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Mohs hardness

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Mohs 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 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]

Minerals

The Mohs scale of mineral hardness is based on the ability of one natural sample of matter to scratch another mineral. 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.[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 nail, 5.5.[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
2.5–3 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
6 orthoclase, titanium, manganese, germanium, niobium, rhodium, uranium
6–7 glass, fused quartz, iron pyrite, silicon, ruthenium, iridium, tantalum, opal, peridot
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):[11]

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

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.
de:Härte#Härteprüfung nach Mohs
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