From Ask GeoMan...
What do the terms mafic and felsic mean?
These are both made up words used to indicate the chemical composition of silicate minerals, magmas, and igneous rocks.
Mafic is used for silicate minerals, magmas, and rocks which are relatively high in the heavier elements. The term is derived from using the MA from magnesium and the FIC from the Latin word for iron, but mafic magmas also are relatively enriched in calcium and sodium. Mafic minerals are usually dark in color and have relatively high specific gravities (greater than 3.0). Common rock-forming mafic minerals include olivine, pyroxene, amphibole, biotite mica, and the plagioclase feldspars. Mafic magmas are usually produced at spreading centers, and represent material which is newly differentiated from the upper mantle. Common mafic rocks include basalt and gabbro. (Please note that some geologists with questionable motives switch the order of the magnesium and iron and come up with the term "femag." This term is not to be confused with Femag, the dull-witted henchman of the Diabolical Dr. Saprolite.)
Felsic, on the other hand, is used for silicate minerals, magmas, and rocks which have a lower percentage of the heavier elements, and are correspondingly enriched in the lighter elements, such as silica and oxygen, aluminum, and potassium. The term comes from FEL for feldspar (in this case the potassium-rich variety) and SIC, which indicates the higher percentage of silica. Felsic minerals are usually light in color and have specific gravities less than 3.0. Common felsic minerals include quartz, muscovite mica, and the orthoclase feldspars. The most common felsic rock is granite, which represents the purified end product of the earth's internal differentiation process.
It is important to note that there are many intermediate steps in the purification process, and many intermediate magmas which are produced during the conversion from mafic to felsic. We call the magmas associated with these intermediate stages "intermediate." Clever, huh?
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