3.4 Classification of Igneous Rocks
As has already been described, igneous rocks are classified into four categories, based on either their chemistry or their mineral composition: felsic, intermediate, mafic, and ultramafic. The diagram in Figure 3.16 can be used to help classify igneous rocks by their mineral composition. An important feature to note on this diagram is the red line separating the non-ferromagnesian silicates in the lower left (K-feldspar, quartz, and plagioclase feldspar) from the ferromagnesian silicates in the upper right (biotite, amphibole, pyroxene, and olivine). In classifying intrusive igneous rocks, the first thing to consider is the percentage of ferromagnesian silicates. That’s relatively easy in most igneous rocks because the ferromagnesian minerals are clearly darker than the others. At the same time, it’s quite difficult to estimate the proportions of minerals in a rock.
Based on the position of the red line in Figure 3.16, it is evident that felsic rocks can have about 1% to 20% ferromagnesian silicates (the red line intersects the left side of the felsic zone 1% of the distance from the top of the diagram, and it intersects the right side of the felsic zone 20% of the distance from the top). Intermediate rocks have between 20% and 50% ferromagnesian silicates, and mafic rocks have 50% to 100% ferromagnesian silicates. To be more specific, felsic rocks typically have biotite and/or amphibole; intermediate rocks have amphibole and, in some cases, pyroxene; and mafic rocks have pyroxene and, in some cases, olivine.
![Figure 3.16 A simplified classification diagram for igneous rocks based on their mineral compositions [SE]](https://opentextbc.ca/physicalgeologyearle/wp-content/uploads/sites/145/2016/06/ingeous-rocks2.png)
Figure 3.16 A simplified classification diagram for igneous rocks based on their mineral compositions [SE]Exercises
Exercise 3.5 Mineral proportions in igneous rocksThe dashed black lines in the diagram represent four igneous rocks. Complete the table by estimating the mineral proportions of the four rocks (to the nearest 10%).
Hint: Rocks b and d are the easiest; start with those.
Figure 3.17 A guide to estimating the proportions of dark minerals in light-colored rocksExercises
Exercise 3.6 Proportions of Ferromagnesian SilicatesThe four igneous rocks shown below have differing proportions of ferromagnesian silicates. Estimate those proportions using the diagrams in Figure 3.17, and then use Figure 3.16 to determine the likely rock name for each one.
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| ___% | ___% | ___% | ___% |
| __________ | __________ | __________ | __________ |
In general, the size of crystals is proportional to the rate of cooling. The longer it takes for a body of magma to cool, the larger the crystals will be. It is not uncommon to see an intrusive igneous rock with crystals up to a centimetre long. In some situations, especially toward the end of the cooling stage, the magma can become water rich. The presence of liquid water (still liquid at high temperatures because it is under pressure) promotes the relatively easy movement of ions, and this allows crystals to grow large, sometimes to several centimetres (Figure 3.18). As already described, if an igneous rock goes through a two-stage cooling process, its texture will be porphyritic (Figure 3.15).
![Figure 3.18 A pegmatite with mica, quartz, and tourmaline (black) from the White Elephant mine, South Dakota [from http://en.wikipedia.org/wiki/Pegmatite#mediaviewer/File:We-pegmatite.jpg]](https://opentextbc.ca/physicalgeologyearle/wp-content/uploads/sites/145/2016/06/pegmatite2.jpg)
Figure 3.18 A pegmatite with mica, quartz, and tourmaline (black) from the White Elephant mine, South Dakota [from http://en.wikipedia.org/wiki/Pegmatite#mediaviewer/File:We-pegmatite.jpg]