LESSON 4: HOW CAN WE TELL THE HARDNESS OF ROCKS?

AIMS:  1.  What is Moh's Scale of Hardness?

            2.  What is the scratch test?

MOTIVATION:  Have a sample of the following rocks on display for the children to touch: The first two rocks (talc and feldspar) were taken from

http://ivyhall.district96.k12.il.us/4th/kkhp/RocksandMinerals/PinkFeldspar.html

and the third picture, quartz, is from http://casdn.neu.edu/~geology/department/staff/colgan/iceland/minerals/quartz.htm

The fourth sample, mica, is from http://cobweb.net/~bug2/rock4.htm

The fifth sample, granite, is from http://ivyhall.district96.k12.il.us/4th/kkhp/RocksandMinerals/rocks.html

PROCEDURE: 

1.  Have students use their fingernails to see if they can make a scratch mark on any of the rock samples.  Have them log on to http://ivyhall.district96.k12.il.us/4th/kkhp/RocksandMinerals/RockQuiz.html and read how they can judge the hardness of the various objects they will use to test their rocks.

The following Moh's Scale of Hardness was taken from

http://cte.jhu.edu/techacademy/fellows/brannon/webquest/rmform.htm

and the picture samples are from http://ivyhall.district96.k12.il.us/4th/kkhp/RocksandMinerals/RockQuiz.html

The hardness of various rocks and minerals can be used as an another method of identification. 
In the 19th. century, a German mineralogist- Friedrich Mohs, devised a scale of hardness for minerals.

Moh's Hardness Scale:

Hardness

Mineral

Scratched by . . . .

1

talcTalc

soft pencil lead

2

gypsumGypsum

fingernail; blackboard chalk

3

calciteCalcite

copper penny

4

fluoriteFluorite

iron nail; brass

5

apatiteApatite

steel knife blade

6

feldsparOrthoclase

window glass; steel file

7

quartzQuartz

flint sandpaper

8

topazTopaz

spinel

(available in rock shops)

9

corundumCorundum

emery sandpaper

10

diamondDiamond

carborundum sandpaper

2.  Students will perform a scratch test on the five rock samples.  See the following lab activity:

LAB ACTIVITY:

Problem:  How hard is a rock?

Hypothesis:  We think if we use our fingernail, penny, and an iron nail, then we can determine how hard a rock is.

Materials:  penny, nail, crayons, 5 rock samples, fingernail

Procedure:  1.  Label each rock 1, 2, 3, 4, 5.  2.  Try to scratch each rock.  Use your fingernail first, then the penny, and last, the nail.  3.  Record what happened.  Put an X on the chart of the rock was scratched.

Observations:  Use the chart to record your observations.

Rock

Fingernail

Penny

Nail

#1

     

#2

     

#3

     

#4

     

#5

     

Answer the following questions:

1.  How many rocks could you scratch with your fingernail?_________ Penny?________ Nail?_________

Conclusion: 

1.  Which rock was the hardest?  How do you know? ________________________________

_________________________________________________________________________

2.  Which rock is the softest?  How do you know?___________________________________

_________________________________________________________________________

_________________________________________________________________________

Click here for student samples of the Lab Activity.

Follow-up:  This concludes the unit.  From here, please click on assessments for culminating activities.

 

OVERVIEW

ASSESSMENTS

RUBRICS

STUDENT SAMPLES

 

Rocks and Minerals

LESSON 1- WHAT ARE ROCKS AND MINERALS?

ROCKMAP.GIF (8575 bytes)

 

LESSON 2- HOW ARE ROCKS FORMED?

LESSON 3- WHAT IS THE ROCK CYCLE?

LESSON 4- HOW CAN WE TELL THE HARDNESS OF ROCKS?