Mafic

Basalt

Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron, and hence is a portmanteau of "magnesium" and "ferric".^[1] Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include basalt, dolerite and gabbro.

Chemically, mafic rocks are on the other side of the rock spectrum from the felsic rocks. The term roughly corresponds to the older basic rock class.

Mafic lava, before cooling, has a low viscosity, in comparison to felsic lava, due to the lower silica content in mafic magma. Water and other volatiles can more easily and gradually escape from mafic lava, so eruptions of volcanoes made of mafic lavas are less explosively violent than felsic-lava eruptions. Most mafic-lava volcanoes are shield volcanoes, like those in Hawaii.

Rock texture	Name of mafic rock
Pegmatitic	Gabbro pegmatite
Coarse grained (phaneritic)	Gabbro
Coarse grained and porphyritic	Porphyritic gabbro
Medium grained	Diabase or Dolerite, Microgabbro
Fine grained (aphanitic)	Basalt
Fine grained and porphyritic	Porphyritic basalt
Pyroclastic	Basalt tuff or breccia
Vesicular	Vesicular basalt
Amygdaloidal	Amygdaloidal basalt
Many small vesicles	Scoria
Glassy texture	Tachylyte, sideromelane, palagonite

References

↑ Schlumberger: Oilfield Glossary

Common igneous rocks classified by silicon dioxide content

Type	Ultramafic <45% SiO₂	Mafic 45–52% SiO₂	Intermediate 52–63% SiO₂	Intermediate–felsic 63–69% SiO₂	Felsic >69% SiO₂

Volcanic rocks: Subvolcanic rocks: Plutonic rocks:	Komatiite, Picrite basalt Kimberlite, Lamproite Peridotite	Basalt Diabase (Dolerite) Gabbro	Andesite Diorite	Dacite Granodiorite	Rhyolite Aplite—Pegmatite Granite

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Mafic

See also

References