Mohs' scale of mineral hardness

Mohs' scale of mineral hardness The hardness of a mineral is mainly controlled by the strength of the bonding between the constituent atoms and partly by the composition and size of the atoms. It is a measure of the resistance of the mineral to abrasion and scratching.

The element carbon exists in two distinct forms—graphite and diamond—which have identical compositions but different structures, according to their conditions of crystallization, yet graphite is one of the softest minerals known whereas diamond is by far the hardest and, indeed, is the hardest substance yet discovered.

In 1822 the Austrian mineralogist Friedrich Mohs proposed a scale of relative hardness based on fairly common minerals (Mohs' scale). The 10-point scale is: 1, talc; 2, gypsum; 3, calcite; 4, fluorite; 5, apatite; 6, orthoclase; 7, quartz; 8, topaz; 9, corundum; 10, diamond.

Each mineral will scratch the ones below it (i.e. with a lower number) on the scale and will be scratched by those above. The intervals are not equal; 9–10 is much greater than 1–9. (On the Rosiwall absolute hardness scale diamond is 140 000; corundum, 1000; topaz, 175; and gypsum, 1.25.) On Mohs' scale a finger nail is 2.5; a copper coin 3.5; a knife blade 5.5; and a metal file 6.5–7.

Hardness is a fairly useful diagnostic property for use in field identification of minerals.

R. Bradshaw