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In the description of igneous rocks (see the articleon pegmatites of Tanakamiyama, Japan, this issue),two commonly used terms are felsic and mafic.

These are both used to indicate the chemical com-position of igneous rocks, the silicate minerals thatcomprise them, and the magmas from which theyform (Best 1982; Le Maitre et al. 2002). Felsicis used todescribe rocks containing greater than 66 weight per-cent silica (silicon concentration reported as a neutraloxide, SiO

2). The term maficis used to describe igne-

ous rocks with 4552 weight percent silica. Felsic rocks

are usually also enriched in sodium and potassiumand depleted in iron, magnesium, and calcium rela-tive to mafic rocks. The mineralogy of an igneous rockdepends largely on the chemistry of its parent magmabut is also influenced by temperature and pressureconditions during crystallization. Because of such dif-ferences, rocks formed from felsic and mafic magmas havecontrasting mineralogies. Key minerals in felsic rocks aresodium and potassium feldspars, quartz, feldspathoids, andmuscovite. Indeed, the term felsicis a mnemonic, based onthis mineralogy, formed from (fe) for feldspar, (l) for lenad(a.k.a. feldspathoid), and (s) for silica, plus (-ic) a suffixmeaning having the character of. Likewise, mafic rocks are

dominantly composed of iron- and magnesium-rich silicates,specifically olivine, pyroxenes, amphiboles, and biotite. Theterm mafic comes from (ma) for magnesium and (f) from

ferrum, the Latin word for iron, plus (-ic). Calcium-rich pla-gioclase, although not an iron-magnesium silicate, is also acommon constituent in mafic rocks because mafic magmasare enriched in calcium relative to potassium and sodium.

Figure 1 summarizes the principal chemical and miner-alogical characteristics of the spectrum of common igne-ous rock types. Note that rock types that fall between felsicand mafic mineralogy are described as intermediate(5266weight percent silica), and those with less than 45 weightpercent silica are described as ultramafic. The most commonfelsic rocks are granite and rhyolite, whereas the most com-mon mafic rocks are gabbro and basalt. Peridotite, a familyof ultramafic rocks (including dunite, wehrlite, harzburgite,and lherzolite) that dominates the earths upper mantle,consists primarily of olivine and pyroxenes.

Although exceptions abound, there is a general relation-ship between color intensity and the type of igneous rock(felsic, intermediate, or mafic). Because the minerals thatcomprise felsic rocks are often light-colored, felsic rocks are

Felsic&Mafic

John rakovan

Deptmet f GelgyMimi Uieity

oxfd, oi 45056jf@mui.edu

Dr. John Rakovan, an executive editor of Rocks & Minerals,is a professor of mineralogy and geochemistry at Miami

University in Oxford, Ohio.

Word to theWise

Figure 1. Igeu c digm (mdified fm Gtzige etl. 2007). ay eticl lie (e.g., te ed ded lie) tug tedigm ill idicte te miel peet (i eltie mutpptil t te legt f te lie egmet pig tugec miel field), te pecet ilic, d te eltie mutf n, k, C, Fe, d Mg i te c type tt te lie iteect.

Figure 2. Exmple f felic d mfic igeu c fm at-ctic. Left: blt (r Ild); igt: gite (Tyl vlley,Ttctic Muti).

usually light-colored. Likewise, because iron-rich silicatesare typically dark-colored, the mafic rocks that they com-prise are also dark-colored. A comparison of the mostcommon felsic and mafic rock types, granite and basalt,exemplify this color difference nicely (fig. 2). A striking illus-tration of this color relationship is seen in an aerial imageof the Harrat Khaybar volcanic field in Saudi Arabia, where

both felsic and mafic rocks are juxtaposed (fig. 3).

Figure 2

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Color, however, is a complex phenomenon, especially inrocks, and is related to the presence or absence of chromo-phores (color-causing elements) and their oxidation states(valences), pigments, and light-scattering phenomena. Thiscomplexity leads to many exceptions to the relationshipdescribed above. One such exception can be seen by compar-ing the colors of two ultramafic xenoliths in basalt from SanCarlos, Arizona (fig. 4). The upper xenolith, dominated byiron-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 abovegeneralization would predict for an ultramafic rock. In con-trast, the much darker color of the lower xenolith, dominatedby pyroxenes of a different composition, agrees better withthis generalization. Of the eight most abundant elements inthe earths 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 thatalthough the earths mantle is composed of ultramafic rocks,it is slightly depleted in iron relative to the crust (Taylor andMcLennan 2009), and that the ultramafic rocks of the mantle,such as the peridotite in figure 3, are generally lighter in colorthan mafic crustal rocks. Another example is the commonly

dark to very dark color of anorthosite, an igneous rock com-

posed of 90100 percent calcium-rich plagioclase feldspar(i.e., labradorite, bytownite, or anorthite). The dark color isthe result of small amounts of finely disseminated inclusionsof iron and titanium oxides, which act as pigments (DonLindsey, pers. comm., 2009).

Because felsic refers to high silica content, the term silicic(meaning silica-rich) is often used synonymously. It wasonce thought that silicic acid was the dominant form of sili-

con in rocks (this is not the case), so the term acidicis some-times also used as a synonym of felsic. In contrast, maficrocks are sometimes referred to as being basic(i.e., depletedin silicic acid). All of these terms have their greatest signifi-cance in their relationship to earth chemistry, which in turnis related to where and how magmas form and evolve; this isthe essence of igneous petrology (the study of igneous rocksand the conditions in which they form).

aCknowLEDGMEnTsI 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,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: Theircomposition, origin, and evolution. Cambridge, NY: CambridgeUniversity Press. q

Figure 3. aeil imge f te ht kyb lcic field, sudiabi. D e e mfic lcic c (blt). Te ligt-cled lcic e felic (ylite) i cmpiti. Imge cu-tey f te Imge sciece & alyi Lbty, nasa Jspce Cete. atut ptgp Iss016-E-34524.

Figure 4. Mtle xelit (ultmfic) i blt (mfic) fm

s Cl, aiz. Te uppe (gee) xelit i peidtite(iety lezlite) d te le (b) e i pyxeite.

Figure 3

Figure 4