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Mohs scale of mineral hardness

2007 Schools Wikipedia Selection. Related subjects: Mineralogy

   Mohs' scale of mineral hardness characterizes the scratch resistance of
   various minerals through the ability of a harder material to scratch a
   softer material. For instance, if you scratched a diamond along a pane
   of glass, the glass would scratch and it wouldn't harm the diamond in
   any way. It was created, in 1812, by the German mineralogist Friedrich
   Mohs and is one of several definitions of hardness in materials
   science.

   Mohs based the scale on ten minerals that are all readily available
   except the last one, diamond. 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 Mohs scale is 4.5.

   The table below shows comparison with absolute hardness measures by a
   sclerometer. Mohs' is a purely ordinal scale with, for example,
   corundum being twice as hard as topaz, but diamond almost four times as
   hard as corundum. So, a diamond is the hardest substance known to man.
   Hardness              Mineral               Absolute Hardness
      1     Talc

            (Mg[3]Si[4]O[10](OH)[2])
                                                       1
      2     Gypsum (CaSO[4]·2H[2]O)                    2
      3     Calcite (CaCO[3])                          9
      4     Fluorite (CaF[2])                         21
      5     Apatite

            (Ca[5](PO[4])[3](OH-,Cl-,F-))
                                                      48
      6     Orthoclase Feldspar (KAlSi[3]O[8])        72
      7     Quartz (SiO[2])                           100
      8     Topaz (Al[2]SiO[4](OH-,F-)[2])            200
      9     Corundum (Al[2]O[3])                      400
      10    Diamond(C)                               1500

   On the Mohs scale, fingernail has hardness 2.5; copper penny, about
   3.5; a knife blade, 5.5; window glass, 5.5; steel file, 6.5. Using
   these ordinary materials of known hardness can be a simple way to
   approximate the position of a mineral on the scale.

   Some mnemonics traditionally taught to geology students to remember
   this table are "The Girls Can Flirt And Other Queer Things Can Do" or
   "To Get Candy From Aunt Fanny, Quit Teasing Cousin Danny".

   An alternative table is shown below which has been modified to
   incorporate additional substances that may fall in between two levels.

   Source: American Federation of Mineralogical Societies: Mohs Scale of
   Mineral Hardness
   Hardness    Substance or Mineral
      1     Talc
      2     Gypsum
   2.5 to 3 Gold, Silver
      3     Calcite, Copper penny
      4     Fluorite
   4 to 4.5 Platinum
    4 to 5  Iron
      5     Apatite
      6     Orthoclase
     6.5    Iron pyrite
    6 to 7  Glass, Vitreous pure silica
      7     Quartz
   7 to 7.5 Garnet
    7 to 8  Hardened steel
      8     Topaz
      9     Corundum
      11    Fused zirconia
      12    Fused alumina
      13    Silicon carbide
      14    Boron carbide
      15    Diamond
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