What is hardness in mineralogy ?

Hardness : definition

Hardness is the resistance that a mineral offers to scratching. It is a complex anisotropic property which depends on the cohesion of atoms and therefore on the atomic structure of each mineral. Because it is anisotropic, the hardness can vary slightly depending on the faces or directions of a crystal. However, for simple current tests, it is assumed that these fluctuations are negligible (which is true for the majority of minerals), and that the hardness remains constant on the different faces or breaks of the same mineral. It is therefore a characteristic property of a mineral.

It is said that a mineral is harder than another if it scratches it without being scratched by it itself. On this basis, an Austrian mineralogist, F. Mohs, established in 1824 a scale of relative hardness from 10 common standard minerals : talc (hardness 1), gypsum (hardness 2), calcite (hardness 3), fluorite (hardness 4), apatite (hardness 5), orthoclase (hardness 6), quartz (hardness 7), topaz (hardness 8), corundum (hardness 9) and diamond (hardness 10). The hardness of minerals is therefore assessed in a relative manner from this scale, called the Mohs scale, universally used because it is simple and very practical.

Measuring the absolute hardness of minerals is more complex to perform. It is known as Vickers hardness and assessed by a weighted microdurimeter. The Vickers hardness (noted VHN for Vickers' Hardness Number) is necessarily given with this weight. The diagram presented here shows that the Mohs hardness scale is not linear in Vickers hardness beyond corundum, diamond is indeed much harder.

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