Rock Cycle - Minerals (3B)

  • Discovering that not all crystals are minerals.
  • Comparing organic and inorganic crystals.
  • crystal
  • inorganic
  • organic

Students look at organic and inorganic crystals.



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.

  1. 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.
  2. 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.
  3. 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.
  4. 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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