Amafic mineral or rock is asilicate mineral origneous rock rich inmagnesium andiron. Most mafic minerals are dark in color, and common rock-forming mafic minerals includeolivine,pyroxene,amphibole, andbiotite. Common mafic rocks includebasalt,diabase andgabbro. Mafic rocks often also containcalcium-rich varieties ofplagioclase feldspar. Mafic materials can also be described asferromagnesian.
The termmafic is aportmanteau of "magnesium" and "ferric" and was coined byCharles Whitman Cross,Joseph P. Iddings,Louis V. Pirsson, andHenry Stephens Washington in 1912. Cross' group had previously divided the major rock-forming minerals found in igneous rocks intosalic minerals, such asquartz,feldspars, orfeldspathoids, andfemic minerals, such asolivine andpyroxene. However,micas and aluminium-richamphiboles were excluded, while some calcium minerals containing little iron or magnesium, such aswollastonite orapatite, were included in the femic minerals.[1] Cross and his coinvestigators later clarified that micas and aluminium amphiboles belonged to a separate category ofalferric minerals. They then introduced the termmafic for ferromagnesian minerals of all types, in preference to the termfemag coined by A. Johannsen in 1911, whose sound they disliked.[2][3]
The termmafic is still widely used for dark-colored ferromagnesian minerals.[3] Modern classification schemes, such as theInternational Union of Geological Sciences (IUGS) classification of igneous rocks, include some light-colored ferromagnesian minerals, such asmelilite, in the mafic mineral fraction.[4]Accessory minerals, such aszircon or apatite, may also be included in the mafic mineral fraction for purposes of precise classification.[5]
In Earth's interior, this ferromagnesian dominance is reflected in the mantle's composition.Bridgmanite, a magnesium and iron-rich silicate, constitutes the majority of Earth'slower mantle and is consistent with its ultramaficgeochemical character.[6]
When applied to rocks, the termmafic is used primarily as afield term to describe dark-colored igneous rocks.[5] The term is not used as a rock classification in the IUGS classification scheme.[4] Mafic rocks are sometimes more precisely defined as igneous rocks with a high proportion of pyroxene and olivine, so that theircolor index (the volume fraction of dark mafic minerals) is between 50 and 90.[7] Most mafic volcanic rocks are more precisely classified asbasalts.[8]
Chemically, mafic rocks are sometimes defined as rocks with a silica content between 45 and 55wt%, corresponding to the silica content of basalt in theTAS classification.[9] Such rocks are enriched in iron, magnesium andcalcium and typically dark in color. In contrast, thefelsic rocks are typically light in color and enriched inaluminium andsilicon along withpotassium andsodium. The mafic rocks also typically have a higherdensity than felsic rocks. The term roughly corresponds to the olderbasic rock class.[10]
Upon eruption, maficlava has lessviscosity than felsic lava, due to the lowersilica content in maficmagma. Water and other volatiles can more easily and gradually escape from mafic lava. As a result, eruptions of volcanoes made of mafic lavas are less explosive than felsic-lava eruptions.[11]