Mohs scale of mineral hardness
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The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German mineralogist Friedrich Mohs and is one of several definitions of hardness in materials science.[1] The method of comparing hardness by seeing which minerals can scratch others, however, is of great antiquity, having first been mentioned by Theophrastus in his treatise On Stones in ca 300 BC, followed by Pliny the Elder in his Naturalis Historia circa A.D. 77.[2][3][4]
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Minerals
The Mohs scale of mineral hardness is based on the ability of one natural sample of matter to scratch another. The samples of matter used by Mohs are all minerals. Minerals are pure substances found in nature. Rocks are made up of one or more minerals.[5] As the hardest known naturally occurring substance when the scale was designed, diamonds are at the top of the scale. The hardness of a material is measured against the scale by finding the hardest material that the given material can scratch, and/or the softest material that can scratch the given material. For example, if some material is scratched by apatite but not by fluorite, its hardness on the Mohs scale would fall between 4 and 5.[6]
The Mohs scale is a purely ordinal scale. For example, corundum (9) is twice as hard as topaz (8), but diamond (10) is almost four times as hard as corundum. The table below shows comparison with absolute hardness measured by a sclerometer, with pictorial examples.[7][8]
Since the invention of the scale, there have been reports of materials harder than the highest mineral on the scale, diamonds; so the Mohs scale may be changed in the future.[9]
| Mohs hardness | Mineral | Absolute Hardness | Image |
|---|---|---|---|
| 1 | Talc (Mg3Si4O10(OH)2) | 1 |  |
| 2 | Gypsum (CaSO4·2H2O) | 3 |  |
| 3 | Calcite (CaCO3) | 9 |  |
| 4 | Fluorite (CaF2) | 21 |  |
| 5 | Apatite (Ca5(PO4)3(OH-,Cl-,F-)) | 48 |  |
| 6 | Orthoclase Feldspar (KAlSi3O8) | 72 |  |
| 7 | Quartz (SiO2) | 100 |  |
| 8 | Topaz (Al2SiO4(OH-,F-)2) | 200 |  |
| 9 | Corundum (Al2O3) | 400 |  |
| 10 | Diamond (C) | 1600 |  |
On the Mohs scale, a pencil "lead" (graphite) has a hardness of 1; a fingernail, 2.2-2.5; a copper penny, 3.2-3.5; a pocketknife 5.1; a knife blade, 5.5; window glass plate, 5.5; and a steel file, 6.5.[10] A streak plate (unglazed porcelain) has a hardness of 7.0. Using these ordinary materials of known hardness can be a simple way to approximate the position of a mineral on the scale.[1]
Intermediate hardness
The table below incorporates additional substances that may fall between levels :
| Hardness | Substance or mineral |
|---|---|
| 0.2-0.3 | Cs, Rb |
| 0.5-0.6 | Li, Na, K |
| 1 | Talc, graphite |
| 1.5 | Ga, Sr, In, Sn, Ba, Tl, Pb |
| 2 | hexagonal BN [11], Ca, Se, Cd, sulfur, Te, Bi |
| 2.5 to 3 | Mg, Au, Ag, Al, Zn, La, Ce |
| 3 | Calcite, Cu, As, Sb, Th, Dentin |
| 4 | Fluorite, Fe, Ni |
| 4 to 4.5 | Pt, Steel |
| 5 | Apatite, Co, Zr, Pd, Tooth enamel |
| 5.5 | Be, Mo, Hf |
| 6 | Orthoclase, Ti, Mn, Ge, Nb, Rh, uranium |
| 6 to 7 | Glass, fused quartz, Iron pyrite, Si, Ru, Ir, Ta |
| 7 | Quartz, vanadium, Os, Re |
| 7.5 to 8 | Hardened steel, Tungsten, emerald |
| 8 | Topaz, Cubic Zirconium |
| 8.5 | Chrysoberyl, Cr |
| 9 | Corundum, Carborundum (SiC), Tungsten carbide, titanium carbide |
| <10 | Rhenium diboride, Tantalum carbide, titanium diboride, Boron [12][13] |
| 10 | Diamond |
| >10 | Nanocrystalline diamond |
See also
- Hardness
- Brinell hardness
- Vickers hardness
- Hardnesses of the elements (data page)
- Knoop hardness test
References
Notes
- ^ a b Encyclopædia Britannica. 2009. Encyclopædia Britannica Online. 22 Feb. 2009 "Mohs hardness."
- ^ Theophrastus on Stones
- ^ Pliny the Elder.Naturalis Historia.Book 37.Chap. 15. ADamas: six varieties of it. Two remedies.
- ^ Pliny the Elder.Naturalis Historia.Book 37.Chap. 76. The methods of testing precious stones.
- ^ Learn science, Intermediate p. 42
- ^ American Federation of Mineralogical Societies. "Mohs Scale of Mineral Hardness"
- ^ Amethyst Galleries' Mineral Gallery What is important about hardness?
- ^ Inland Lapidary Mineral Hardness and Hardness Scales
- ^ T. Irifune, A Kurio, S. Sakamoto, T. Inoue, H. Sumiya "Ultrahard polycrystalline diamond from graphite" Nature 421 (2003) 599
- ^ ""The Hardness of Minerals and Rocks" by William S. Cordua". Lapidary Digest. 1998. http://www.gemcutters.org/LDA/hardness.htm. Retrieved 2007-08-19. Hosted at International Lapidary Association
- ^ L. I. Berger "Semiconductor materials" CRC Press, 1996 ISBN 0849389127, 9780849389122 (available on google books), p. 126
- ^ Solozhenko, V. L.; Kurakevych O. O.; Oganov A. R. (2008). "On the hardness of a new boron phase, orthorhombic γ-B28". Journal of Superhard Materials 30: 428–429. doi:10.3103/S1063457608060117.
- ^ E. Yu. Zarechnaya (2009). "Superhard Semiconducting Optically Transparent High Pressure Phase of Boron". Phys. Rev. Lett. 102: 185501. doi:10.1103/PhysRevLett.102.185501.
Sources
- Mohs hardness of elements is taken from G.V. Samsonov (Ed.) in Handbook of the physicochemical properties of the elements, IFI-Plenum, New York, USA, 1968.
- Cordua, William S. "The Hardness of Minerals and Rocks". Lapidary Digest, c. 1990.
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