What is Mohs Hardness Scale?
One of the most important tests for identifying mineral specimens is the Mohs Hardness Test. This test
compares the resistance of a mineral to being scratched by ten reference minerals known as the
Mohs Hardness Scale (see Table 1). The test is useful because most specimens of a given mineral are very
close to the same hardness.
This makes hardness a reliable diagnostic property for most minerals.
Friedrich Mohs, a German mineralogist, developed the scale in 1812. He selected ten minerals of distinctly
different hardness that ranged from a very soft mineral (talc) to a very hard mineral (diamond). With the
exception of diamond, the minerals are all relatively common and easy or inexpensive to obtain.
Making Hardness Comparisons
"Hardness" is the resistance of a material to being scratched. The test is conducted by placing a sharp point
of one specimen on an unmarked surface of another specimen and attempting to produce a scratch. Here are the
four situations that you might observe when comparing the hardness of two specimens:
- If Specimen A can scratch Specimen B, then Specimen A is harder than Specimen B.
- If Specimen A does not scratch Specimen B, then Specimen B is harder than Specimen A.
- If the two specimens are equal in hardness then they will be relatively ineffective at
scratching one another. Small scratches might be produced, or it might be difficult to
determine if a scratch was produced.
- If Specimen A can be scratched by Specimen B but it can not be scratched by Specimen C, then the
hardness of Specimen A is between the hardness of Specimen B and Specimen C.
Mohs Hardness Testing Procedure
- Begin by locating a smooth, unscratched surface for testing.
- With one hand, hold the specimen of unknown hardness firmly against a table top so that the surface to be tested is exposed and accessible. The table top supports the specimen and helps you hold it motionless for the test.
- Hold one of the standard hardness specimens in the other hand and place a point of that specimen against the selected flat surface of the unknown specimen.
- Firmly press the point of the standard specimen against the unknown specimen, and firmly drag the point of the standard specimen across the surface of the unknown specimen.
- Examine the surface of the unknown specimen. With a finger, brush away any mineral fragments or powder that was produced. Did the test produce a scratch? Be careful not to confuse mineral powder or residue with a scratch. A scratch will be a distinct groove cut in the mineral surface, not a mark on the surface that wipes away.
- Conduct the test a second time to confirm your results.
| When conducting the test, place the unknown specimen on a table top and firmly hold it in place with one hand. Then place a point of the reference specimen against a flat, unmarked surface of the unknown specimen. Press the reference specimen firmly against the unknown, and deliberately drag it across the flat surface while pressing firmly. To avoid injury drag the known specimen away from your body and parallel to the fingers that are holding the unknown specimen. |
Mohs Hardness Testing Tips
- A list of minerals in order of hardness can be a handy reference. If you determine that a specimen has a hardness of Mohs 4 you can quickly get a list of potential minerals.
- Practice and experience will improve your abilities when doing this test. You will become faster and more confident.
- If the hardness of the unknown specimen is about 5 or less, you should be able to produce a scratch without much exertion. However, if the unknown specimen has a hardness of about 6 or greater, then producing a scratch will require some force. For those specimens, hold the unknown firmly against the table, place the standard specimen against it, press firmly with determination, then holding pressure slowly drag the standard specimen across the surface of the unknown.
- Don't be fooled by a soft standard specimen producing a mark on a hard unknown. That mark is like what a piece of chalk produces on a blackboard. It will wipe off without leaving a scratch. Wipe your finger across the tested surface. If a scratch was produced there will be a visible groove. If marks wipe away then a scratch was not produced.
- Some hard materials are also very brittle. If one of your specimens is breaking or crumbling rather than scratching, you will have to be very careful while conducting the test. Testing tiny or granular specimens can be difficult.
- Some specimens contain impurities. If the results of your test are not visibly conclusive, or if the information from your test does not conform with other properties, do not hesitate to do the test again. It is possible that a small piece of quartz (or another impurity) was embedded in one of your specimens.
- Don't be wimpy! This is a very common problem. Some people casually rub one specimen back and forth against another and then look for a mark. That is not how the test is done. It is done with a single, determined motion with the goal of cutting a scratch.
- Be careful. When you hold the unknown specimen against the table, position it so that the known specimen will not be pulled across one of your fingers.
- This test should be done on a lab table or work bench with a durable surface or a protective covering. Don't do this type of testing on fine furniture.
- Test tiny particles or grains by placing them between two pieces of an index mineral and scraping them together. If the grains are harder than the index mineral scratches will be produced. If the grains are softer they will smear.
Hardness of Common Objects
|
Mohs Hardness of Common Objects
|
| fingernail |
2 to 2.5 |
| copper |
3 |
| nail |
4 |
| glass |
5.5 |
| knife blade |
5 to 6.5 |
| steel file |
6.5 |
| streak plate |
6.5 to 7 |
| quartz |
7 |
|
Some people use a few common objects for quick hardness tests. For example, a geologist
in the field might always carry a pocket knife. The knife can be used for a quick hardness
test to determine if a specimen is harder or softer than Mohs 5 to 6.5.
Before using these objects as quick testing tools it is a good idea to confirm their hardness.
Some knives have harder steel than others. Test yours and then you know its hardness.
These common objects can also be useful if you don't have a set of reference minerals. We included quartz in this list because it is a ubiquitous mineral. In the field you are often no more than a few steps away from a piece of quartz.
Hardness Picks
An alternative to using the reference minerals for testing is a set of "hardness picks."
These picks have sharp metal points that you can use for very accurate testing. The picks
allow much more control, and their sharp points can be used to test small mineral grains in
a rock.
The sharp picks can be used easily and either produce a scratch if they are harder than the
specimen being tested or leave behind a tiny streak of metal if they are softer. Examine the
test site with a hand lens to see the results of your test.
We have used hardness picks and think that they do a great job. They are easier to use and
more accurate than testing with specimens. They can be resharpened when they dull. The only
downside is their price (about $80 per set).
Harder than Diamond, Softer than Talc?
Diamond is not the hardest substance known, but the materials that are harder are much more rare.
Researchers have reported that wurtzie boron nitride and lonsdaleite can be harder than diamond. [1]
It is unlikely that you will find a mineral that is softer than talc. However, a few metals are softer. These include: caesium, rubidium, lithium, sodium and potassium.
You will probably never need to test their hardness.
[2]
Is the Mohs Scale Linear?
Mohs Hardness Scale is not linear. The steps in the scale have gaps of variable size between them.
"For instance, the progression from calcite to fluorite (from 3 to 4 on the Mohs scale) reflects an
increase in hardness of approximately 25 percent; the progression from corundum to diamond, on the
other hand (9 to 10 on the Mohs scale), reflects a hardness increase of more than 300 percent." [3]
About Hardness Tests
The hardness test developed by Friedrich Mohs was the first known test to assess resistance of a material
to scratching. It is a very simple but inexact comparative test. Perhaps its simplicity has enabled
it to become the most widely used hardness test.
Since the Mohs Scale was developed in 1812, many different hardness tests have been invented. These include
tests by Brinell, Knoop, Rockwell, Shore and Vickers. Each of these tests uses a tiny "indenter" that is
applied to the material being tested with a carefully measured amount of force. Then the size or the depth of
the indentation and the amount of force are used to calculate a hardness value.
Because each of these tests uses a different apparatus
and different calculations they can not be directly compared to one another. So if the Knoop hardness test was
done the number is usually reported as a "Knoop hardness". For this reason, Mohs hardness test results should
also be reported as a "Mohs hardness."
Why are there so many different hardness tests? The type of test used is determined by the size, shape and other
characteristics of the specimens being tested. Although these tests are quite different from the Mohs test there
is some correlation between them.
[2]
Hardness, Toughness and Strength
When testing for hardness, remember that you are testing "the resistance to scratching." During the test, some
materials might fail in other ways. They could break, deform, or crumble instead of scratching. Hard materials often break
when subjected to stress. This is a lack of toughness. Other materials might deform or crumble when subjected
to stress. These materials lack strength. Always keep in mind that you are testing for the resistance to being
scratched. Don't be fooled by other types of failure in the specimen being tested.
Uses for Hardness Tests
The Mohs Hardness Test is almost exclusively used to determine the relative hardness of mineral specimens. This
is done as part of a mineral identification procedure in the field, in a classroom, or in a laboratory when easily
identified specimens are being examined or where more sophisticated tests are not available.
In industry, other hardness tests are done to determine the suitability of a material for a specific industrial process or a specific end-use application. Hardness testing is also done in manufacturing processes to confirm that hardening treatments
such as annealing, tempering, work hardening or case hardening have been done to specification.
Some Notes on Spelling
Mohs Hardness Scale is named after its inventor, Friedrich Mohs. This means that an apostrophe is not needed when typing the name of the test. "Moh's" and "Mohs' " are
incorrect.
Google is really smart about these names. You can even type "Moe's Hardness Scale"
as a query and Google knows to return results for "Mohs Hardness Scale". :-)
Contributor: Hobart King
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| A laboratory Mohs Hardness Scale kit containing: (1) talc; (2) gypsum; (3) calcite; (4) fluorite; (5) apatite; (6) orthoclase; (7) quartz; (8) topaz; and (9) corundum. Diamond is not included in most kits to keep the cost down. Also a diamond specimen would be so small that it would need to be mounted in a handle to be useful. Purchase a Mohs Hardness Kit. |
Mohs Hardness of Common Minerals |
| Hardness picks are easy to use. They have a brass stylus and an alloy "pick" that is used for hardness testing. Place the sharp point of a pick on your unknown specimen and drag it across the surface. It will either produce a scratch, slide across the surface or leave a trace of metal. They are supplied with a hardness of 2 (a plastic point), 3 (a copper point) and 4 through 9 (carefully produced alloys). These hardness picks are available in the Geology.com store. |
| Mohs Hardness Scale References |
[1] Scientists Discover Material Harder Than Diamond - Lisa Zyga,
website article on Phys.org, February, 2009.
[2] Mohs Scale of Mineral Hardness: Wikipedia article, accessed October, 2012.
[3] Material Hardness: website article, Center for Advanced Life Cycle Engineering, University of Maryland, accessed October, 2012.
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