4.4 Igneous Minerals

Common Minerals in Igneous Rocks

We have just explored how different types of molten materials (i.e., mafic and felsic) will cause the color of igneous rocks to also be different. But the main factor that actually causes this specific difference between dark rocks and light rocks are the minerals that compose each rock.

Felsic rocks mostly contain minerals that are lighter in color. The name originates from FELdspar and SIliCa.

Mafic rocks mostly contain minerals that are darker in color. The name originates from MAgnesium and FerrIC iron.

Classification of igneous rocks by composition. Darker rocks are ultramafic or mafic whereas lighter rocks are intermediate/felsic.
Figure 4.4.1 Classification of igneous rocks by composition. Darker rocks are ultramafic or mafic, whereas lighter rocks are intermediate/felsic. Y-axis refers to the abundance of Fe and Mg-containing minerals in each rock type. Image by Karla Panchuk (Physical Geology, First University of Saskatchewan Edition) CC BY 4.0

Because this is a predictable pattern, we can usually guess which igneous rock we are looking at based on the minerals it has–and vice versa!

Next, we will examine the mineralogical composition of igneous rocks. Based on whether an igneous rock is mafic, intermediate, or felsic, a different abundance of minerals will be present. We will review some of the different minerals that are present in common igneous rocks. [1]

Mineral Appears As Chemical Formula
ORTHOCLASE/ ALKALI FELDSPAR

Orthoclase - a pink to tan colored mineral
Figure 4.4.2 “Orthoclase” by Photographer: John Bosworth is licensed under CC BY 4.0
 Pinkish-tan grains or coloring KAlSi3O8
MUSCOVITE MICA

Muscovite Interactive Model. Muscovite is flaky, gray to white, and micaceous.
Figure 4.4.3 Muscovite. Click on this image to go to a 3D interactive model by rocksandminerals (CC BY)
 Shiny, flaky flecks KAl2(AlSi3O10)(F,OH)2
BIOTITE MICA
Biotite interactive model. biotite is black, flaky and micaceous.
Figure 4.4.4 Biotite. Click on this image to go to a 3D interactive model by rocksandminerals (CC BY)
 Black flecks, in small amounts K(Mg,Fe)2–3Al1–2Si2–3O10(OH,F)2
QUARTZ

Cluster of white to transparent crystals
Figure 4.4.5 3D Interactive Model of Quartz – Click to explore, by University Libraries at Virginia Tech, CC BY
 Clear to grayish crystals SiO2
PLAGIOCLASE FELDSPAR
White opaque crystal in a rhombohedral shape
Figure 4.4.6 3D Interactive model of plagioclase – Click to Explore, by rockdoc, CC BY 4.0
 White grains and crystals NaAlSi3O8 – CaAl2Si2O8
AMPHIBOLE
File:Magnesio-hornblende (cropped).png Dark gray to green prismatic crystals
Figure 4.4.7 “File:Magnesio-hornblende (cropped).png” by Creator:Robert Lavinsky is licensed under CC BY-SA 3.0
 Dark gray to blackish grains Mg7Si8O22(OH)2 -Fe7Si8O22(OH)2
PYROXENE
Tan-Greenish pyroxene mineral with distinctive cleavage planes
Figure 4.4.8 Enstatite pyroxene mineral. Image by Rob Lavinsky, CC BY-SA 3.0, Wikimedia
 Dark gray grains Mg2Si2O6 – Fe2Si2O6
OLIVINE
Olivine Interactive Model. Olivine is light to dark green, granular clusters of crystals
Figure 4.4.9 Olivine. Click on this image to go to a 3D interactive model by UQ School of Earth and Environmental Science (CC BY)
 Dark green grains (in small amounts) (Mg,Fe)2SiO4

We are now familiar with the 8 primary minerals in volcanic rocks! Using this information, we can start to identify these minerals in actual samples of igneous rocks and determine what category of rock we are examining!

Mineral composition of igneous rocks where mafic rocks have minerals with more Mg,Fe, Ca elements and Felsic rocks have minerals with more Si,Na,K elements.
Figure 4.4.10 Mineral Content of different types of igneous rocks, where the y-axis is the percent of mineral makeup generally found in a given composition of the rock. Image by Julie Anne Workman, Wikimedia Commons, CC BY-SA 3.0

Let’s Review!

Igneous rock close up with pinkish grains, grayish-clear grains, white grains, and black flecks.
Figure 4.4.11 Close up of an igneous rock. Image by James St John, Flickr CC-BY 2.0
Coarse-grained igneous rock with large blackish grains in a white matrix.
Figure 4.4.12 Igneous Rock Closeup #2. Image by Thomas Leonard Watson, Wikimedia Commons, Public Domain

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