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.[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[edit]

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 hardnessMineralChemical formulaAbsolute hardnessImage
1TalcMg3Si4O10(OH)21Talc block.jpg
2GypsumCaSO4·2H2O3Gypse Arignac.jpg
3CalciteCaCO39Calcite-sample2.jpg
4FluoriteCaF221Fluorite with Iron Pyrite.jpg
5ApatiteCa5(PO4)3(OH,Cl,F)48Apatite crystals.jpg
6FeldsparKAlSi3O872OrthoclaseBresil.jpg
7QuartzSiO2100Quartz Brésil.jpg
8TopazAl2SiO4(OH,F)2200Topaz cut.jpg
9CorundumAl2O3400Cut Ruby.jpg
10DiamondC1600Rough 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, 5.5[clarification needed]; 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[edit]

The table below incorporates additional substances that may fall between levels:

HardnessSubstance or mineral
0.2–0.3caesium, rubidium
0.5–0.6lithium, sodium, potassium
1talc
1.5gallium, strontium, indium, tin, barium, thallium, lead, graphite, ice[10]
2hexagonal boron nitride,[11] calcium, selenium, cadmium, sulfur, tellurium, bismuth
2.5–3magnesium, gold, silver, aluminium, zinc, lanthanum, cerium, Jet (lignite)
3calcite, copper, arsenic, antimony, thorium, dentin
4fluorite, iron, nickel
4–4.5platinum, steel
5apatite (tooth enamel), cobalt, zirconium, palladium, obsidian (volcanic glass)
5.5beryllium, molybdenum, hafnium
6orthoclase, titanium, manganese, germanium, niobium, rhodium, uranium
6–7glass, fused quartz, iron pyrite, silicon, ruthenium, iridium, tantalum, opal, peridot, tanzanite
7osmium, quartz, rhenium, vanadium
7.5–8emerald, hardened steel, tungsten, spinel
8topaz, cubic zirconia
8.5chrysoberyl, chromium, silicon nitride, tantalum carbide
9–9.5corundum, silicon carbide (carborundum), tungsten carbide, titanium carbide
9.5–10boron, boron nitride, rhenium diboride, stishovite, titanium diboride
10diamond, carbonado
>10nanocrystalline diamond (hyperdiamond, ultrahard fullerite)

Hardness (Vickers)[edit]

Comparison between Hardness (Mohs) and Hardness (Vickers):[12]

Mineral
name
Hardness (Mohs)Hardness (Vickers)
kg/mm2
Graphite1–2VHN10=7–11
TinVHN10=7–9
Bismuth2–2½VHN100=16–18
GoldVHN10=30–34
SilverVHN100=61–65
Chalcocite2½–3VHN100=84–87
Copper2½–3VHN100=77–99
GalenaVHN100=79–104
Sphalerite3½–4VHN100=208–224
Heazlewoodite4VHN100=230–254
Carrollite4½–5½VHN100=507–586
Goethite5–5½VHN100=667
Hematite5–6VHN100=1,000–1,100
ChromiteVHN100=1,278–1,456
Anatase5½–6VHN100=616–698
Rutile6–6½VHN100=894–974
Pyrite6–6½VHN100=1,505–1,520
Bowieite7VHN100=858–1,288
EuclaseVHN100=1,310
ChromiumVHN100=1,875–2,000

See also[edit]

References[edit]

  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. Retrieved 2007-08-19.  Hosted at International Lapidary Association
  10. ^ http://www.messenger-education.org/library/pdf/ice_mineral.pdf
  11. ^ Berger, Lev I. (1996). Semiconductor Materials (First ed.). Boca Raton, FL: CRC Press. p. 126. ISBN 978-0849389122. 
  12. ^ http://www.mindat.org

Further reading[edit]