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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 Talc block.jpg
2 Gypsum (CaSO4·2H2O) 3 Gypsum Australia.jpg
3 Calcite (CaCO3) 9 Calcite-sample2.jpg
4 Fluorite (CaF2) 21 Fluorite with Iron Pyrite.jpg
5 Apatite (Ca5(PO4)3(OH-,Cl-,F-)) 48 Apatite crystals.jpg
6 Orthoclase Feldspar (KAlSi3O8) 72 Mineraly.sk - ortoklas.jpg
7 Quartz (SiO2) 100 Quartz Brésil.jpg
8 Topaz (Al2SiO4(OH-,F-)2) 200 Topaz cut.jpg
9 Corundum (Al2O3) 400 Cut Ruby.jpg
10 Diamond (C) 1600 Rough diamond.jpg

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

References

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Notes

  1. ^ a b Encyclopædia Britannica. 2009. Encyclopædia Britannica Online. 22 Feb. 2009 "Mohs hardness."
  2. ^ Theophrastus on Stones
  3. ^ Pliny the Elder.Naturalis Historia.Book 37.Chap. 15. ADamas: six varieties of it. Two remedies.
  4. ^ Pliny the Elder.Naturalis Historia.Book 37.Chap. 76. The methods of testing precious stones.
  5. ^ Learn science, Intermediate p. 42
  6. ^ American Federation of Mineralogical Societies. "Mohs Scale of Mineral Hardness"
  7. ^ Amethyst Galleries' Mineral Gallery What is important about hardness?
  8. ^ Inland Lapidary Mineral Hardness and Hardness Scales
  9. ^ T. Irifune, A Kurio, S. Sakamoto, T. Inoue, H. Sumiya "Ultrahard polycrystalline diamond from graphite" Nature 421 (2003) 599
  10. ^ ""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
  11. ^ L. I. Berger "Semiconductor materials" CRC Press, 1996 ISBN 0849389127, 9780849389122 (available on google books), p. 126
  12. ^ 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. 
  13. ^ 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.

Simple English

File:Knoop-and Mohs-
The mohs scale, named after scientist Friedrich Mohs

Mohs scale of mineral hardness is named after the scientist, Friedrich Mohs, who invented a scale of hardness based on the ability of one mineral to scratch another. Rocks are made up of one or more minerals. According to the scale, Talc is the softest: it can be scratched by all other materials. Gypsum is harder: it can scratch talc but not calcite, which is even harder. The hardness of a mineral is mainly controlled by the strength of the bonding between the atoms and partly by the size of the atoms. It is a measure of the resistance of the mineral to scratching.

Contents

The hardest mineral

As it says in Mohs scale, the diamond is always at the most top of the scale, being the hardest mineral. There are ten minerals in Mohs scale, talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum, and for last and the hardest, diamond. But since the Mohs scale was made long ago, it is not exactly correct - for example, many have found and reported minerals higher in hardness than the diamond. So the Mohs scale may not be most completely perfect, but it is still quite trustworthy.

Hardness of some other items

2.5 Fingernail
2.5–3 Gold, Silver
3 Copper penny
4-4.5 Platinum
4-5 Iron
5.5 Knife blade
6-7 Glass
6.5 Iron pyrite
7+ Hardened steel file
[1]

Other websites

References

  1. Mohs scale of Hardness, http://www.amfed.org/t_mohs.htm


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