Earth Science 11 – Earth materials
Lab Activity – Studying Rocks in Thin Sections
Heath Earth Science - pg.80-81
Today’s Objectives:
 Differentiate between rocks and minerals, including:
 Classify rocks as igneous, sedimentary, or metamorphic using texture and composition
Have you ever tried to look through a rock? In addition to looking at hand-held rock
samples, sometimes geologists need to see through the rock in order to study it. They do this by
making thin sections. Thin sections are slices of rock so thin that light actually passes through
them! Thin sections are analyzed by using microscopes. The magnified view in the microscope
makes it possible to see minerals that are not large enough to view in the hand-held sample and to
more easily identify the larger minerals that can be seen.
In this lab, you will look at some diagrams of thin sections of rock. You will study minerals
that are found in the rocks as well as identify the rocks containing the minerals.
Lab Skills and Objectives
 To observe and interpret several diagrams of rock thin sections.
 To classify and identify the minerals and rocks represented by the diagrams.
Materials
 Metric ruler
 Figure 5.22, pg. 80, Heath Earth Science
 Figure 5.23, pg. 81, Heath Earth Science
 Mineral Composition chart, pg. 61, Heath Earth Science
Procedure
1) Look at Rock A and Rock B in figure 5.23. Use the key to determine the name of each
mineral. List the names of the minerals found in Rock A and Rock B in the table below.
Rock A minerals
% of each mineral
Rock B minerals
% of each mineral
present
present
Total %:
100
Total %:
100
2) Use the chart in Figure 5.22 to estimate the percent of each mineral present in Rock A and
Rock B. For example, does the amount of quartz in Rock A look as if it occupies 10% of the
diagram? 20%? 40%? Repeat your estimate for each of the minerals found in Rock A and
Rock B. Record the percentages in the table. Your values should total 100% for each rock.
3) Using a metric ruler, measure the diameter of the circular diagram for Rock C. Record your
measurement.
Diameter of circular diagram for Rock C: __________
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Earth Science 11 – Earth materials
4) Look at the mineral grains in Rock C. Measure the width (in centimeters) in any direction
across five different mineral grains. Record your data in the table below. Calculate the
average width for the grains.
Grain 1
Grain 2
Grain 3
Grain 4
Grain 5
Average
diameter (cm) diameter (cm) diameter (cm) diameter (cm) diameter (cm) diameter of
grains (cm):
5) Answer the questions in Analysis and Conclusions.
Analysis and Conclusions
1) Grain or crystal size can provide clues to rock types. Typically, sedimentary grains are
rounded and found in a cement matrix. Igneous grains fit together in a jigsaw puzzle
fashion. Metamorphic grains exhibit linear patterns and foliation (foliation is a parallel
arrangement of minerals such as alternating layers of dark and light minerals). Look at
Rocks A to D. Which of the rocks shown is sedimentary? Metamorphic? Igneous? Answer
below.
Rock A: _____________ Rock B: _____________ Rock C: ____________ Rock D: _____________
2) Determine the actual size of an average grain of Rock C. The actual diameter of the rock
sample shown in each diagram is 0.5 centimeters. Use your average grain diameter data
and the magnified diameter you recorded. Show your work.
Actual sample diameter:
___0.5 cm__
Recorded sample diameter:
__________
Magnification factor:
___________ (recorded sample diam. /actual sample diam.)
Recorded average grain diameter:
__________
Actual average grain diameter:
__________ (recorded avg. grain diam. /mag. factor)
3) Using your answer to question 2, determine the name of the average grain size in Rock C
according to the information provided below:
 Diameter of clay-sized particles: 0.00001 ~ 0.0004 cm
 Diameter of silt-sized particles: 0.0004 ~ 0.006 cm
 Diameter of sand-sized particles: 0.006 ~ 0.2 cm
 Diameter of pebble-sized particles: 0.2 ~ 6.4 cm
Name of average grain size in Rock C: _______________ (clay, silt, sand, or pebble)
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Earth Science 11 – Earth materials
4) In Rock A and Rock B, which two minerals did you estimate make up over 50% of the rock?
Which minerals make up the remainder of the rock? Using the graph on page 61,
determine which rock in the graph most closely matches the mineral composition of Rock A
and Rock B. Answer in the table below.
Rock A
Rock B
Two minerals making up over
50% of Rock
Other Minerals
Name of Rock
Explain your choices for the names of the rocks in the space below.
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
5) Igneous rocks are commonly grouped into mafic rocks and felsic rocks based on their
chemical composition. Mafic rocks are dark in color because they contain a number of dark
minerals such as amphibole, pyroxene, olivine, biotite, as well as plagioclase feldspar. Felsic
rocks are light in color and contain minerals such as quartz and orthoclase feldspar as well
as a few dark minerals such as biotite and amphibole. Based on these definitions, which
rock, A or B, is mafic? Which rock is felsic? Explain your answer.
Mafic rock:______A or B________ Felsic Rock:____A or B_________
Explain your choices below.
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
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Earth Science 11 – Earth materials
6) Compare the diagrams for Rock C and Rock D. Look at each rock’s texture (size, shape,
orientation of grains and contact points with other crystals). How does the texture differ in
the two rock samples?
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
7) Based on your observations during this activity, your textbook, and using the rock
classification tables in your notes, classify and name each of the five rock samples in this
lab.
Rock Type (igneous, sedimentary,
metamorphic)
Rock Name
Rock A
Rock B
Rock C
Rock D
Rock E
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