Download Specific Gravity

Earth Science
LAB # 8
SPECIFIC GRAVITY OF ROCKS & MINERALS
Name ___________________________
Lab Instructor _________________________
Lab Day: M T W TR
INTRODUCTION:
Density is one of the most fundamental properties of geological materials, from rocks to water to
air. Minerals on Earth have a rather limited range, from about 2.0 g/cm3 for some zeolites to 7.6
g/cm3 for galena (22.6 g/ cm3 for native iridium). Rocks, which are masses of various minerals,
have an even more restricted range of density. Density is easy to measure and can be used to
help teach the use of graphs and math.
NOTE: The word density is used here, even though what will actually be measured is "specific
gravity". Density is mass per unit volume (such as g/ cm3). Specific gravity is a dimensionless number
(no units), since it is the ratio of the weight of a rock to a weight of an equal volume of water (weight
units cancel).
MATERIALS:
1. A single pan balance or balances, number depending on how many are available and how
you want to run the exercise. Check the maximum weight the balances can measure, since
this limits the maximum weight of rocks that can be weighed (most are ~500 g).
2. Paper for data tables.
3. A collection of different rock types and sizes. A good size range is from golf ball size to half
the size of a fist.
4. A beaker or jar of water big enough to hold the largest rock easily.
5. Some thin wire, fishing line, or thick thread (not string) to make a sling to hold the rocks
suspended in the water.
6. Here is what the set up looks like:
First measure the mass in air
String
Table
Second, measure the mass in water
Floor
PROCEDURE:
1. Obtain a rock sample.
2. Weigh the rock sample (Including String and tape) and record the mass in the data table.
3. Tape the string on the balance pan, and let the rock hang from the string so that the rock
is submerged in the beaker of water. (Be sure that the rock is not touching any
portion of the beaker.)
4. Weigh the rock in the water, record the mass in the data table.
5. Calculate the specific gravity of the sample using the following equation and write it in the
data table.
Specific Gravity =
(Mass in air)
[(Mass in air) - (Mass in water)]
6. Repeat procedures #1-5 for 3 other types of rocks or minerals.
7. When everyone in your class has finished complete the Class Data Table #2
8. Using Excel, create a column graph of the Class Data Table #2 AVERAGES and print
one per person.
9. Complete the critical thinking questions using complete sentences on a separate piece of
paper.
Table #1
Sample
Rock type
A
Magnetite
B
Potassium Feldspar
C
Granite
D
Basalt
Rock weight
in air (g)
Rock weight in
water (g)
Potassium
Feldspar
Specific Gravity
Specific
Gravity
Table #2- Class Data
Group
1
2
3
4
5
6
7
Averages
Magnetite
Specific Gravity
Specific Gravity
Granite
Basalt
Specific Gravity
CRITICAL THINKING QUESTIONS: Answer the following questions in complete sentences.
1. Is there a correlation between the density of a mineral or rock and its size? In other words
does size have anything to do with density? Explain. _______________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
2. Looking at the masses of the rocks in air as compared to their masses in water? Explain
why the mass of the rock in water was less. (Hint: Think of Archimedes) _______________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
3. What is the density of water? What would happen to an object in water if its density was
greater that that of water? Explain _____________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
4. Which mineral did you find to have the greatest specific gravity? _____________________
____________________________________________________________________________
5. Which rock did you find to have the greatest specific gravity? ________________________
____________________________________________________________________________
6. In which portion of the Earth’s crust is composed mostly of Granite? Basalt? ___________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
7. Based on your answer to questions 5 and 6 which type of crust would you expect to “sink”
lower upon the asthenosphere (partially melted upper portion of the Mantle) Why? ______
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
8. In this investigation we actually determined the specific gravity of the rocks. Define
specific gravity and explain how determining could be useful in identifying rocks and
minerals. __________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
9. Given the Accepted Densities of the following calculate the Percent Error for your Averages
in Table #2.
% Error =
Difference between Accepted and Measured
Accepted
Average Specific
Gravity
Magnetite
Feldspar
Granite
Basalt
Accepted Specific
Gravity
x 100
% Error