Rock Cycle - Minerals (2B)
 Lab 

   
OBJECTIVES:
  • Exploring how minerals can grow.
  • Observing and recording mineral growth.
VOCABULARY:
  • crystal
  • mineral 
MATERIALS:
  • clean baby food jars
  • masking tape, crayons, water, ammonia
  • laundry bluing
  • salt
  • white porous rock or other porous material (charcoal works well)

Students make a crystal garden.


Internal structure of halite 

BACKGROUND:

Minerals are naturally occurring, inorganic combinations of one or more elements. Minerals are crystalline, which means they have an internal atomic structure organized in a regular, repeating pattern. Organic substances can also form crystals, however, these are not minerals.


halite

The natural growth (shape) of a mineral is called its crystal form. This shape is based on the internal arrangement of atoms although not visible to the eye, is reflected by the mineralís appearance. For example, halite (salt) occurs in cubic crystals because the sodium and chloride atoms which compose halite are organized in a cubic structure. Minerals show recognizable crystal forms only when they have open space to grow. This is uncommon; most minerals form in confined spaces, and take on the shape available to them.

PROCEDURE:
  1. Remind the students that minerals grow as crystals. Tell them that today they are going to grow crystals.
      
  2. In this experiment, students will make a solution that will grow into a "garden" of crystals. Note that not all crystals are minerals. For example, solid sugar is crystalline, but is organic, so it is not a mineral.

    Use the following recipe for the crystal solution. Make the appropriate amount for your classroom if you do not want your students to mix their own solutions. WARNING: The use of ammonia can be dangerous around children. Per student, mix:

    5 ml water
    5 ml ammonia
    5 ml laundry bluing
    5 ml salt
      

  3. Have the students carefully place several small pieces of sponge, porous rock or broken brick in their jars. We prefer white porous rocks (available at nurseries), because they make the garden look attractive. Make sure the rocks are no more than 1cm. high.
      
  4. Pour enough of the mixture into the jar to almost cover the rocks. Leave a 5 mm air pocket at the top of the jar.
      
  5. Add a few drops of food coloring to make it "pretty." Students can put one drop of red, one drop of blue on the rocks. Do not put 2 different color drops on each other. Do not add more than 4-5 drops.
      
  6. Place the jar in a warm (but not hot) area, such as the sunny part of a windowsill. Crystal growth rate depends on temperature. The lower the heat, the slower the crystals will grow. However, slow growing crystals will become larger than fast-growing crystals. You many want your students to experiment by putting some jars in warm parts of the classroom and some in the cool parts.
      
  7. Crystals will form and become visible within a week. If the sponges or rocks dry out, you can get the crystals to further grow by adding a mixture of equal parts water and ammonia to the fluid around the rocks. Add the mixture carefully, a little at a time.
      
  8. The crystals formed are very delicate, so do not let the children move the jars until the assignment is completed. If the children bring the jars home, tell them to carry them very carefully.
      
  9. Ask the students to observe the jars each day. They should record what they see by drawing pictures in the boxes on their worksheet.
      
  10. If the students put different types of rocks on the bottom of their jars, the class can also observe and discuss what type of base makes the best crystals. Likewise, if the jars were placed in different parts of the classroom, the class can observe and discuss why some crystals grew better or larger than others.

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