How a Minderal May Have Many Colors

By Richard Bagger

Purpose

To show how a colorless mineral may have many different colored varieties.

Background

Although minerals such as quartz and calcite are colorless when absolutely pure, there are many varieties of these minerals that exist in a wide range of colors. These color variations of these minerals that exist in a wide range of colors. These color variations can be explained as being caused by the presence of a tiny amount of dissolved "impurity." The "impurity" is usually a metallic compound.

 Materials

• Nichrome wire inoculating loop
• Bunsen burner
• Powdered borax
• Cobalt nitrate
• Manganese dioxide
• Copper sulfate
• Ferric chloride
• Nickel chloride
• Goggles

Procedure and Interpretations

1. Heat the loop in the outer part of the bunsen burner flame until it is red hot, then dip it into the powdered borax. 
2. Heat the loop in the Bunsen burner flame until the borax fuses to a clear colorless glassy "bead." If the bead does not fill the loop, dip the loop into the borax powder again, and heat it again, repeating the process until the bead is large enough. This borax bead represents a mineral which is colorless when it is pure.
3. Touch the hot borax bead to a tiny amount - just a few grains - of cobalt nitrate, and heat it in the outer part of the Bunsen burner flame. 
4. Remove the bead from the flame, let it cool, examine its color, and record it in the data table below:
5. To see what colors other metallic "impurities" produce. It is necessary to use a new clear borax bead for each new substance. Remove the old bead by heating it until it melts and then quickly flicking the wire loop over a waste container to eject the melted bead. 
6. Repeat procedures A - E with each of the following "impurities"
   1. Managanese dioxide
   2. Copper sulfate
   3. Ferric chloride
   4. Chromium nitrate
   5. Nickel chloride

Questions

1. What does the borax bead represent?
2. From this experiment what conclusion can you draw about the color of pure borax?
3. What conclusion can you draw with respect to the color of the borax and the metallic impurities?
4. How does this experiment help to explain the many variations in the colors of quartz or calcite?
5. What evidence is there in the data table to indicate that the color of the bead is caused by the metallic element and not by the non-metallic part of the "impurity"?
6. What additional tests would you have to make to prove that the color of a bead depends on the particular metallic element in the "impurity"?
7. How can a "borax bead test" be used to determine which metal is present in a mineral specimen?