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