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) |
|---|---|---|
| 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 |
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.
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.
Will quartz scratch topaz (Figure 12a)?
Answer
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, which of the following minerals is the hardest, and which is the softest: galena, hematite, pyrite?
Answer
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.
OpenLearn - Minerals and the crystalline state
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