Talk It Over

Minerals are chemicals that are present in rocks and soil. They have a uniform chemical composition and a definite crystalline structure. Some are made of a single element, such as gold. Others are compounds of two of more elements. Magnetite is a good example. It is made of iron and oxygen.

If you had a mineral that you couldn't identify, what observations and tests could you use to find its name?

Get

• Several samples of unknown minerals*
• Bright light
• Magnifying glass
• Penny
• Steel nail
• Glass plate (from a picture frame is fine)
• 1 bathroom tile
• Kitchen scale or balance (metric)
• Small yogurt container, empty, washed and dried
• Small bowl
• Water
• Measuring cup (metric)

Go

Here I will suggest four properties you can test for and use to distinguish minerals: luster, hardness (on the Mohs scale), streak color, and specific gravity. There are many others you may want to learn about and use in your project, but these will get you started.

• Luster: Observe each sample carefully under a bright light, using a magnifying glass or hand lens. Look at its texture and surface appearance. Decide which of the following descriptions fits it best:
  • Metallic (shiny like a metal)
  • Shiny (like a diamond)
  • Glassy
  • Resinous (looks like sap, gum, pitch, or tar)
  • Pearly
  • Oily or greasy
  • Silky (appears to have fibers like a woven fabric)
• Hardness: Next, determine how hard or soft the mineral is, using the Mohs scale of hardness.

  

• This scale is relative. You assign a number to a mineral by seeing what scratches what. To get the range of hardness for a sample do this:
  1. Try scratching it with your fingernail. If you can, the Mohs scale hardness value is 2.5 or less.
  2. Try scratching with a penny. If you can, the value is 3.5 or less.
  3. Try scratching with a steel nail. If you can, the value is 5.5 or less.
  4. See whether the mineral will scratch glass. If it will, the Mohs value is 6 or more.
• Streak color: Use the back of a bathroom tile as your streak plate. Rub the mineral across it and examine the color of the streak.
• Specific gravity: To find a mineral's specific gravity (or its density, the ratio of its mass to its volume), do this:
  1. Weigh the mineral on the balance or kitchen scale. Record the weight in grams.

     

  2. Set the small yogurt container in the bowl. Fill the yogurt container with water to the brim, but don't let it run over.
  3. Carefully drop the mineral into the water. Water will spill from the yogurt container into the bowl.

     

  4. When water stops spilling over, remove the container from the bowl. Pour the water from the bowl into the measuring cup. Measure and record the amount of water in milliliters. This equals the volume of the mineral.

     

  5. The specific gravity of the mineral is its weight in grams divided by its volume in milliliters. (Note: Specific gravity is defined in relation to the density of water, which is 1g/1ml. Therefore, it has no units.) In the example, the specific gravity is 2.5 times that of water, or 2.5g/ml. Use the calculator to find the specific gravity of each mineral you test. For example, if the weight of a sample is 25 grams and its volume is 10 milliliters, its specific gravity is 25 ÷ 10 = 2.5.

     Some common minerals and their characteristics for these properties are shown in this table:

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