Determining the hardness of an unknown rock or mineral is often very useful in the identification process. Hardness is a measure of a mineral's resistance to abrasion and is measured against a standard scale - Mohs Scale of Hardness. Mohs Scale was named after Frederick Mohs (1773-1839), a German minerologist. It consists of 10 fairly common minerals (except for the diamond) of known hardness which are numerically ordered from the softest (1) to the hardest (10). They are:
|1. Talc (H=1)||2. Gypsum (H=2)||3. Calcite (H=3)||4. Fluorite (H=4)||5. Apatite (H=5)|
|6. Orthoclase (H=6)||7. Quartz (H=7)||8. Topaz (H=8)||9. Corundum (H=9)||10. Diamond (H=10)|
As common sense dictates, Mohs Scale is based on the fact that a harder material will scratch a softer one. By using a simple scratch test, you can determine the relative hardness of an unknown mineral.
How to Perform the Test
- Select a fresh, clean surface on the specimen to be tested.
- Hold the specimen firmly and attempt to scratch it with the point of an object of known hardness. In this example, we use a sharp quartz (H=7) crystal .
- Press the point of the crystal firmly against the surface of the unidentified specimen.
- If the "tool" (in this case the quartz crystal) is harder, you should feel a definite "bite" into the surface of the specimen.
- Look for an etched line. It is a good idea to rub the observed line with your finger to ensure that it is actually etched into the surface of the specimen. In this case, the crystal left a deep, definite scratch in the surface. Because the specimen was scratched by the quartz crystal, we know its hardness is less than that of the quartz—less than H=7.
- If there is any question about the result of the test, repeat it being sure to use a sharp point and a fresh surface.
"Tools" for Testing Hardness
Believe it or not, most people do not normally carry around samples of the 10 minerals on the Mohs Scale! However, there are several simple "tools" people often have with them that can be useful in determining the relative hardness of an unknown mineral specimen.
Your fingernail has a hardenss of 2.5. If you can scratch the surface of an unknown specimen with it, you will immediately know that its hardness is less than 2.5. In other words, it is slightly harder than gypsum (H=2) but softer than calcite (H=3).
A penny has a hardness of 3.0 - slightly harder than your fingernail. So, if you can't scratch the specimen with your fingernail (H=2.5), but a penny does the job, you immediately know that it is at least as hard as calcite (H=3).
The steel blade of the average knife usually has a hardness of about 5.5. If a penny does not scratch your unknown specimen but the knife blade does, then you can correctly conclude that it is harder than calcite (H=3) but softer than orthoclase (H=6).
Example: You select one of the minerals from Mohs Scale that looks like the one pictured here and find that it can be scratched by the knife (H=5.5) but not by the penny (H=3). Therefore, you are able to conclude that the specimen has a relative hardness between 3.0 and 5.5. The minerals from Mohs Scale that fall into that relative hardness range are calcite (H=3), fluorite (H=4) and apatite (H=5). By using your powers of observation and your knowledge of other physical characteristics of minerals such as crystalline structure, color, streak etc. you are able to conclude that your sample is calcite!