Mohs scale of mineral hardness

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An example of the Mohs scale
The mohs scale, named after Friedrich Mohs

Mohs scale of mineral hardness is named after the scientist, Friedrich Mohs, who invented a scale of hardness based on the ability of one mineral to scratch another. Rocks are made up of one or more minerals. According to the scale, Talc is the softest: it can be scratched by all other materials. Gypsum is harder: it can scratch talc but not calcite, which is even harder. The hardness of a mineral is mainly controlled by the strength of the bonding between the atoms and partly by the size of the atoms. It is a measure of the resistance of the mineral to scratching.

The Mohs scale is for natural minerals. For manufactured products other measures of hardness are better.[1]

Diamond is always at the top of the scale, being the hardest mineral. There are ten minerals in Mohs scale, talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum, and for last and hardest, diamond. Because the Mohs scale was made long ago, it is not exactly correct - for example, several minerals are now known to be harder than the diamond. The Mohs scale may not be perfect, but field geologists still find it very useful.

Mohs hardness Mineral Chemical formula Absolute hardness[2] 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

Hardness of some other items[change | change source]

2.5 Fingernail
2.5–3 Gold, Silver
3 Copper penny
4-4.5 Platinum
4-5 Iron
5.5 Knife blade
6-7 Glass
6.5 Iron pyrite
7+ Hardened steel file
[3]

References[change | change source]

  1. NDT Resource Center: hardness. [1]
  2. Applied Mineralogy: applications in industry and environment. Swapna Mukherjee 2011 [2]
  3. Mohs scale of Hardness, http://www.amfed.org/t_mohs.htm

Other websites[change | change source]