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Scratch: How do you Determine the Hardness of a Mineral?

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Author: Janice VanCleave

Problem

How do you determine the hardness of a mineral?

Materials

  • sharpened No.2 pencil

Procedure

  1. Hold the pencil against a table with one hand.
  2. Scratch

  3. With your other hand, scratch the pencil lead with a fingernail.
  4. Observe the ease or difficulty of making a scratch in the pencil lead.

Results

Your fingernail easily cuts a groove in the pencil lead.

Why?

Hardness is one of the most useful properties used to identify minerals. The hardness of a mineral is its resistance to being scratched. A hardness scale was invented in 1822 by Frederick Mohs (1773–1839), a German chemist. He arranged ten common minerals from the softest to the hardest. He gave the softest mineral, talc, the number 1, and the hardest mineral, diamond, the number 10.

The hardness value of a mineral is determined by how easily it can be scratched when rubbed by another mineral or material of known hardness. The rule is that a mineral can scratch any mineral or material with a lower hardness number. For example, the mineral apatite, which has a hardness of 5, will scratch fluorite with a hardness of 4 or anything softer. Apatite can be scratched by orthoclase, which has a hardness of 6, or by anything harder.

Pencil lead is really not made of lead but is a mixture of the mineral graphite and clay.

Your fingernail has a hardness of about 2. Since your nail can scratch the graphite-clay mixture in the pencil lead, the hardness of the lead must be less than 2. Pure graphite is known to have a hardness of 1, but when mixed with clay its hardness increases.

The Mobs' Hardness Scale

  1. Talc
  2. Gypsum
  3. Calcite
  4. Fluorite
  5. Apatite
  6. Orthoclase
  7. Quartz
  8. Topaz
  9. Corundum
  10. Diamond

Let's Explore

  1. As the amount of clay mixed with graphite increases, the hardness of the pencil lead increases. Do pencil numbers correspond with the Mohs' hardness scale? That is, is a No.1 pencil softer than a No.2 or No.3 pencil? Repeat the experiment twice, first using a No. 1 pencil, then using a No.3 pencil.
  2. Is gypsum harder than your fingernail? Gypsum is a mineral that has about the same hardness as school chalk. While chalk is not a mineral, it can be substituted for gypsum in this experiment if a sample of gypsum is not available. Repeat the original experiment, using a piece of gypsum or chalk instead of the pencil lead. If chalk is used, repeat again using different brands of chalk. Estimate the hardness of each brand. Science Fair Hint: Tape or glue the pencils and gypsum or chalk to poster board in order from softest to hardest. Print the estimated hardness under each sample. Use the poster as part of a project display.
  3. A form of gypsum can be made by adding water to plaster of paris. Make a sample of homemade gypsum by mixing together in a paper cup 2 tablespoons (30 ml) of plaster of paris with 1 tablespoon (15 ml) of tap water. NOTE: Do not wash plaster down the drain. It can clog the drain. When the mixture dries (after about 20 minutes), peel off and discard the paper cup from around the solid piece of gypsum. Natural gypsum is given a rating of 2 on the Mohs' scale. How hard is your homemade gypsum? Repeat the original experiment, replacing the pencil with the homemade gypsum. Your results can determine whether the sample has a hardness of less than your fingernail, whichis 2.

Scratch

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