Mohs scale of mineral hardness

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.^[1] 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 in ca 300 BC, followed by Pliny the Elder in his Naturalis Historia circa A.D. 77.^[2][3][4]

Minerals

Mohs based the scale on ten minerals that are all readily available. 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.^[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.^[6][7]

Since the invention of the scale, there have been reports of materials harder than the highest mineral on the scale, diamonds; so the Mohs scale may be changed in the future.^[8]

Mohs hardness	Mineral	Absolute Hardness	Image
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 Feldspar (KAlSi3O8)	72
7	Quartz (SiO2)	100
8	Topaz (Al2SiO4(OH-,F-)2)	200
9	Corundum (Al2O3)	400
10	Diamond (C)	1600

On the Mohs scale, a pencil "lead" (graphite) has a hardness of 1; a fingernail, 2.2-2.5; a copper penny, 3.2-3.5; a pocketknife 5.1; a knife blade, 5.5; window glass plate, 5.5; and a steel file, 6.5.^[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

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

See also

• Hardness
• Brinell hardness
• Vickers hardness
• Hardnesses of the elements (data page)
• Knoop hardness test

References

Notes

Sources

• Mohs hardness of elements is taken from G.V. Samsonov (Ed.) in Handbook of the physicochemical properties of the elements, IFI-Plenum, New York, USA, 1968.
• Cordua, William S. "The Hardness of Minerals and Rocks". Lapidary Digest, c. 1990.