GEOSCI/ENVIRON 118 – Laboratory 4 “Igneous rocks”, page 1
Igneous Rocks
GEOSCI/ENVIRON 118 Laboratory
Name:________________________________
Section:________
The purpose of this laboratory is to further acquaint you with the minerals that
comprise igneous rocks and to examine the diversity of igneous rock types.
Task 1: Description of minerals that commonly occur in igneous rocks
This exercise will reacquaint you with several minerals that you learned to recognize in the
previous laboratory session, while introducing these in the context of Bowen’s reaction
series (see page 4 of this handout and/or the poster to be shown by your GSI). In Table 1
below, provide a summary of the diagnostic properties that help you identify this mineral.
This will include: color, cleavage, hardness, etc. Note that these specimens are much larger
than the ones that typically comprise igneous rocks, and as such, it is important to learn to
recognize the diagnostic features in naturally occurring, typical crystals within a rock. This
will be aided if you spend some time re-examining and describing these specimens.
Mineral
Olivine
Pyroxene
Amphibole
Plagioclase
(Ca-rich)
Plagioclase
(Na-rich)
Biotite
K-Feldspar
Muscovite
Quartz
TABLE 1: ROCK FORMING MINERALS
Color
Cleavage Hardness Other Properties
GEOSCI/ENVIRON 118 – Laboratory 4 “Igneous rocks”, page 2
Task 2: Igneous Rock – Textures and Composition
Now that you know how to identify the most common igneous minerals, we are going to
look at rocks – aggregates of minerals that reflect the temperature and composition of a
magma. This exercise will test your ability to identify these minerals in various rock
specimens and then to classify these into subdivisions of the Igneous Rock types.
For each specimen, record the following information:
Rock Color: As a first step, rock composition is reflected by its color. For this exercise,
classify each rock as: dark/mafic, grey-green/intermediate, or pink-white/felsic. Note that
this will provide an effective means of separating your rock types into 3 basic groups that
are helpful later on, when you further subdivide them into specific rock names.
Rock Textures (see figure at the bottom of page 4): Describe the texture of the rock
(coarse/phaneritic; fine/aphanitic; glassy; porphyritic, etc.) or other special textural features.
Minerals Present: This will require that you focus on the individual crystals that comprise
the rock as these represent discrete minerals. Identify and tabulate all of the minerals that
are present within each specimen. For this you will rely on the knowledge you developed in
last week’s lab and in Task 1. For each mineral that you observe, make some estimate of its
abundance (abundant, common, sparse, rare). The relative abundance of different minerals
will be important in your final designation of the rock type (see the diagram on page 3).
Other Features: These should include any other features that you note that may be useful
to you in the future when you identify this rock type.
Extrusive versus Intrusive: On the basis of the textural features that you observed above,
for each rock specimen, do you think that it was extrusive or intrusive?
Felsic versus Mafic: Based on your description of rock color and the constituent minerals,
for each rock specimen do you think that it is mafic or felsic?
Rock Name: Based on all of the above observations, name the specimens provided by your
instructor (see attached table).
GEOSCI/ENVIRON 118 – Laboratory 4 “Igneous rocks”, page 3
The following figure shows the relative mineral abundance in igneous rocks ranging from
most felsic (left) to ultramafic (right). You can find such a figure also in the book by
Marshak that we use in GEOSCI/ENVIRON 119. This figure is taken from a web site of
Prof. Lynn Fichter of James Mason University:
http://csmres.jmu.edu/geollab/Fichter/IgnRx/IgnRx.html
GEOSCI/ENVIRON 118 – Laboratory 4 “Igneous rocks”, page 4
The following figure is the well-known reaction series of Bowen. It is well-illustrated in
many textbooks (in all its complexities), including the book by Marshak that we use in
GEOSCI/ENVIRON 119. This figure is taken from a web site of Prof. Lynn Fichter of
James Mason University: http://csmres.jmu.edu/geollab/Fichter/RockMin/RockMin.html
Spec#
1
2
3
Color
Texture
Minerals
Other features
Maf/
Ext /
Fel
Intr/
Interm.
Rock Name
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7
8
9
10
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