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Mohs Hardness Scale


A rapid hardness test for field and classroom use


Mohs Hardness Scale

Mohs Hardness Kit: 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 Mineral Hardness Kit.

What is Mohs Hardness Scale?




Mohs Hardness Scale

Mineral
Hardness
Talc 1
Gypsum 2
Calcite 3
Fluorite 4
Apatite 5
Orthoclase 6
Quartz 7
Topaz 8
Corundum 9
Diamond 10

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 at left). 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:

  1. If Specimen A can scratch Specimen B, then Specimen A is harder than Specimen B.

  2. If Specimen A does not scratch Specimen B, then Specimen B is harder than Specimen A.

  3. 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.

  4. If Specimen A can be scratched by Specimen B but it cannot be scratched by Specimen C, then the hardness of Specimen A is between the hardness of Specimen B and Specimen C.


Mohs hardness test

Mohs hardness test: 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 Procedure

Mohs Hardness of Common Minerals

Alphabetical
Anhydrite3 to 3.5
Apatite5
Arsenopyrite5.5 to 6
Augite5.5 to 6
Azurite3.5 to 4
Barite2.5 to 3.5
Bauxite1 to 3
Beryl7.5 to 8
Biotite2.5 to 3
Bornite3 to 3.25
Calcite3
Cassiterite6 to 7
Chalcocite2.5 to 3
Chalcopyrite3.5 to 4
Chlorite2 to 2.5
Chromite5.5 to 6
Chrysoberyl8.5
Cinnabar2 to 2.5
Copper2.5 to 3
Cordierite7 to 7.5
Corundum9
Cuprite3.5 to 4
Diamond10
Diopside5.5 to 6.5
Dolomite3.5 to 4
Enstatite5 to 6
Epidote6 to 7
Fluorite4
Galena2.5+
Garnet6.5 to 7.5
Glauconite2
Gold2.5 to 3
Graphite1 to 2
Gypsum1.5 to 2
Halite2 to 2.5
Hematite5 to 6.5
Hornblende5 to 6
Ilmenite5 to 6
Jadeite6.5 to 7
Kyanite4.5 to 7
Limonite1 to 5
Magnesite3.5 to 5
Magnetite5 to 6.5
Malachite3.5 to 4
Marcasite6 to 7.5
Molybdenite1 to 2
Monazite5 to 5.5
Muscovite2 to 3
Nepheline5.5 to 6
Nephrite5 to 6
Olivine6.5 to 7
Orthoclase6 to 6.5
Plagioclase6 to 6.5
Prehnite6 to 6.5
Pyrite6 to 6.5
Pyrophyllite1 to 2
Pyrrhotite3.5 to 4.5
Quartz7
Rhodochrosite3.5 to 4
Rhodonite5.5 to 6.5
Rutile6 to 6.5
Serpentine3 to 5
Siderite3.5 to 4.5
Sillimanite6.5 to 7.5
Silver2.5 to 3
Sodalite5.5 to 6
Sphalerite3.5 to 4
Spinel7.5 to 8
Spodumene6.5 to 7
Staurolite7 to 7.5
Sulfur1.5 to 2.5
Sylvite2
Talc1
Titanite5 to 5.5
Topaz8
Tourmaline7 to 7.5
Turquoise5 to 6
Uraninite5 to 6
Witherite3 to 3.5
Wollastonite4.5 to 5.5
Zircon7.5
Zoisite6 to 7
Decreasing Hardness
Diamond10
Corundum9
Chrysoberyl8.5
Topaz8
Beryl 7.5 to 8
Spinel7.5 to 8
Zircon7.5
Cordierite7 to 7.5
Staurolite7 to 7.5
Tourmaline7 to 7.5
Quartz7
Garnet6.5 to 7.5
Jadeite6.5 to 7
Sillimanite6.5 to 7.5
Olivine6.5 to 7
Spodumene6.5 to 7
Marcasite6 to 7.5
Cassiterite6 to 7
Epidote6 to 7
Zoisite6 to 7
Orthoclase6 to 6.5
Plagioclase6 to 6.5
Prehnite6 to 6.5
Pyrite6 to 6.5
Rutile6 to 6.5
Diopside5.5 to 6.5
Rhodonite5.5 to 6.5
Arsenopyrite5.5 to 6
Augite5.5 to 6
Chromite5.5 to 6
Hematite5.5 to 6.5
Nepheline5.5 to 6
Sodalite5.5 to 6
Magnetite5 to 6.5
Enstatite5 to 6
Hornblende5 to 6
Ilmenite5 to 6
Nephrite5 to 6
Turquoise5 to 6
Uraninite5 to 6
Monazite5 to 5.5
Titanite5 to 5.5
Apatite5
Wollastonite4.5 to 5.5
Kyanite4.5 to 7
Fluorite4
Magnesite3.5 to 5
Pyrrhotite3.5 to 4.5
Siderite3.5 to 4.5
Azurite3.5 to 4
Chalcopyrite3.5 to 4
Cuprite3.5 to 4
Dolomite3.5 to 4
Malachite3.5 to 4
Rhodochrosite3.5 to 4
Sphalerite3.5 to 4
Serpentine3 to 5
Anhydrite3 to 3.5
Witherite3 to 3.5
Bornite3 to 3.25
Calcite3
Barite2.5 to 3.5
Biotite2.5 to 3
Chalcocite2.5 to 3
Copper2.5 to 3
Gold2.5 to 3
Silver2.5 to 3
Galena2.5+
Muscovite2 to 3
Chlorite2 to 2.5
Cinnabar2 to 2.5
Halite2 to 2.5
Glauconite2
Sylvite2
Sulfur1.5 to 2.5
Gypsum1.5 to 2
Limonite1 to 5
Bauxite1 to 3
Graphite1 to 2
Molybdenite1 to 2
Pyrophyllite1 to 2
Talc1

Mohs Hardness Testing Tips

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.

Mohs hardness picks

Mohs hardness picks: 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 selected alloys). They are great for testing small specimens or for testing small grains embedded in a rock. These hardness picks are available in the Geology.com store.

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 wurtzite 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: cesium, rubidium, lithium, sodium, and potassium. You will probably never need to test their hardness. [2]

Mohs - Vickers comparison scale

Mohs - Vickers hardness comparison: This chart compares the hardness of index minerals of the Mohs hardness scale (an integer scale) with their Vickers hardness (a continuous scale). Mohs hardness is a resistance to being scratched, while Vickers hardness is a resistance to indentation under pressure. The graph shows the great difference between the Vickers hardness of corundum and diamond - which are only one unit apart on the Mohs hardness scale.

Mohs Scale of Hardness Compared to Others



Mineral Hardness Scales
Mineral MohsVickers
(kg/mm2)
Talc 127
Gypsum 261
Calcite 3157
Fluorite 4315
Apatite 5535
Orthoclase 6817
Quartz 71161
Topaz 81567
Corundum 92035
Diamond 1010000

When Friedrich Mohs developed his hardness scale in 1812, very little information about mineral hardness was available. He simply selected ten minerals that varied in hardness and arbitrarily placed them on an integer scale from 1 to 10. It was a relative scale in which a mineral of unknown hardness could be tested against a group of ten index minerals to see where it positioned on the scale.

The Mohs scale has stood the test of time and has been widely used throughout the world for over 200 years - mainly because it is easy-to-do, inexpensive and people quickly understand it. Other hardness tests have been devised but none of them are in as widespread use.

A “Mohs hardness” is a relative integer-scale comparison of “resistance to being scratched.” Most other hardness scales use “resistance to indentation under a stylus to which a specific amount of pressure is applied for a specific length of time.” Although these tests differ from Mohs hardness in their procedure, they are all tests of the resistance to atoms being dislodged from their positions by pressure against the surface of a mineral specimen.

One of these scales is the Vickers Hardness Scale. In the Vickers test, the size of the indentation is microscopically estimated and used to calculate a hardness value. The Vickers hardness values form a continuous scale which provides more information about the hardness of minerals when compared to the integer values of the Mohs scale. A table comparing the Mohs scale minerals to their Vickers hardness is shown here along with a graph of the data. The graph shows that in terms of Vickers hardness, the gaps between the integer values of the Mohs scale are not uniform in width. In addition the gaps between minerals of higher Mohs hardness are much broader than those between the softer minerals. In terms of Vickers hardness, diamond is enormously harder than corundum.

Mineral collection

The best way to learn about minerals is to study with a collection of small specimens that you can handle, examine, and observe their properties. Inexpensive mineral collections are available in the Geology.com Store.

Hardness Variations in a Single Mineral

Although reference books and websites often list a single hardness for each mineral, many minerals have variable hardness. They have greater or lesser hardness depending upon the direction in which they are being scratched.

A well-known example of a mineral with variable hardness is kyanite. Kyanite frequently occurs in blade-shaped crystals. These crystals have a hardness of about 5 if they are tested parallel to the long axis of the crystal, and a hardness of about 7 if they are tested parallel to the short axis of a crystal. Why? These different directions encounter different bonding environments in the kyanite crystal. The bonds that resist scratching parallel to the long axis of the bladed crystal are weaker than those encountered when scratching across the width of the crystal. Intermediate hardnesses are encountered in other directions.

Another example is diamond. The people who cut diamonds have known about its variable hardness for hundreds of years. They know that parallel to the octahedral crystal faces, a diamond crystal is almost impossible to saw and very difficult to polish. The diamond can be broken in this direction by cleaving, and the best method for cutting it in this direction is with a laser. The softest and best direction to saw or polish a diamond crystal is parallel to its cubic crystal faces. This information is critical knowledge for the craftsmen who plan the design of a faceted diamond. Understanding it and working with it saves time, saves money and creates a better product with less waste.

Weathering can also influence the hardness of a mineral specimen. Weathering changes a mineral's composition, with the weathering product usually softer than the original material. When testing the hardness or streak or other property of a mineral, the best way to test is on a freshly broken surface with expected luster that has not been exposed to weathering.

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.

Information Sources
[1] Scientists Discover Material Harder Than Diamond - Lisa Zyga, website article on Phys.org, February 2009.

[2] Mohs Scale of Mineral Hardness: Wikipedia article, last accessed July 2016.

[3] Material Hardness: website article, Center for Advanced Life Cycle Engineering, University of Maryland, last accessed July 2016.


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."   :-)

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