Crystals

Therese Donatello              Archbishop Weber H. S.
                               5252 W. Palmer St.
                               Chicago IL 60639
                               (312) 637-7500

Objectives:

The Jr. High students will be able to:
1.  Identify  patterns in solids.
2.  Compare the patterns to basic models of the six crystal systems.
3.  Distinguish the various crystals that make up rocks.
4.  Realize that gem stones are usually made from minerals and sometimes 
    from rocks.

Materials Needed:

large mineral specimens                      eye droppers
rock specimens                               water
magnifying glasses                           stereoscopes
stereo pictures of rocks, minerals & gems    stereo glasses
large & small styrofoam balls                toothpicks
sodium chloride                              scissors
epsom salts                                  glue
glass slides                                 thumbtacks
candles                                      cardboard
matches                                      petri dish
forceps                                      beads of various sizes

Stategy:

1.  Review of solids
    a. Place a petri dish containing different sizes of beads on the overhead 
       projector.
    b. Ask the students how the particles would move in a gas, a liquid, and a 
       solid.
    c. When the particles in the solid are moving slowly show how they start
       arranging themselves into an orderly pattern.  Tell them this orderly 
       pattern is seen in solids called crystals.
    d. Point out that particles in a solid are held close together in fixed
       positions by forces that are stronger than those between particles in
       a liquid or a gas.

2.  Patterns
    a. Using 4 large and 4 small styrofoam balls have the students attach them
       to each other with toothpicks so that no 2 balls of the same size are
       attached to each other.
    b. Ask the students to name the shape of the structure created.
    c. Ask them to arrange the balls again into a different structure.
    d. Ask them what shape the second structure would have if they used more 
       balls in the same way.
    e. Make sure that they see a pattern in the structures they make.


3.  Crystal formation
    a. Using forceps take one pinch of sodium chloride and place it on the 
       slide.  On another slide do the same thing with the epsom salts.
    b. Observe each material under the stereoscope.
    c. Draw the structures seen on each slide.
    d. Using an eye dropper place a drop of water on each slide and stir until
       the solid dissolves.
    e. Light the candle with the match and using forceps to hold the slide
       gently heat the slide until the water evaporates.
    f. Observe each slide again.  Then draw the structures again and compare
       them to the original drawings.
    g. Point out that the crystal patterns are the same but the sizes may 
       differ.

4.  Crystal systems
    a. Cut out the patterns for the 6 crystal systems and fold and glue them to 
       form three dimensional figures.
    b. Cut out the pattern for the contact goniometer and glue it to the 
       cardboard.  Attach the pointer to the measuring part with a thumbtack.
    c. Place one side of the goniometer on one side of the crystal model and
       the pointer on an adjacent side.  Read the angle on the goniometer.
    d. Crystal system angles
       (1) The isometric, orthorhombic, and tetragonal systems have 90 degree
           angles.
       (2) The hexagonal system has 60 degree angles.
       (3) The monoclinic system has two 90 degree angles and on oblique 
           angle.
       (4) The triclinic system has three oblique angles.

5.  Crystal comparisons
    a. Take a large quartz crystal and count the number of sides and measure 
       the angles between two adjacent sides using the goniometer.
    b. Compare the readings for the models and determine what system quartz
       belongs.
    c. Do the same thing for the calcite crystal.

6.  Stereo pictures
    a. Place each lens of the stereo glasses over the same section of each
       picture.
    b. Move the lenses to match the distance between your eyes.  The pictures
       should look 3 dimensional.  Ask if they can see the number of sides and
       angles on the pictures of minerals.
    c. Look at pictures of rocks in the same manner.  Ask if the crystals in 
       the rocks all look the same.  They should not look the same since rocks 
       are mixtures of minerals.
    d. Look at pictures of gems in the same way.  Ask if they think gems are
       rocks or minerals.  Most are minerals since the properties will be the 
       same.  This makes it easier to work with the material.

7.  Identifying crystals in rocks
    a. Using a magnifying glass look at a piece of granite.  Ask if any of the
       crystals compare to the models studied previously.  They should be able
       to see the quartz crystals.
    b. Do the same thing with marble.  They should be able to distinguish
       calcite.

Performance Assessment:

Samples have been brought back from the recently discovered planet "Olympia".
Your job is to set up a plan by which you can: (1) classify the samples as 
minerals or rocks; (2) place any crystals in the correct crystal system.
Students should have plans that show that various minerals make up rocks and
by measuring the angles on the crystals and then comparing them to the crystal
models they can be placed in the correct system.

References:

Horowitz, Irving.  Earth Science Investigations.  Amsco School Publications,
Inc. New York: 1973