The Mohs hardness scale is a qualitative test that measures the hardness of a mineral by its ability to visibly scratch softer minerals. The scale isn’t perfect, but it’s a great tool for quick identification of rocks in the field. Here a look at the scale, how ordinary minerals and objects rank, and how to perform the Mohs test.
The Mohs Hardness Scale for Minerals
Minerals are chemically pure solids that occur in nature. The Mohs scale ranks minerals on a scale from 1 to 10, in which 1 is the softest mineral and 10 is the hardest mineral. Diamond is the hardest mineral, with a Mohs hardness of 10. Talc is the softest mineral, with a hardness of 1.
The Mohs scale is ordinal, where the relative hardness values between one number and another are not uniform For example, diamond (10) is four times harder than corundum (9), while corundum is twice as hard as topaz (8).
| Mohs Hardness | Mineral | Chemical Formula | Absolute Hardness |
|---|---|---|---|
| 10 | Diamond | C | 1500 |
| 9 | Corundum | Al2O3 | 400 |
| 8 | Topaz | Al2SiO4(OH−,F−)2 | 200 |
| 7 | Quartz | SiO2 | 100 |
| 6 | Orthoclase feldspar | KAlSi3O8 | 72 |
| 5 | Apatite | Ca5(PO4)3(OH−,Cl−,F−) | 48 |
| 4 | Fluorite | CaF2 | 21 |
| 3 | Calcite | CaCO3 | 14 |
| 2 | Gypsum | CaSO4·2H2O | 2 |
| 1 | Talc | Mg3Si4O10(OH)2 | 1 |
Mohs Hardness Scale of Common Materials
Where, exactly, a specimen falls on the Mohs scale it not determined solely by its chemical formula. Minerals may have multiple crystals structures and usually contain additional elements. But, this table is a good comparison of common values.
| Mohs Hardness | Materials |
|---|---|
| 0.2–0.3 | cesium, rubidium |
| 0.5–0.6 | lithium, sodium, potassium, candle wax |
| 1 | talc |
| 1.5 | gallium, strontium, indium, tin, barium, thallium, lead, graphite, ice |
| 2 | calcium, selenium, cadmium, sulfur, tellurium, bismuth, gypsum |
| 2-2.5 | fingernail, mica, rock salt (halite), amber |
| 2.5-3 | gold, silver, zinc, lanthanum, aluminum, cerium, jet, pearl |
| 3 | calcite, copper, chalk, dentin, thorium, arsenic, antimony |
| 3.5 | platinum, sphalerite, chalcopyrite |
| 4 | fluorite, nickel, iron |
| 4-4.5 | ordinary steel |
| 5 | apatite, tooth enamel, obsidian, palladium, zirconium |
| 5.5 | glass, beryllium, cobalt, hafnium, molybdenum, turquoise, lapis lazuli, opal |
| 6 | orthoclase, uranium, titanium, germanium, niobium, manganese |
| 6-7 | iron pyrite, silicon, ruthenium, opal, peridot, tanzanite, iridium, jade, rhodium, pyrite, garnet, jasper, chrysoprase, chalcedony |
| 7 | quartz, citrine, amethyst, osmium, rhenium, vanadium, unglazed porcelain, zircon |
| 7.5-8 | beryl, emerald, zircon, spinel, tungsten |
| 8 | topaz, cubic zirconia, spinel, hardened steel |
| 8.5 | chrysoberyl, silicon nitride, tantalum carbide, chromium |
| 9 | rubies and sapphires (corundum), tungsten carbide, aluminum oxide (alumina), titanium nitride |
| 9-9.5 | moissanite (silicon carbide or carborundum), boron carbide, zirconium carbide |
| 9.5- almost 10 | boron nitride, boron, boron carbide, stishovite, titanium diboride |
| 10 | diamond, carbonado (black diamond) |
From the table, note which pure elements are soft and which are hard. Metals that becomes liquids near room temperature are the softest. Nonmetals are often softer than metals, particularly transition metals. While diamond is pure carbon and is a nonmetal, other carbon allotropes are softer.
Perform the Mohs Hardness Test
The Mohs hardness test is easily performed. You need examples of items with known hardness values. Handy materials include:
- your fingernails (2.5)
- a copper coin or piece of chalk (3.5)
- a pocket knife or piece of glass (4.5-5.5)
- a quartz point or unglaze porcelain (7.0)
- a hardened drill bit (8.5)
Once you have a collection of materials of representative hardness values, perform the test:
- Choose a fresh, clean surface on the material you are testing.
- See if you can scratch it with the point of an object with known hardness. This object is your “tool.” If you feel it “bite” into the surface of the test material, it’s probably harder than your sample.
- Inspect your sample for a scratch or etched line. Turn it under good lighting so a scratch is more visible. Run your fingernail across the sample and see if you feel a scratch.
- If the tool scratches the sample, then the sample has a lower Mohs hardness value than the tool. For example, if you scratch a sample with a quartz point, then the sample has a hardness less than 7.
- If the tool does not scratch the sample, then it is harder than the tool. This is because materials of equal hardness scratch one another. For example, if you cannot scratch a sample with a quartz point, its hardness is greater than 7.
Use other tools you have available to narrow down the possible hardness value. For example, if quartz scratches the sample, you know it is harder than 7. See if a hardened drill bit scratches it. If it does not, then the hardness is between 7 and 8.5. If no scratch appears, the samples is harder than 8.5.
Mohs Scale Uses
The Mohs hardness scale is useful in roughly identifying minerals in the field with scratch kits. It also helps predict the best type of mill for a given product. In the electronics industry, the Mohs scale helps in the evaluation of hardness of LCD cover glass and OLED encapsulation. However, hardness and toughness are two different mechanical properties, so a material that is hard according to the Mohs scale may still easily shatter.
History
The Mohs hardness tests takes its name for German geologist and mineralogist Friedrich Mohs. Mohs published the scale in 1822 in his Treatise on Mineralogy. However, the scratch test has been around for a long time. Theophrastus mentions comparing minerals by scratching them in his c. 300 BC treatise On Stones.
See the appearance of the birthstone minerals before they get cut into gemstones and see their chemical formulas and Mohs hardness values.
Limitations of the Mohs Hardness Scale
Limitations of the Mohs scale include its lack of precision and that fact that it is a potentially destructive test. For example, if you test a gemstone by seeing whether or not corundum scratches it, any mineral softer than 9 sustains a permanent mark.
Alternatives to the Mohs Test
There are alternative to both the Mohs scale and the Mohs test. Other scales include the Vickers, Brinell, Rockwell, and Rosiwall scales. Other tests include in the Knoop hardness test and Meyer hardness test.
References
- Mohs, Friedrich (1825) [Original German publication in 1822]. Treatise on Mineralogy: Or, The Natural History of the Mineral Kingdom. Translated by Haidinger, William. United Kingdom: A. Constable and Company.
- Mukherjee, Swapna (2012). Applied Mineralogy: Applications in Industry and Environment. Springer Science & Business Media. ISBN 978-94-007-1162-4.
- Smith, R.L.; Sandland, G.E. (1922). “An Accurate Method of Determining the Hardness of Metals, with Particular Reference to Those of a High Degree of Hardness.” Proceedings of the Institution of Mechanical Engineers. Vol. I: 623–641.
- Zhang, P. (September 2011). “General relationship between strength and hardness”. Materials Science and Engineering A. 529: 62. doi:10.1016/j.msea.2011.08.061