mafic

Mafic is a term used in geology and petrology to describe a silicate mineral or igneous rock that is rich in magnesium and iron and relatively low in silica compared with felsic varieties. The word mafic combines elements of magnesium and iron. Mafic rocks are typically darker in color and denser than felsic rocks.

In terms of mineralogy and chemistry, mafic rocks contain essential ferromagnesian minerals such as olivine and pyroxene, with plagioclase feldspar often calcium-rich. They usually have lower silica content, roughly in the range of about 45 to 52 percent SiO2 for many basaltic rocks, and higher proportions of magnesium and iron than silica. Accessory minerals may include magnetite, ilmenite, and amphibole.

Common mafic rocks include basalt and basaltic andesite in the extrusive (volcanic) realm, and gabbro and diabase (also called dolerite) in intrusive settings. Mafic rocks form from relatively hotter, less viscous magmas compared with felsic rocks, and they crystallize at higher temperatures. They often originate from partial melting of the mantle and are prevalent at mid-ocean ridges and oceanic islands, as well as in certain continental settings.

Mafic rocks sit between felsic and ultramafic on the compositional spectrum. They are distinguished by their darker color, higher density, and mineral assemblages dominated by magnesium- and iron-rich minerals, reflecting their geochemical derivation and tectonic environment.