Mohs scale of mineral hardness


The Mohs scale of mineral hardness is a qualitative ordinal scale characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. Created in 1812 by German geologist and mineralogist Friedrich Mohs, it is one of several definitions of hardness in materials science, some of which are more quantitative. The method of comparing hardness by observing which minerals can scratch others is of great antiquity, having been mentioned by Theophrastus in his treatise On Stones,, followed by Pliny the Elder in his Naturalis Historia,. While greatly facilitating the identification of minerals in the field, the Mohs scale does not show how well hard materials perform in an industrial setting.

Usage

Despite its lack of precision, the Mohs scale is relevant for field geologists, who use the scale to roughly identify minerals using scratch kits. The Mohs scale hardness of minerals can be commonly found in reference sheets.
Mohs hardness is useful in milling. It allows assessment of which kind of mill will best reduce a given product whose hardness is known. The scale is used at electronic manufacturers for testing the resilience of flat panel display components.
As an example, most modern smartphone displays use Gorilla Glass that scratches at level 6 with deeper grooves at level 7.

Minerals

The Mohs scale of mineral hardness is based on the ability of one natural sample of mineral to scratch another mineral visibly. The samples of matter used by Mohs are all different minerals. Minerals are chemically pure solids found in nature. Rocks are made up of one or more minerals. As the hardest known naturally occurring substance when the scale was designed, diamonds are at the top of the scale. The hardness of a material is measured against the scale by finding the hardest material that the given material can scratch, or the softest material that can scratch the given material. For example, if some material is scratched by apatite but not by fluorite, its hardness on the Mohs scale would fall between 4 and 5. "Scratching" a material for the purposes of the Mohs scale means creating non-elastic dislocations visible to the naked eye. Frequently, materials that are lower on the Mohs scale can create microscopic, non-elastic dislocations on materials that have a higher Mohs number. While these microscopic dislocations are permanent and sometimes detrimental to the harder material's structural integrity, they are not considered "scratches" for the determination of a Mohs scale number.
The Mohs scale is a purely ordinal scale. For example, corundum is twice as hard as topaz, but diamond is four times as hard as corundum. The table below shows the comparison with the absolute hardness measured by a sclerometer, with pictorial examples.
Mohs hardnessMineralChemical formulaAbsolute hardnessImage
1TalcMg3Si4O1021
2GypsumCaSO4·2H2O2
3CalciteCaCO314
4FluoriteCaF221
5ApatiteCa5348
6Orthoclase feldsparKAlSi3O872
7QuartzSiO2100
8TopazAl2SiO42200
9CorundumAl2O3400
10DiamondC1500

On the Mohs scale, a streak plate has a hardness of approximately 7.0. Using these ordinary materials of known hardness can be a simple way to approximate the position of a mineral on the scale.

Intermediate hardness

The table below incorporates additional substances that may fall between levels:
HardnessSubstance or mineral
0.2–0.3caesium, rubidium
0.5–0.6lithium, sodium, potassium
1talc
1.5gallium, strontium, indium, tin, barium, thallium, lead, graphite, ice
2hexagonal boron nitride, calcium, selenium, cadmium, sulfur, tellurium, bismuth, gypsum
2–2.5halite, fingernail
2.5–3gold, silver, aluminium, zinc, lanthanum, cerium, jet
3calcite, copper, arsenic, antimony, thorium, dentin
3.5platinum
4fluorite, iron, nickel
4–4.5ordinary steel
5apatite, zirconium, palladium, obsidian
5.5beryllium, molybdenum, hafnium, glass, cobalt
6orthoclase, titanium, manganese, germanium, niobium, uranium
6–7fused quartz, iron pyrite, silicon, ruthenium, iridium, tantalum, opal, peridot, tanzanite, rhodium, jade
7osmium, quartz, rhenium, vanadium
7.5–8emerald, beryl, zircon, tungsten, spinel
8topaz, cubic zirconia, enhanced hardened steel such as REX 121 steel
8.5chrysoberyl, chromium, silicon nitride, tantalum carbide
9corundum, tungsten carbide, titanium nitride
9–9.5silicon carbide, tungsten carbide, tantalum carbide, zirconium carbide, alumina, beryllium carbide, titanium carbide, aluminum boride, boron carbide.
9.5–near 10boron, boron nitride, rhenium diboride, stishovite, titanium diboride, moissanite
10diamond, carbonado

Comparison with Vickers scale

Comparison between Mohs hardness and Vickers hardness:
Mineral
name		Hardness Hardness
Graphite1–2VHN10 = 7–11
Tin1.5VHN10 = 7–9
Bismuth2–2.5VHN100 = 16–18
Gold2.5VHN10 = 30–34
Silver2.5VHN100 = 61–65
Chalcocite2.5–3VHN100 = 84–87
Copper2.5–3VHN100 = 77–99
Galena2.5VHN100 = 79–104
Sphalerite3.5–4VHN100 = 208–224
Heazlewoodite4VHN100 = 230–254
Carrollite4.5–5.5VHN100 = 507–586
Goethite5–5.5VHN100 = 667
Hematite5–6VHN100 = 1,000–1,100
Chromite5.5VHN100 = 1,278–1,456
Anatase5.5–6VHN100 = 616–698
Rutile6–6.5VHN100 = 894–974
Pyrite6–6.5VHN100 = 1,505–1,520
Bowieite7VHN100 = 858–1,288
Euclase7.5VHN100 = 1,310
Chromium8.5VHN100 = 1,875–2,000