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Minerals and the crystalline state

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# 3.6 Hardness

Hardness is loosely defined as the resistance of a material to scratching or indentation. The absolute hardness of a material can be determined precisely, using a mechanical instrument to measure the indentation of a special probe into a crystal surface. However, you can get a general idea of a mineral's relative hardness by undertaking a few simple scratch tests.

The nineteenth-century German mineralogist, Friedrich Mohs, devised a useful scale of mineral hardnesses, consisting of well-known minerals ranked in order of increasing hardness, from talc, with a hardness of 1, to diamond, with a hardness of 10 (Table 2).

Table 2 Mohs' hardness scale.
Mohs' hardness Reference mineral Non-mineral example (hardness in brackets)
1talc
2gypsum
fingernail (2.5)
3calcite
copper coin Footnotes   1 (3.5)
4fluorite
5apatite
6orthoclase feldspar Footnotes   2
hardened steel (6.5)
7quartz
8topaz
9corundum
10diamond

## Footnotes

Footnotes   1 Many of today's 'copper coins' are copper-plated steel and are harder below the copper coating. Footnotes   2 Other types of feldspar may have a slightly greater hardness, between 6 and 6.5. Back to main text

Compared with an absolute hardness scale, Mohs' scale is highly non-linear (diamond is about four times harder than corundum; Figures 12c and b) but, because the scale uses familiar minerals, it provides a quick and easy reference for geologists in the field.

Figure 12 Three very hard minerals: (a) topaz (5 cm long); (b) corundum (variety ruby) (1 cm); (c) diamond (6 mm).

Minerals with a hardness of less than 2.5 may be scratched by a fingernail (Video 4), whereas those with a hardness of less than 3.5 may be scratched by a copper coin (Video 5), and so on.

Video 4 Scratch test on gypsum. (There is no voice-over in this video.)
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