Mafic
Basalt
Mafic is an adjective describing a silicate mineral or igneous rock that is rich in magnesium and iron, and is thus 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, diabase and gabbro. Mafic rocks often also contain calcium-rich varieties of plagioclase feldspar.
Chemically, mafic rocks are enriched in iron, magnesium and calcium and typically dark in color. In contrast the felsic rocks are typically light in color and enriched in aluminium and silicon along with potassium and sodium. The mafic rocks also typically have a higher density than felsic rocks. The term roughly corresponds to the older basic rock class.
Mafic lava, before cooling, has a low viscosity, in comparison with felsic lava, due to the lower silica content in mafic magma. 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 explosively violent than felsic-lava eruptions. Most mafic-lava volcanoes are shield volcanoes,[citation needed] 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 |
See also
- QAPF diagram
- List of minerals
- List of rock types
- Bowen's reaction series
References
^ Schlumberger: Oilfield Glossary
