Understanding The Mohs Scale of Hardness

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Created in 1812 by German mineralogist Friedrich Mohs, the Mohs Scale of Mineral Hardness measures the scratch resistance of various minerals.  This scale is based on the ability of one mineral to scratch another, which is intended to measure hardness.  This does not, however, take into consideration things such as brittleness.  Therefore, just because a mineral may be hard to scratch does not mean that the particular mineral is not delicate or fragile.

This can be a confusing concept to understand at first, because a mineral such as titanium, which is regarded to be one of the strongest metals in the world, only has a Mohs Scale rating of 6, while steel would have a score of around 8 on the same scale.  Therefore, when using the Mohs Scale, it is important to remember that this scale does not measure things such as strength or breakability.

The Mohs Scale is a scale that rates a mineral a score of between 1 and 10, with softer minerals earning a low score and hard minerals earning a high score.  The traditional scale is as follows:

1. Talc

2. Gypsum

3. Calcite

4. Fluorite

5. Apatite

6. Orthoclase Feldspar

7. Quartz

8. Topaz

9. Corundum

10. Diamond

The way this scale works is by comparing the scratch resistance of one mineral to another.  Therefore, a mineral such as quartz, which has a Mohrs Scale score of 7, will be able to scratch the surface of any mineral which has a lower score, but will not be able to scratch a mineral with a higher score.

Mineralogists, geologists, and recreational rock hunters rely on this scale to make a quick identification of a mineral while in the field.  This is done by scratching the surface of a specimen with various objects of which the Mohs rating is known.  For instance, the human fingernail has a Mohs hardness rating of 2.5, meaning that only minerals such as talc and gypsum can be scratched with the fingernail.  A mineralogist may also use other objects to scratch the surface of a mineral specimen, such as a copper penny (which has a hardness rating of 3.5), a knife blade (which has a hardness rating of 5.5), or a steel file (which has a hardness rating of 6.5).  

While there are materials in the natural world that fall below a score of 1 and above a score of 10, the Mohs Scale is a reliable, simple, and easy method for making a quick identification when no other means of identification are available.


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