Name:
Date:
Geology 12: Igneous Textures and Composition Practice
1. A fine-grained igneous rock, when examined using a microscope, is found to contain 30% dark
ferromagnesian (mafic) minerals. Which other minerals would you expect to find in this rock?
a) Plagioclase feldspar only
b) Plagioclase feldspar and muscovite
c) Potassium feldspar, muscovite and quartz
d) Plagioclase feldspar, potassium feldspar and quartz
*Use the table below to answer questions 2-3.
2. Which of the igneous rocks above would be considered ultramafic?
a) W
b) X
c) Y
d) Z
3. Rock W is shown right. Identify its MCI group (Felsic, Intermediate, Mafic,
Ultramafic), texture, and environment of formation. (3 Marks)
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4. Describe the igneous rock texture of the rock shown right and hypothesize
where/how it was formed. (2 Marks)
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5. Which of the following minerals would crystallize first from a mafic magma? Why
is this? (2 Marks)
a) Quartz
b) Pyroxene
c) Biotite Mica
d) Sodium-rich feldspar
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Geology 12: Igneous Textures and Composition Practice
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6. The two rocks shown right could both be classified as:
a) Mafic and intrusive
b) Mafic and extrusive
c) Felsic and intrusive
d) Felsic and extrusive
Rock A
7. Name and provide a couple of words to identify the minerals present in Rock B.
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8. Examine the rock pictured right.
a) Identify its MCI group. (1 Mark)
Rock B
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b) Determine the percent abundance of the minerals found in this rock.
(2 Marks)
Plagioclase Feldspar: ________________________________________
Amphibole: _______________________________________________
c) Name the texture(s) present. (1 Mark)
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d) Based on this texture, how did the rock form? (1 Mark)
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e) The name of the rock is: (1 Mark)
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9. A sample of course-grained rock was collected from a Kilauea lava flow. Geologists think that it is an
ultramafic xenolith brought up from Earth’s mantle. The rock has the following composition:
How can geologists tell that the material in the xenolith must have formed at a temperature above
1000°C?
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Geology 12: Igneous Textures and Composition Practice
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