An introduction to minerals and rocks under the microscope
An introduction to minerals and rocks under the microscope

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An introduction to minerals and rocks under the microscope

1.3.6 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.

Figure 11 Three very hard minerals: (a) topaz (5 cm long); (b) corundum (variety ruby) (1 cm); (c) diamond (6 mm).

The 19th 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). Compared with an absolute hardness scale, Mohs' scale is highly non-linear (diamond is about four times harder than corundum; Figure 11c and b) but, because the scale uses common minerals, it provides a quick and easy reference for geologists in the field. Minerals with a hardness of less than 2.5 may be scratched by a fingernail, whereas those with a hardness of less than 3.5 may be scratched by a copper coin, and so on.

Table 2 Mohs' hardness scale

Mohs' hardness Reference mineral Non-mineral example (hardness in brackets)
1 talc
2 gypsum
 fingernail (2.5)
3 calcite
 copper coin1 (3.5)
4 fluorite
5 apatite
 window glass/ordinary knife blade (5.5)
6 orthoclase feldspar2
 hardened steel (6.5)
7 quartz
8 topaz
9 corundum
10 diamond
1 Note that many of today's 'copper coins' are copper-plated steel and are harder below the copper coating.
2 Other types of feldspar may have a slightly greater hardness, between 6 and 6.5.
  • Will quartz scratch topaz (Figure 11a)?

  • The hardness of quartz is 7, whereas topaz has a hardness of 8, so topaz will scratch quartz but not the other way round.

Hardness should not be confused with toughness, which is the resistance of a material to breaking. Many minerals are hard, but they may not be tough. Diamond, for example, is the hardest known material, but it is not tough: it will shatter if dropped onto a hard surface.

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