BACKGROUND:
Crystals consist of solid matter that has a regular,
internal pattern of atoms: a crystalline structure. Individual crystals are
bounded by smooth, planar surfaces called faces, that meet at specific
angles. For example, in halite crystals, the faces meet at right angles and
can be seen by its cubic crystals.
Many times, however, the internal pattern is present
but not visible. This may happen if the crystal grows in a confined space,
and could not assume its preferred shape. Students usually think that all
minerals have nice crystal shapes. This is not true. Only minerals that had
"room to grow" will produce nice crystals.
Minerals are the basic building block of rocks.
Minerals are inorganic crystals, which are not derived from any
living organisms. Crystals can also form from organic
compounds. For example, sugar (which comes from plants) can form crystals, but since sugar is composed
of organic material, these crystals are not minerals. Minerals have to be
naturally created or else they are classified as man-made substances.
PROCEDURE:
- Pass around the geode from the module to demonstrate crystal growth. The
quartz crystals in the geode are a good example of crystals that grew
into an open space. You may wish to describe how the geode formed. As
water percolated through a void (a cavity) in a rock, it precipitated a
compound (see the picture above). In the case of a quartz geode, Si
(silicon) + O (oxygen) slowly formed quartz around the edge of the void.
Little crystals grew and grew, until there was no room. Geodes that
completely "fill up" are called thunder eggs.
- Next, have students examine the samples of epsom salt, table salt
(halite), and sugar under the microscope. Instruct them to draw what
they see in their workbooks. Point out that halite and epsom salt are
minerals, but sugar is not, because it is organic. Make clear, however,
that all three specimens are crystalline.
- Instruct students to look at the crystal specimens of quartz and
gypsum and try to determine how they are different. They may distinguish
hardness, color, shininess (luster) or other properties. Next have them
examine the massive examples of gypsum and quartz. Tell them to try to
identify each specimen. Emphasize with the students that although many
substances can form crystals they don't always do so, which may make
identification more difficult.
- As an added portion of this lab you might want to dissolve some Epsom
salt in hot water to make a supersaturated solution. Then when it
is still warm, pour a thin layer in a watch glass and watch it crystallize
under the microscope. If you add this component it would be
difficult to do during a 50 minute lab period, but it you are doing this
in the classroom it is interesting for children to check the progress of
the crystals.
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