Mohs scale of mineral hardness

The Mohs scale of mineral hardness (/moʊz/) 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.[1] 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, c. 300 BC, followed by Pliny the Elder in his Naturalis Historia, c. AD 77.[2][3][4] 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.[5]

Mohs hardness kit, containing one specimen of each mineral on the ten-point hardness scale

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.[6] The scale is used at electronic manufacturers for testing the resilience of flat panel display components (such as cover glass for LCDs or encapsulation for OLEDs).

As an example, most modern smartphone displays use Gorilla Glass that scratches at level 6 with deeper grooves at level 7 (on the Mohs scale of hardness). [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.[8] "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.[9]

The Mohs scale is a purely ordinal scale. For example, corundum (9) is twice as hard as topaz (8), but diamond (10) is four times as hard as corundum. The table below shows the comparison with the absolute hardness measured by a sclerometer, with pictorial examples.[10][11]

Mohs hardness	Mineral	Chemical formula	Absolute hardness[12]	Image
1	Talc	Mg3Si4O10(OH)2	1
2	Gypsum	CaSO4·2H2O	2
3	Calcite	CaCO3	14
4	Fluorite	CaF2	21
5	Apatite	Ca5(PO4)3(OH^−,Cl^−,F^−)	48
6	Orthoclase feldspar	KAlSi3O8	72
7	Quartz	SiO2	100
8	Topaz	Al2SiO4(OH^−,F^−)2	200
9	Corundum	Al2O3	400
10	Diamond	C	1500

On the Mohs scale, a streak plate (unglazed porcelain) 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.[1]

Intermediate hardness

The table below incorporates additional substances that may fall between levels:[13]

Hardness	Substance or mineral
0.2–0.3	caesium, rubidium
0.5–0.6	lithium, sodium, potassium
1	talc
1.5	gallium, strontium, indium, tin, barium, thallium, lead, graphite, ice[14]
2	hexagonal boron nitride,[15] calcium, selenium, cadmium, sulfur, tellurium, bismuth, gypsum
2–2.5	halite (rock salt), fingernail[16]
2.5–3	gold, silver, aluminium, zinc, lanthanum, cerium, jet
3	calcite, copper, arsenic, antimony, thorium, dentin
3.5	platinum
4	fluorite, iron, nickel
4–4.5	ordinary steel
5	apatite (tooth enamel), zirconium, palladium, obsidian (volcanic glass)
5.5	beryllium, molybdenum, hafnium, glass, cobalt
6	orthoclase, titanium, manganese, germanium, niobium, uranium
6–7	fused quartz, iron pyrite, silicon, ruthenium, iridium, tantalum, opal, peridot, tanzanite, rhodium, jade
7	osmium, quartz, rhenium, vanadium
7.5–8	emerald, beryl, zircon, tungsten, spinel
8	topaz, cubic zirconia, enhanced hardened steel such as REX 121 steel
8.5	chrysoberyl, chromium, silicon nitride, tantalum carbide
9	corundum (includes sapphire and ruby), tungsten carbide, titanium nitride
9–9.5	silicon carbide (carborundum), tungsten carbide, tantalum carbide, zirconium carbide, alumina, beryllium carbide, titanium carbide, aluminum boride, boron carbide.[note 1][17][18]
9.5–near 10	boron, boron nitride, rhenium diboride (a-axis),[19] stishovite, titanium diboride, moissanite (crystal form of silicon carbide)
10	diamond, carbonado

Comparison with Vickers scale

This list is incomplete; you can help by expanding it.

Comparison between Mohs hardness and Vickers hardness:[20]

Mineral name	Hardness (Mohs)	Hardness (Vickers) (kg/mm^2)
Graphite	1–2	VHN10 = 7–11
Tin	1.5	VHN10 = 7–9
Bismuth	2–2.5	VHN100 = 16–18
Gold	2.5	VHN10 = 30–34
Silver	2.5	VHN100 = 61–65
Chalcocite	2.5–3	VHN100 = 84–87
Copper	2.5–3	VHN100 = 77–99
Galena	2.5	VHN100 = 79–104
Sphalerite	3.5–4	VHN100 = 208–224
Heazlewoodite	4	VHN100 = 230–254
Carrollite	4.5–5.5	VHN100 = 507–586
Goethite	5–5.5	VHN100 = 667
Hematite	5–6	VHN100 = 1,000–1,100
Chromite	5.5	VHN100 = 1,278–1,456
Anatase	5.5–6	VHN100 = 616–698
Rutile	6–6.5	VHN100 = 894–974
Pyrite	6–6.5	VHN100 = 1,505–1,520
Bowieite	7	VHN100 = 858–1,288
Euclase	7.5	VHN100 = 1,310
Chromium	8.5	VHN100 = 1,875–2,000

See also

• Brinell scale
• Hardnesses of the elements (data page)
• Knoop hardness test
• Meyer hardness test
• Pencil hardness
• Rockwell scale
• Scratch hardness

Notes

1. The hardness level of the carbides of the following elements falls between 9 and 10:[17][18] W, Ta, Zr, Be, Ti, Si, B.

References

1. "Mohs hardness" in Encyclopædia Britannica Online
2. Theophrastus on Stones. Farlang.com. Retrieved on 2011-12-10.
3. Pliny the Elder. Naturalis Historia. Book 37. Chap. 15. ADamas: six varieties of it. Two remedies.
4. Pliny the Elder. Naturalis Historia. Book 37. Chap. 76. The methods of testing precious stones.
5. Hardness Archived 2014-02-14 at the Wayback Machine. Non-Destructive Testing Resource Center
6. "Size reduction, comminution - grinding and milling". PowderProcess.net. Retrieved 27 October 2017.
7. https://www.youtube.com/watch?v=7jD5Gkh4K34&feature=youtu.be&t=101
8. American Federation of Mineralogical Societies. "Mohs Scale of Mineral Hardness". amfed.org
9. Geels, Kay. "The True Microstructure of Materials", pp. 5–13 in Materialographic Preparation from Sorby to the Present. Struers A/S, Copenhagen, Denmark - archived Mar 7 2016
10. Amethyst Galleries' Mineral Gallery What is important about hardness?. galleries.com
11. Mineral Hardness and Hardness Scales Archived 2008-10-17 at the Wayback Machine. Inland Lapidary
12. Mukherjee, Swapna (2012). Applied Mineralogy: Applications in Industry and Environment. Springer Science & Business Media. p. 373. ISBN 978-94-007-1162-4.
13. Samsonov, G.V., ed. (1968). "Mechanical Properties of the Elements". Handbook of the Physicochemical Properties of the Elements. New York: IFI-Plenum. p. 432. doi:10.1007/978-1-4684-6066-7. ISBN 978-1-4684-6068-1.
14. "Ice is a mineral" Archived 2015-10-30 at the Wayback Machine in Exploring Ice in the Solar System. messenger-education.org
15. Berger, Lev I. (1996). Semiconductor Materials (First ed.). Boca Raton, FL: CRC Press. p. 126. ISBN 978-0849389122.
16. "Mohs Hardness Scale: Testing the Resistance to Being Scratched". geology.com.
17. "Material Hardness Tables, Ted Pella, Inc". www.tedpella.com. Retrieved 2019-05-09.
18. "Hardness table" (PDF). Retrieved 2019-05-09.
19. Levine, Jonathan B.; Tolbert, Sarah H.; Kaner, Richard B. (2009). "Advancements in the Search for Superhard Ultra-Incompressible Metal Borides" (PDF). Advanced Functional Materials. pp. 3526–3527. doi:10.1002/adfm.200901257. Archived from the original (PDF) on 2016-03-04. Retrieved 2015-12-08.
20. Ralph, Jolyon. "Welcome to mindat.org". mindat.org. Hudson Institute of Mineralogy. Retrieved April 16, 2017.

Further reading

• Cordua, William S. "The Hardness of Minerals and Rocks". Lapidary Digest, c. 1990.