Minerals and the crystalline state
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)
fingernail (2.5)
copper coin1 (3.5)
window glass/ordinary knife blade (5.5)
6orthoclase feldspar2
hardened steel (6.5)
1 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.

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.

Download this video clip.Video player: Video 4
Video 4 Scratch test on gypsum. (There is no voice-over in this video.)
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Download this video clip.Video player: Video 5
Video 5 Scratch test on calcite. (There is no voice-over in this video.)
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  • Will quartz scratch topaz (Figure 12a)?

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

  • Using evidence from the Digital Geology Kit [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] , which of the following minerals is the hardest, and which is the softest: galena, hematite, pyrite?

  • Pyrite is the hardest (6-6.5), galena the softest (2.5). Hematite has hardness 5-6.

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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