Rock Topics

Composition (Part 1)

• Igneous rocks are also classified based on the minerals that make them up.  Igneous minerals occur in groups called mineral assemblages.

Mafic Minerals

• Dark-colored minerals such as olivine, pyroxene, amphibole, biotite, and calcium-rich plagioclase feldspar tend to occur together.  They form dark-colored igneous rocks described as mafic.

Mafic Minerals
Typical minerals found in mafic rocks (Photographs by Parvinder Sethi)

Mafic minerals include olivine (top left), pyroxene (top right), amphibole (bottom left), and biotite (bottom right).  These minerals are dark-colored and often occur together in mafic igneous rocks.

Gabbro(Photograph by Parvinder Sethi)

Gabbro, shown here, is an example of a mafic igneous rock.  Note the dark, bluish-gray  crystals of calcium-rich plagioclase feldspar.