Mohs scale
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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, 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]
Minerals
Mohs based the scale on ten minerals that are all readily available. As the hardest known naturally occurring substance when the scale was designed, diamonds are at 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. [5]
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.[6][7]
However given that new minerals found from asteroids or formed during volcanic eruptions are proven to be harder than the highest mineral on the scale, diamonds, it may be contentious in the future that the Mohs scale as it is won't be changed. [8]
| 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 | 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, 2.5; a copper penny, about 3.5; a knife blade, 5.5; window glass, 5.5; and a steel file, 6.5.[9] Using these ordinary materials of known hardness can be a simple way to approximate the position of a mineral on the scale.
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 [10], Ca, Se, Cd, sulfur, Te, Bi |
| 2.5 to 3 | Mg, Au, Ag, Al, Zn, La, Ce |
| 3 | Calcite, Cu, As, Sb, Th |
| 4 | Fluorite, Fe, Ni |
| 4 to 4.5 | Pt, Steel |
| 5 | Apatite, Co, Zr, Pd |
| 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 to 7.5 | Garnet |
| 7 to 8 | Hardened steel, Tungsten |
| 8 | Topaz |
| 8.5 | Chrysoberyl, Cr |
| 9 | Corundum, Carborundum (SiC), Tungsten carbide |
| <10 | Rhenium diboride, Tantalum carbide, Boron [11] |
| 10 | Diamond |
| >10 | nanocrystalline diamond |
See also
References
- ^ 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.
- ^ 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
- ^ New Scientist, Diamond no longer nature's hardest material. Feb. 16 2009.
- ^ ""The Hardness of Minerals and Rocks" by William S. Cordua". Lapidary Digest. 1998. 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. (2008). "On the hardness of a new boron phase, orthorhombic γ-B28". Journal of Superhard Materials. 30: 428–429. doi:10.3103/S1063457608060117.
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- 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.
