Final answer:
Ferromagnesian minerals are those that contain a significant amount of iron and magnesium, contributing to their dark color and prevalence in the Earth's mantle and oceanic crust.
Step-by-step explanation:
Ferromagnesian minerals are rich in iron and magnesium, which is reflected in their name; "ferro" signifies iron, and "magnesian" pertains to magnesium. These minerals are important for many geological processes and make up a significant portion of the Earth's mantle and oceanic crust. Common ferromagnesian minerals include olivine, pyroxene, amphibole, and biotite mica, all of which typically appear darker due to their high iron and magnesium content.