Some examples of ferromagnesian minerals in mafic rock include olivine, pyroxene and amphibole. Rocks containing pyroxene and amphibole often also contain feldspar, a non-ferromagnesian mineral.
Olivine is made of fayalite, with a chemical formula of Fe2SiO4, and forsterite, with a chemical formula of Mg2SiO4. Pyroxene can be composed of enstatite (MgSiO3) and ferrosilite (FeSiO3). Igneous amphibole is made of hornblende, which is composed of an iron-magnesium silicate, a calcium-iron-magnesium silicate and an aluminum-iron-magnesium silicate. All of these minerals contain iron and magnesium. Mafic rock is a dark-colored category of igneous rock, the type of rock that forms from solidified magma.