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MINERAL PROPERTIES: HARDNESS


Hardness plays a major role in identifying a mineral. It can make the identification process much simpler by considerably narrowing a search.

Hardness is defined by how well a substance will resist scratching by another substance. For example, if mineral A scratches mineral B, and mineral B does not scratch mineral A, then mineral A is harder than mineral B. If mineral A and B both scratch each other, then their hardness is equal. A scale to measure hardness was devised by Austrian mineralogist Frederick (Friedrich) Mohs in 1822, and is the standard scale for measuring hardness. The scale consists of numbers one through ten; 1 being the softest and 10 being the hardest. Each number represents a different mineral - each harder than the previous. The 10 minerals are:

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

All conceivable minerals fit in this scale, since Talc is the softest known mineral and Diamond the hardest. To demonstrate how to use the scale, understand the following example: Suppose a mineral scratches Fluorite, but not Apatite, then it has a hardness between 4 and 5.

Several common household items have a fixed hardness, and can be used to test for hardness:

Fingernail
Penny 3
Knife blade
Glass
Steel file
Streak plate (floor tile)

Hardness is almost always rounded off to the nearest half number.

There are various hardness testing kits. One type consists of 10 metal rods, each one containing a fragment of one of the minerals in the Moh's scale. Another type consists of large, low cost specimens of the Moh's minerals, labeled and stored in a wooden compartment box. The Diamond is either absent or a chip attached to a small metal rod. (The Diamond is really unnecessary, since no minerals are between hardness 9 and 10.)

A mineral is struck with a metal rod or "testing mineral" to test its hardness.
It is tested in the manner of the following example:

Action Conclusion
Mineral struck with rod or mineral number 4 (Fluorite) from the testing kit. Mineral gets scratched. Mineral must be less than or equal to 4.
Mineral struck with rod or mineral number 2 (Gypsum). Mineral does not get scratched. Mineral must be between 2 and 4.
Mineral struck with rod or mineral number 3 (Calcite). Mineral gets scratched. Mineral must be between 2 and 3.

Two minerals with equal hardness will scratch each other. This gives an advantage to the hardness testing kit that includes real minerals over rods. One can scratch the mineral from the kit instead of scratching a nice specimen. In addition, one can also get more exact results by seeing if both minerals scratch each other.

Minerals can be damaged and lose value if not scratched properly. If a mineral testing kit is composed of minerals (as opposed to rods), it is preferable for the testing kit mineral to be scratched over the specimen. If this cannot be done, than the specimen has to be scratched. This should be done in an area where a scratch will be less noticeable, since it will make a permanent mark.

Hardness can be easily detected without a "kit". All one needs to know is the hardness of certain items (including the ones mentioned above) and minerals in his collection. These can be used instead of purchasing a kit.


How to test using hardness

Hardness testing is done by "scratching" one mineral with the other. To get the most accurate results, a sharp edge should be scratched against a smooth surface, on a small an area as possible. The scratch should not be conducted on a surface that is coated, chipped, or weathered, for it will give inaccurate results.

When a mineral is scratched, a permanent indentation is created. Powder of the softer mineral will come off, and it will cover the scratch area. This powder needs to be brushed away to see if the mineral really got scratched, or if the powder of the softer mineral that was swiped across the specimen being tested created a scratch-like marking. When minerals of similar hardness are scratched together, it is difficult to tell which mineral (if not both of them) is really getting scratched because of this.

Most minerals are anisotropic to a minor extent, meaning their hardness varies in different directions. Kyanite is famous for this habit. When scratched in one direction, it exhibits a hardness of 4 to 5. When struck from the perpendicular direction, it exhibits a hardness of 6 to 7. Kyanite is the only mineral exhibiting such strong anisotropism. In virtually all minerals, the anisotropism is so weak that it cannot be determined.

Micromounts and small embedded crystals are very hard to determine in terms of hardness. One may not be able to test for hardness because of the small size.



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