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Mohs
scale of mineral hardness
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Mohs'
scale of mineral hardness characterizes the scratch
resistance of various minerals through the ability of
a harder material to scratch a softer. 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.
|
Hardness |
Mineral |
Absolute
Hardness |
1 |
Talc
(Mg3Si4O10(OH)2)
|
1 |
2 |
Gypsum (CaSO4•2H2O) |
2 |
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) |
1500 |
On the Mohs scale, fingernail has
hardness 2; copper penny, about 3; a knife blade, 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.
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 and up |
Hardened steel |
| 8 |
Topaz |
| 9 |
Corundum |
| 10 |
Garnet |
| 11 |
Fused zirconia |
| 12 |
Fused alumina |
| 13 |
Silicon
carbide |
| 14 |
Boron carbide |
| 15 |
Diamond |
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