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Ferromagnesian minerals are minerals that have iron or magnesium in their structure. The greater specific gravity is between 3.2 and 3.6. Some examples are pyroxene, biotite, garnet, and olvine.
Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include basalt, diabase and gabbro. Mafic rocks often also contain calcium-rich varieties of plagioclase feldspar.
