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Igneous Compositions
Igneous rocks are composed mainly of silicate minerals. Chemical analyses
show that silicon and oxygen are by far the most abundant constituents of igneous
rocks. These two elements, plus ions ofaluminum (Al), calcium (Ca), sodium (Na),
potassium (K), magnesium (Mg), and iron (Fe), make up roughly 98 percent, by weight,
of most magmas. In addition, magma contains small mounts of many other elements,
including titanium and manganese, and trace amounts of much rarer elements such as
gold, silver, and uranium. As magma cools and solidifies, these elements combine to
form two major groups of silicate minerals. The dark (ferromagnesian) silicates are rich
in iron and/or magnesium and comparatively low in silica. Olivine, pyroxene,
amphibole, and biotite mica are the common dark silicate minerals of Earth’s crust. By
contrast, the light (nonferromagnesian) silicates contain greater amounts of potassium,
sodium, and calcium rather than iron and magnesium. As a group, nonferromagnesian
minerals are richer in silica than the dark silicates. The light silicates include quartz,
muscovite mica, and the most abundant mineral group, the feldspars. Feldspars make up
at least 40 percent of most igneous rocks. Thus, in addition to feldspar, igneous rocks
contain some combination of the other light and/or dark silicates listed above.

Task 3. Look at Figure 14.1. Analyse the figure and then try to continue the
following sentences:
1. Rocks that contain substantial dark silicate minerals and calcium-rich plagioclase
feldspar (but no quartz) are said to have a … composition.
2. Rocks with a composition between granitic and basaltic rocks are said to have an
composition.
3. Rocks with a composition between granitic and basaltic rocks after the common
volcanic rock an desite are said to have … composition.
4. Another important igneous rock, …, contains mostly olivine and pyroxene and
thus falls on the opposite side of the compositional spectrum from granitic rocks.

Task 4. Discuss the following problems:
1. Two basic settings of igneous rocks.
2. Dark (ferromagnesian) silicates in igneous rocks.
3. Light (nonferromagnesian) silicates in igneous rocks.

Task 5. Write a brief summary of the text from Task 2.

Individual work

Task 1. Read the text and translate it into Ukrainian (in written form).
Build up a list of key terms to the text.

Other Compositional
Groups
Rocks with a composition between granitic and basaltic rocks are said to have an
intermediate rocks contain at least 25% dark silicate minerals, mainly amphibole,
pyroxene, and biotite mica with the other dominant mineral being plagioclase feldspar.
This important category of igneous rocks is associated with volcanic activity that is
typically confined to the margins of the continents. Another important igneous rock,
peridotite, contains mostly olivine and pyroxene and thus falls on the opposite side of
the compositional spectrum from granitic rocks. Because peridotite is composed almost
entirely of ferromagnesian minerals, its chemical composition is referred to as
ultramafic. Although ultramafic rocks are rare at Earth’s surface, peridotite is the main

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