Mafic: Definition & Composition

Instructor: Amy Lange

Amy has taught university-level earth science courses and has a PhD in Geology.

Mafic is a term used by geologists to describe igneous rocks and minerals. This lesson will examine the definition of this term and how to relates to earth processes.

Rock Terminology

While non-geologists may look at a rock and just see a rock, geologists are likely thinking of one of the hundreds of descriptive terms they use to describe rocks. Geologists don't intend to be confusing by using so many terms, but rather the terms are used for different properties. They have different terminology for describing chemical composition, texture, mineral proportions, and sometimes even location. This lesson is on mafic, which is one of the terms used to describe the chemical composition of minerals and the rocks in which these minerals are found. Mafic rocks also happen to be the most abundant rock type that makes up the earth's crust. Thus, understanding what mafic rocks are and how they form is very important to understanding the earth's crust.

Mafic Minerals and Rocks

The term mafic is derived from a combination of magnesium and ferric (ferric is from the Latin word for iron). Mafic was originally reserved to describe minerals that had a high percentage of iron and magnesium. However, the term is now commonly used to both describe igneous rocks that contain a high percentage of mafic minerals and magmas that will eventually crystallize a high percentage of mafic minerals. Remember that igneous rocks are those that are formed from cooled magma.

The most common mafic minerals include biotite, hornblende, pyroxene, and olivine.

Large Hornblende Crystal
hornblende crystal

The chemical formulas of these minerals show that they are high in iron and magnesium. For instance, olivine is (Mg,Fe)2SiO4 and hornblende is Ca2(Mg,Fe,Al)5 (Al,Si)8O22(OH)2. The elements in parentheses mean that the proportions of those elements may vary depending on the exact mineral. But overall, these equations show that these minerals have a high proportion of magnesium (Mg) and iron (Fe). How you can tell is to compare the number of metal ions in each chemical formula. Most elements in mineral formulas are metals except oxygen and silica.

Olivine Crystals
Olivine crystals

If we look at the chemical formula for olivine, (Mg,Fe)2SiO2, you'll see that the only metals in the formula are iron and magnesium, so it obviously is a mafic mineral! Let's look at a non-mafic mineral for a comparison. Orthoclase is a mineral with chemical formula KAlSi3O8. You'll notice that its formula has no magnesium or iron, which means Orthoclase is not a mafic mineral.

Large Biotite Crystal
biotite crystal

The high magnesium and iron content of these minerals will give them a dark color. In turn, the high percentage of mafic minerals in a mafic rock will give the overall rock a dark color as well. Felsic rocks are the opposite of mafic rocks, in that they have a low concentration of magnesium- and iron-rich minerals. These rocks are alternatively distinguished by their light color.

Dark-Colored Basalt
dark colored basalt

Mafic and felsic can only be used to describe igneous rocks. Igneous rocks are those that crystallize from magma, or liquid rock.

Light-colored Rhyolite
Light colored rhyolite

Formation and Location of Mafic Rocks

From experimental studies of magma and crystal formation, we know that mafic minerals crystallize at higher temperatures than felsic minerals. This is commonly displayed in the Bowen's Reaction Series, which graphically demonstrates the sequence of mineral crystallization within magmas. The more mafic minerals crystallize first, followed by the more felsic minerals. Thus, mafic rocks form at higher temperatures than felsic rocks.

Bowens Reaction Series
Bowens Reaction Series

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