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 mineralogist Friedrich Mohs and is one of several definitions of hardness in materials science.  The method of comparing hardness by seeing which minerals can scratch others, however, is of great antiquity, having first been mentioned by Theophrastus in his treatise On Stones, circa 300 BC, followed by Pliny the Elder in his Naturalis Historia, circa 77 AD.

The Mohs scale of mineral hardness is based on the ability of one natural sample of matter to scratch another. The samples of matter used by Mohs are all minerals. Minerals are pure substances found in nature. Rocks are made up of one or more minerals. 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.

The Mohs scale is a purely ordinal scale. For example, corundum (9) is twice as hard as topaz (8), but diamond (10) is almost four times as hard as corundum. The table below shows comparison with absolute hardness measured by a sclerometer, with pictorial examples. Since the invention of the scale, there have been reports of materials harder than the highest mineral on the scale, diamond; so the Mohs scale may be changed in the future.

Mohs original scale is as follows:

Hardness

Material

Absolute Hardness

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 (KAlSi3O8)

72

7

Quartz (SiO2)

100

8

Topaz (Al2SiO4(OH-,F-)2)

200

9

Corundum (Al2O3)

400

10

Diamond (C)

1500

To give some examples from everyday life:

  • a fingernail has a hardness of 2
  • a copper penny, about 3
  • a knife blade, 5
  • window glass, 5.5
  • and a steel file, 6.5
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