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MINERAL PROPERTIES and identification procedures 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 or very similar. A scale to measure hardness was devised by Austrian mineralogist Frederick (Friedrich) Mohs in 1822, and is currently 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: 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:
Hardness is almost always rounded off to the nearest half number. Although the Mohs scale is very useful, it is not linear. The minerals chosen were only selected because of their popularity. Number 10 on the scale (diamond) is 140 times harder then number 9 (corundum), whereas 4 (fluorite) is only 1.11 times harder than 3 (calcite). A proportional measurement, called absolute hardness, was more recently devised, but is only used by scientists who need accurate results. The Mohs scale, although flawed, still satisfies all mineral collectors. How to use hardness as an identification mark There are various hardness testing kits. One type consists of 10 metal rods, each one containing a fragment of one of the ten minerals in the Moh's scale. Another type consists of large, low cost specimens of the ten 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.
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. We can add another step to the chart above for those that have hardness testing kits composed of minerals rather than rods:
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 not make a noticeable 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, which is an inessential investment. How to test using hardness Hardness testing is done by "swiping" one mineral with the other. The swipe should be strong enough to make a scratch, but not so much as to damage the specimen. To get the most accurate results, a sharp edge should be swiped 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. Neither should it be conducted on a visible surface; a bad scratch on the face of a mineral can diminish its value. 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 must 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. All minerals are anisotropic, 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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