Download W Felsic - Miami University

W ord to the Wise
& Mafic
Felsic
John rakovan
Department of Geology
Miami University
Oxford, Ohio 45056
[email protected]
I
n 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.
Figure 1 summarizes the principal chemical and mineralogical characteristics of the spectrum of common igneous rock types. Note that rock types that fall between felsic
and mafic mineralogy are described as intermediate (52–66
weight percent silica), and those with less than 45 weight
percent silica are described as ultramafic. The most common
felsic rocks are granite and rhyolite, whereas the most common mafic rocks are gabbro and basalt. Peridotite, a family
of ultramafic rocks (including dunite, wehrlite, harzburgite,
and lherzolite) that dominates the earth’s upper mantle,
consists primarily of olivine and pyroxenes.
Although exceptions abound, there is a general relationship between color intensity and the type of igneous rock
(felsic, intermediate, or mafic). Because the minerals that
comprise felsic rocks are often light-colored, felsic rocks are
Dr. John Rakovan, an executive editor of Rocks & Minerals,
is a professor of mineralogy and geochemistry at Miami
University in Oxford, Ohio.
Figure 2
Figure 1. Igneous rock diagram (modified from Grotzinger et
al. 2007). Any vertical line (e.g., the red dashed line) through the
diagram will indicate the minerals present (in relative amounts
proportional to the length of the line segment passing through
each mineral field), the percent silica, and the relative amounts
of Na, K, Ca, Fe, and Mg in the rock type that the line intersects.
Figure 2. Examples of felsic and mafic igneous rocks from Antarctica. Left: basalt (Ross Island); right: granite (Taylor Valley,
Transantarctic Mountains).
usually light-colored. Likewise, because iron-rich silicates
are typically dark-colored, the mafic rocks that they comprise are also dark-colored. A comparison of the most
common felsic and mafic rock types, granite and basalt,
exemplify this color difference nicely (fig. 2). A striking illustration of this color relationship is seen in an aerial image
of the Harrat Khaybar volcanic field in Saudi Arabia, where
both felsic and mafic rocks are juxtaposed (fig. 3).
Volume 84, November/December 2009 559
Figure 3
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
in silicic acid). All of these terms have their greatest significance 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.
Figure 4
Figure 3. Aerial image of the Harrat Khaybar volcanic field, Saudi
Arabia. Dark areas are mafic volcanic rocks (basalts). The lightcolored volcanics are felsic (rhyolite) in composition. Image courtesy 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 chromophores (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 comparing 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 chromiumrich 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 chromophore 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 com560 ROCKS & MINERALS
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. Understanding 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 glossary of terms. 2nd ed. Cambridge: Cambridge University Press.
Taylor, S. R., and S. M. McLennan. 2009. Planetary crusts: Their
composition, origin, and evolution. Cambridge, NY: Cambridge
University Press.
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