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November-December 2009

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Word to the Wise: Felsic & Mafic

In the description of igneous rocks (see the article on pegmatites of Tanakamiyama, Japan, this issue), two 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 differences, 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.
Dr. John Rakovan, an executive editor of Rocks & Minerals, is a professor of mineralogy and geochemistry at Miami University in Oxford, Ohio.

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