Composition

Igneous Rocks and Their Minerals

An observational evaluation of the composition of an igneous rock sample is most relevant to the crystalline textured igneous rocks. A scale of composition is typically described by the changing fractions of felsic and mafic mineral components.

A small number of minerals or mineral groups constitute a majority of the common igneous rocks with a crystalline texture. On the chart below, these rocks are labeled "Intrusive".

Image credit: Steven Earle, CC by https://creativecommons.org/licenses/by/4.0/ modified by S. Brande

Common Felsic Minerals

Felsic igneous rocks are composed of a small group of silicate minerals dominated by

quartz
plagioclase group feldspar
potassium (K) feldspar
muscovite group mica

These minerals do vary in color but because they largely lack large amounts of iron (Fe) and magnesium (Mg), igneous rocks dominated with these minerals typically are lighter in tone (not color).

Click any image to enlarge.

Image credit: R. Weller, Cochise College, modified by S. Brande

Common Mafic Minerals

Mafic igneous rocks are composed of another small group of silicate minerals dominated by

pyroxene group
amphibole group
biotite group
olivine group minerals

These minerals also vary in color but because they contain larger amounts of iron (Fe) and/or magnesium (Mg), igneous rocks dominated with these minerals typically are darker in tone (not color).

Click any image to enlarge.

augite (darker)

Image credit: Augiasz / CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/)

hornblende + biotite (darker)

Image credit: James St. John / CC BY (https://creativecommons.org/licenses/by/2.0)

biotite (dark)

Image credit: R. Weller, Cochise College, modified by S. Brande

olivine (green)

Image credit: James St. John / CC BY (https://creativecommons.org/licenses/by/2.0)

Rock Color by Minerals

Rock Color

Igneous rocks may be composed of a few or many different minerals that vary in color.

Focus on the two categories of minerals in igneous rocks - felsic and mafic. Minerals in these categories (above) differ in color. Rocks with these minerals vary in color according the abundances of the rock's component minerals.

Here is a visual chart to help calibrate your eye to the classification of igneous rock by mineral composition.

Igneous Rock Color Variation

Felsic-rich Rock Colors

Click any image to enlarge.

purple

image credit: R. Weller, Cochise College

pink

image credit: R. Weller, Cochise College

red-orange

image credit: R. Weller, Cochise College

Igneous Rock Color Variation

Mafic-rich Rock Colors

Click any image to enlarge.

green

image credit: R. Weller, Cochise College

Rock Variation by Tone

Rock Tone

Igneous rocks that are phaneritic exhibit crystals large enough to be individually visible with the naked eye. When a phaneritic igneous rock exhibits a mixture of lighter and darker minerals, we can refer to the tone of the rock, its degree of lightness or darkness on a gray scale.

Felsic minerals such as quartz and feldspar may vary in color but both are lighter-toned minerals because these minerals typically contain relatively smaller amounts of iron and magnesium.

Mafic minerals such as augite and hornblende, members of the pyroxene and amphibole groups, are relatively richer in iron and magnesium (and other metals). Thus minerals in these groups appear darker in tone.

When phaneritic igneous rocks are composed of a mixture of felsic and mafic mineral crystals, the mixture may vary greatly in tone (lighter or darker) depending on the percentage of each component. A more formal color index is quantified by the percentage of silica (SiO2) contributed by all the silicate minerals.

An analogy is a mixture of salt (white) and pepper (black).

mostly salt and a little pepper = felsic
much salt and some pepper = intermediate
about equal (or less) salt and pepper (or more) = mafic

As this online treatment is more visually and qualitatively based, it will not use any numerical divisions. The goal here is to form the visual concepts for igneous rocks across broad categories of variation.