Geology Reference
In-Depth Information
Mineral composition of igneous rocks
The composition, or mineral content, of an igneous rock is determined by the elements
that exist in the liquid from which it crystallizes. Most magmas are composed primarily
of silica (usually as the mineral
quartz,
formed by silicon and oxygen) with portions of
other elements such as aluminum, calcium, magnesium, potassium, sodium, and iron.
The relative amounts of these elements in a magma determine which minerals are cre-
ated, as well as the color of the igneous rock.
The terms listed here are used to indicate the differences in composition of igneous
rocks and describe both intrusive and extrusive rocks:
Felsic:
These rocks contain more than 65 percent silica, with portions of other ele-
ments such as aluminum, potassium, and sodium. Felsic rocks are usually light in
color, dominated by quartz, potassium feldspar, and sodium-plagioclase feldspar
minerals. Examples of felsic rocks are granite and rhyolite.
Intermediate:
This category of rocks contains between 55 and 65 percent silica
with portions of other elements such as aluminum, calcium, sodium, iron, and
magnesium. Intermediate rocks are a nearly equal mix of light- and dark-colored
minerals, such as amphibole and various plagioclase feldspars. Examples of inter-
mediate rocks are andesite and diorite.
Maic:
Mafic rocks have between 45 and 55 percent silica with portions of other ele-
ments such as aluminum, calcium, iron, and magnesium. They are usually fairly
dark in color, with minerals such as pyroxene and calcium-plagioclase feldspar.
Examples of mafic rocks are basalt and gabbro.
Ultramafic:
With very low amounts of silica (less than 45 percent) mixed with mag-
nesium, iron, aluminum, and calcium, ultramafic rocks are dark and not very com-
mon. They contain olivine and pyroxene minerals, and examples include komatiite
and peridotite.
Table 7-1 provides a quick overview of the composition, color, and names of various ig-
neous rocks.
