Mafic

A shorthand term used by geologists to describe silicate minerals, magmas, and rocks that are relatively high in heavier elements.

• Ma = Magnesium
• Fic = Ferric / Ferrous (Iron)

Mafic rocks represent the Oceanic Crust.

Chemical Composition

• High in: Magnesium and Iron, resulting in a high density (3 g/cm3) and a dark color.
• Low in: Silica ($\mathrm{SiO}{\phantom{A}}_2$), typically contain 45% to 52% Silica.

Because silica acts as a "thickener" in magma, the lack of silica makes mafic lava have a lower viscosity.

Mineralogy

In the Bowen's Reaction Series mafic minerals crystallize at high temperatures (1000°C - 1200°C).

1. Pyroxene: The most common dark mineral in mafic rocks.
2. Calcium-Plagioclase: A feldspar, but specifically the Calcium-rich variety (Anorthite/Bytownite). It is often dark grey or dark blue.
3. Olivine: Often present as green phenocrysts (crystals) inside the black rock.
4. Amphibole: Sometimes present if the magma was slightly wetter or cooler.

Rock Types

• Basalt: The most common rock on Earth's surface (covers 70%)
• Gabbro: The coarse-grained equivalent, found deep in the oceanic crust.

Question and Answers

What two elements combine to form the term "Mafic"?::Magnesium (Ma) and Iron/Ferric (Fic).

What is the typical range of Silica (${\text{SiO}}_2$) content in Mafic rocks?::45% to 52%.

What is the typical density of a Mafic rock compared to a Felsic rock?
?
Mafic is High Density ($\approx 3.0{\text{ g/cm}}^3$) due to iron/magnesium.
Felsic is Low Density ($\approx 2.7{\text{ g/cm}}^3$).