of finely disseminated inclusions of iron and titanium oxides, which act as pigments (Don Lindsey, pers. comm., 2009). Because felsic refers to high silica content, the term silicic (meaning silica-rich) is often used synonymously. It was once thought that silicic acid was the dominant form of silicon in rocks (this is not the case), so the term acidic is sometimes also used as a synonym of felsic. In contrast, mafic rocks are sometimes referred to as being basic (i.e., depleted Figure 3 in silicic acid). All of these terms have their greatest signifi- cance in their relationship to earth chemistry, which in turn is related to where and how magmas form and evolve; this is the essence of igneous petrology (the study of igneous rocks and the conditions in which they form). ACKNOWledgmeNTS I thank Kendall Hauer and Liz Widom for their careful reviews of this column. REFERENCES Best, M. G. 1982. Igneous and metamorphic petrology. New York: W. H. Freeman and Co. Grotzinger, J., T. H. Jordan, F. Press, and R. Siever. 2007. Under- standing Earth. 5th edition. New York: W. H. Freeman and Co. Le Maitre, R. W., A. Streckeisen, B. Zanettin, M. J. Le Bas, B. Bonin, Figure 4 and P. Bateman, eds. 2002. Igneous rocks: A classification and glos- sary of terms. 2nd ed. Cambridge: Cambridge University Press. Taylor, S. R., and S. M. McLennan. 2009. Planetary crusts: Their Figure 3. Aerial image of the Harrat Khaybar volcanic field, Saudi composition, origin, and evolution. Cambridge, NY: Cambridge Arabia. Dark areas are mafic volcanic rocks (basalts). The light- University Press. q colored volcanics are felsic (rhyolite) in composition. Image cour- tesy of the Image Science & Analysis Laboratory, NASA Johnson Space Center. Astronaut photograph ISS016-E-34524. Figure 4. Mantle xenoliths (ultramafics) in basalt (mafic) from San Carlos, Arizona. The upper (green) xenolith is a peridotite (variety lherzolite) and the lower (brown) one is a pyroxenite. Color, however, is a complex phenomenon, especially in rocks, and is related to the presence or absence of chromo- phores (color-causing elements) and their oxidation states (valences), pigments, and light-scattering phenomena. This complexity leads to many exceptions to the relationship described above. One such exception can be seen by compar- ing the colors of two ultramafic xenoliths in basalt from San Carlos, Arizona (fig. 4). The upper xenolith, dominated by iron-poor olivine (forsterite) with small bits of chromium- rich diopside (emerald-green) and orthopyroxene (very dark green), is light-colored overall; this is not what the above generalization would predict for an ultramafic rock. In contrast, the much darker color of the lower xenolith, dominated by pyroxenes of a different composition, agrees better with this generalization. Of the eight most abundant elements in the earth’s crust (O, Si, Al, Fe, Mg, Ca, K, and Na) and mantle (O, Si, Mg, Fe, Al, Ca, Na, and Cr), iron is the dominant chro- mophore in minerals and rocks. It is interesting to note that although the earth’s mantle is composed of ultramafic rocks, it is slightly depleted in iron relative to the crust (Taylor and McLennan 2009), and that the ultramafic rocks of the mantle, such as the peridotite in figure 3, are generally lighter in color than mafic crustal rocks. Another example is the commonly dark to very dark color of anorthosite, an igneous rock com- 560 ROCKS & MINERALS.

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