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 geologist and mineralogist Friedrich Mohs and is one of several definitions of hardness in materials science, some of which are more quantitative.1 The method of comparing hardness by seeing which minerals can visibly scratch others, however, is of great antiquity, having been mentioned by Theophrastus in his treatise On Stones, c. 300 BC, followed by Pliny the Elder in his Naturalis Historia, c. 77 AD.234 While greatly facilitating the identification of minerals in the field, the Mohs scale is not suitable for accurately gauging the hardness of industrial materials.5

Minerals

The Mohs scale of mineral hardness is based on the ability of one natural sample of mineral to scratch another mineral visibly. The samples of matter used by Mohs are all different minerals. Minerals are pure substances found in nature. Rocks are made up of one or more minerals.6 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 5.7 "Scratching" a material for the purposes of the Mohs scale means creating non-elastic dislocations visible to the naked eye. Frequently, materials that are lower on the Mohs scale can create microscopic, non-elastic dislocations upon materials that have a higher Mohs number. While these microscopic dislocations are permanent and sometimes detrimental to the harder material's structural integrity, they are not considered "scratches" for the determination of a Mohs scale number. 8

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.910

Mohs hardness Mineral Chemical formula Absolute hardness11 Image
1 Talc Mg3Si4O10(OH)2 1 Talc block.jpg
2 Gypsum CaSO4·2H2O 3 Gypse Arignac.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 Feldspar KAlSi3O8 72 OrthoclaseBresil.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, 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, 5.5clarification needed; window glass plate, 5.5; and a steel nail, 5.5.12 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, ice13
2 hexagonal boron nitride,14 calcium, selenium, cadmium, sulfur, tellurium, bismuth
2.5–3 magnesium, gold, silver, aluminium, zinc, lanthanum, cerium, Jet (lignite)
3 calcite, copper, arsenic, antimony, thorium, dentin
4 fluorite, iron, nickel
4–4.5 platinum, steel
5 apatite (tooth enamel), cobalt, zirconium, palladium, obsidian (volcanic glass)
5.5 beryllium, molybdenum, hafnium, glass
6 orthoclase, titanium, manganese, germanium, niobium, rhodium, uranium
6–7 fused quartz, iron pyrite, silicon, ruthenium, iridium, tantalum, opal, peridot, tanzanite
7 osmium, quartz, rhenium, vanadium
7.5–8 emerald, hardened steel, tungsten, spinel
8 topaz, cubic zirconia
8.5 chrysoberyl, chromium, silicon nitride, tantalum carbide
9–9.5 corundum, silicon carbide (carborundum), tungsten carbide, titanium carbide
9.5–10 boron, boron nitride, rhenium diboride, stishovite, titanium diboride
10 diamond, carbonado
>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 VHN10=7–9
Bismuth 2–2½ VHN100=16–18
Gold VHN10=30–34
Silver VHN100=61–65
Chalcocite 2½–3 VHN100=84–87
Copper 2½–3 VHN100=77–99
Galena 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 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 VHN100=1,310
Chromium 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. ^ Non-Destructive Testing Resource Center [1]
  6. ^ Learn science, Intermediate p. 42
  7. ^ American Federation of Mineralogical Societies. "Mohs Scale of Mineral Hardness"
  8. ^ The True Microstructure of Materials [2]
  9. ^ Amethyst Galleries' Mineral Gallery What is important about hardness?. galleries.com
  10. ^ Inland Lapidary Mineral Hardness and Hardness Scales
  11. ^ Applied Mineralogy: Applications in Industry and Environment By Swapna Mukherjee 2011 [3]
  12. ^ William S. Cordua (1998). "The Hardness of Minerals and Rocks". Lapidary Digest. Retrieved 2007-08-19.  Hosted at International Lapidary Association
  13. ^ http://www.messenger-education.org/library/pdf/ice_mineral.pdf
  14. ^ Berger, Lev I. (1996). Semiconductor Materials (First ed.). Boca Raton, FL: CRC Press. p. 126. ISBN 978-0849389122. 
  15. ^ http://www.mindat.org

Further reading

  • 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.