Mohs Scale of Hardness
Created | Updated Oct 8, 2004
Hardness is a measure of the ability of a material to resist wear and tear, i.e. abrasion. The hardness of a mineral largely determines its durability and is related also to the strength and toughness of solid substances. It is important not to confuse hardness of a material with strength. Many hard materials (e.g. diamond, see below) are brittle and will break easily if they are struck. However, in common usage the term 'hardness' is often extended to include strength and toughness.
Strength of Materials
Full discussion of strength of materials is beyond the scope of this entry. Suffice it to say that there are different types of strength:| Strength | Definition |
| tensile | when it is stretched |
| compressive | when it is squashed |
| torsional | when it is twisted |
| shear | when it is cut |
| bending | when it is bent |
Hence, in simple terms, the strength of a material is a measure of its resistance to various forms of deformation.
Hardness
The relative hardness of minerals is determined according to Mohs' Scale, named after the German mineralogist, Friedrich Mohs who devised it in 1812. In the original Mohs Scale, 10 minerals were arranged in order of increasing hardness and were assigned the numbers 1 to 10. These 10 minerals are shown in Column 1 below:
| Mohs Substance | Mohs Number | Common Substance | Simple Classification |
| Talc | 1 | ||
| Gypsum | 2 | ||
| 2.5 | Finger Nail | SOFT | |
| Calcite | 3 | ||
| Fluorite | 4 | 'Copper coin' 1 | |
| Apatite | 5 | ||
| 5.5 | Knife blade or glass | MEDIUM | |
| Orthoclase (feldspar) | 6 | ||
| 6.5 | Steel file | HARD | |
| Quartz | 7 | ||
| Topaz | 8 | ||
| Corundum (emery) | 9 | ||
| Diamond | 10 | ||
A substance with higher Mohs number is capable of scratching a substance with a lower number.
The order of these minerals can be remembered using the mnemonic, The Girls Can Flirt And Other Queer Things Can Do .
Mohs selected these ten minerals either because they were common or readily available. The scale is not a linear one, but is somewhat arbitrary. This means that fluorite, for example, at 4 is not twice as hard as gypsum at 2; nor is the difference between calcite and fluorite similar to the difference between corundum and diamond.
Hardness is used in a rough way to inform mineral identification in the field. When one encounters real minerals out in the field, they can look remarkably alike. This may be due to weathering, or variations in their chemical structure from the ideal, or clathrate inclusions that simply change the colour of the mineral so it doesn't look at all as expected. Sometimes faulting and metamorphism can induce facets and planes in a mineral that aren't at all natural to it so it really looks like something else.
The Extended Mohs Scale
| Mohs Substance | Hardness |
| Liquid | 1 |
| Substance as indicated in the standard scale | 2-6 |
| Vitreous pure silica | 7 |
| Quartz | 8 |
| Topaz | 9 |
| Garnet | 10 |
| Fused zirconia | 11 |
| Fused alumina | 12 |
| Silicon carbide | 13 |
| Boron carbide | 14 |
| Diamond | 15 |
Industrial Measurement of Hardness
In metallurgy and engineering there are much more accurate and precise ways of measuring hardness. One way involves measuring the size of the dent made in the material under a particular pressure, applied from a pyramid-shaped diamond. For all but the hardest materials, a steel ball may be used instead of the diamond. The smaller the dent is, the harder the test material. Hard metals are indented less than soft metals. This test to determine the hardness of metal surfaces is known as the Brinell test, named after the Swedish engineer Johann Brinell. The instrument used to conduct this test is called a sclerometer, and this enables us to create an 'absolute hardness scale'.
An absolute hardness scale looks a little different to the relative scale. It turns out that adjacent minerals are relatively close in hardness; but as hardness increases, the difference in hardness increases greatly, as is seen in the scale below:
An absolute hardness scale
| Substance | Relative Absolute Hardness |
| Talc | 1 |
| Gypsum | 3 |
| Calcite | 9 |
| Fluorite | 21 |
| Apatite | 48 |
| Orthoclase | 72 |
| Quartz | 100 |
| Topaz | 200 |
| Corundum | 400 |
| Diamond | 1600 |
This scale shows, for example, that diamond is four times harder than the penultimate mineral, corundum which itself is twice as hard as topaz.
Other Hardness Tests
Besides the Brinell Test, which employs a hardened steel or carbide ball indenter, there are two other hardness tests in common use, these being the Rockwell Test (involving a diamond cone indenter or a hardened steel ball) and the Vickers Test (diamond indenter in the form of a right pyramid). Discussion of these is beyond the scope of this article, but may be found on the following link (Tests of Hardness).
Materials of Extreme Hardness
There is much interest in the development of super-hard materials - substances which are harder than diamond - for which there are many applications. Super-hard materials could be used to cut steel, which diamond cannot do because it burns when it gets hot. Although it is possible to coat metals with a thin layer of diamond, it seems to be somewhat easier with synthetic materials such as beta-carbon nitride. Mechanical components such as gears and bearings coated with beta-carbon nitride would last much longer than normal parts, and could be used in devices where liquid lubricants are unsuitable. A thin layer of the beta-carbon nitride could also be used to protect the surface of computer discs.Another celebrated material is Borazon, a synthetic compound of boron and nitrogen, was discovered by Dr R Wentorf in 1956. Depending on the source of information this is either harder than diamond or is equal hardness to diamond because each will scratch the other.
