Mohs scale of mineral hardness

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 geologist and 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, c. 300 BC, followed by Pliny the Elder in his Naturalis Historia, c. 77 AD.[2][3][4]

Contents

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 four times as hard as corundum. The table below shows comparison with absolute hardness measured by a sclerometer, with pictorial examples.[7][8]

Mohs hardness Mineral Chemical formula 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, graphite (a principal constituent of pencil "lead") has a hardness of 1.5; 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.[9] 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 caesium, rubidium
0.5–0.6 lithium, sodium, potassium
1 talc
1.5 gallium, strontium, indium, tin, barium, thallium, lead, graphite
2 hexagonal boron nitride,[10] calcium, selenium, cadmium, sulfur, tellurium, bismuth
2.5 to 3 magnesium, gold, silver, aluminium, zinc, lanthanum, cerium, Jet_(lignite) (lignite)
3 calcite, copper, arsenic, antimony, thorium, dentin
4 fluorite, iron, nickel
4 to 4.5 platinum, steel
5 apatite, cobalt, zirconium, palladium, tooth enamel, obsidian (volcanic glass)
5.5 beryllium, molybdenum, hafnium
6 orthoclase, titanium, manganese, germanium, niobium, rhodium, uranium
6 to 7 glass, fused quartz, iron pyrite, silicon, ruthenium, iridium, tantalum, opal
7 quartz, vanadium, osmium, rhenium
7.5 to 8 hardened steel, tungsten, emerald, spinel
8 topaz, cubic zirconia
8.5 chrysoberyl, chromium, silicon nitride
9-9.5 corundum, silicon carbide (carborundum), tungsten carbide, titanium carbide, stishovite
9.5–10 rhenium diboride, tantalum carbide, titanium diboride, boron nitride, boron.[11][12][13][14]
10 diamond
>10 nanocrystalline diamond (hyperdiamond, ultrahard fullerite)

Hardness (Vickers)

Comparison between Hardness (Mohs) and Hardness (Vickers):[15]

Mineral
name		 Hardness (Mohs) Hardness (Vickers)
kg/mm2
Graphite 1–2 VHN10=7–11
Tin 1½–2 VHN10=7–9
Bismuth 2–2½ VHN100=16–18
Gold 2½–3 VHN10=30–34
Silver 2½–3 VHN100=61–65
Chalcocite 2½–3 VHN100=84–87
Copper 2½–3 VHN100=77–99
Galena 2½ VHN100=79–104
Sphalerite 3½–4 VHN100=208–224
Heazlewoodite 4 VHN100=230–254
Carrollite 4½–5½ VHN100=507–586
Goethite 5–5½ VHN100=667
Hematite 5–6 VHN100=1,000–1,100
Chromite 5½ VHN100=1,278–1,456
Anatase 5½–6 VHN100=616–698
Rutile 6–6½ VHN100=894–974
Pyrite 6–6½ VHN100=1,505–1,520
Bowieite 7 VHN100=858–1,288
Euclase 7½ VHN100=1,310
Chromium 9 VHN100=1,875–2,000

See also

References

  1. ^ a b Encyclopædia Britannica. 2009. Encyclopædia Britannica Online. 22 Feb. 2009 "Mohs hardness."
  2. ^ Theophrastus on Stones. Farlang.com. Retrieved on 2011-12-10.
  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?. galleries.com
  8. ^ Inland Lapidary Mineral Hardness and Hardness Scales
  9. ^ William S. Cordua (1998). "The Hardness of Minerals and Rocks". Lapidary Digest. http://www.gemcutters.org/LDA/hardness.htm. Retrieved 2007-08-19.  Hosted at International Lapidary Association
  10. ^ L. I. berger "semiconductor materials" CRC press, 1996 ISBN 0849389127, p. 126
  11. ^ Weintraub E. (1911). "On the properties and preparation of the element boron.". J. Ind. Eng. Chem. 3 (5): 299–301. doi:10.1021/ie50029a007. 
  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 (6): 428–429. doi:10.3103/s1063457608060117. http://mysbfiles.stonybrook.edu/~aoganov/files/JSM-2008-6b-e.pdf. 
  13. ^ Zarechnaya, E. Yu.; Dubrovinsky, L.; Dubrovinskaia, N.; Filinchuk, Y.; Chernyshov, D.; Dmitriev, V.; Miyajima, N.; El Goresy, A. et al. (2009). "Superhard semiconducting optically transparent high pressure phase of boron". Phys. Rev. Lett. 102 (18): 185501. Bibcode 2009PhRvL.102r5501Z. doi:10.1103/physrevlett.102.185501. PMID 19518885. 
  14. ^ Oganov A.R., Solozhenko V.L. (2009). "Boron: a hunt for superhard polymorphs". Journal of Superhard Materials 31: 285–291. http://mysbfiles.stonybrook.edu/~aoganov/files/Boron-history-JSM.pdf. 
  15. ^ http://www.mindat.org

Further reading

Mineral identification