Mohs scale explained
The Mohs scale of mineral hardness is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of minerals through the ability of harder material to scratch softer material.
The scale was introduced in 1812 by the German geologist and mineralogist Friedrich Mohs, in his book "Versuch einer Elementar-Methode zur naturhistorischen Bestimmung und Erkennung der Fossilien";[1] [2] it is one of several definitions of hardness in materials science, some of which are more quantitative.[3]
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, .[4] [5] [6] The Mohs scale is useful for identification of minerals in the field, but is not an accurate predictor of how well materials endure in an industrial setting.[7]
Reference minerals
The Mohs scale of mineral hardness is based on the ability of one natural sample of mineral to visibly scratch another mineral. Minerals are chemically pure solids found in nature. Rocks are mixtures of one or more minerals.
Diamond was the hardest known naturally occurring mineral when the scale was designed, and defines the top of the scale, arbitrarily set at 10. 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 be between 4 and 5.
[8] Technically, "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]
Each of the ten hardness values in the Mohs scale is represented by a reference mineral, most of which are widespread in rocks.
The Mohs scale is an 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 images of the reference minerals in the rightmost column.[10] [11]
Examples
Below is a table of more materials by Mohs scale. Some of them have a hardness between two of the Mohs scale reference minerals. Some solid substances which are not minerals have been assigned a hardness on the Mohs scale. However, if the substance is actually a mixture of other substances, hardness can be difficult to determine or may be misleading or meaningless. For example, some sources have assigned a Mohs hardness of 6 or 7 to granite but it is a rock made of several minerals, each with its own Mohs hardness (e.g. topaz-rich granite contains: topaz — Mohs 8, quartz — Mohs 7, orthoclase — Mohs 6, plagioclase — Mohs 6–6.5, mica — Mohs 2–4).
| Hardness | Substance[13] |
|---|
| 0.2–0.4 | Caesium, potassium, rubidium, butter |
| 0.5–0.6 | Lithium, sodium, graphite, candle wax |
| 1 | Talc |
| 1.5 | Tin, lead, ice, todorokite, wakabayashilite, idrialite, dimorphite |
| 2 | Gypsum, calcium, hardwood, dry ice (solid form of carbon dioxide), glauconite |
| 2–2.5 | Bismuth, plastic |
| 2.5 | Gold, silver, magnesium, zinc, pearl, amber, ivory, finger nail,[14] galena, linarite, ulexite, kinoite, cylindrite |
| 2.5–3 | Copper, aluminium, chalcocite, jet |
| 3 | Calcite, thorium, dentin, chalk,[15] brass, bronze |
| 3.5 | Platinum, adamite, strontianite, roselite, ludlamite |
| 3.5-4 | Sphalerite |
| 4 | Fluorite, iron, nickel, heazlewoodite |
| 4–4.5 | Ordinary steel |
| 4.5 | Conichalcite, duftite, colemanite, lindgrenite |
| 5 | Apatite, tooth enamel, zirconium, obsidian (volcanic glass) |
| 5-5.5 | Goethite |
| 5.5 | Cobalt, beryllium, glass, perovskite, chromite, bavenite, agrellite |
| 5.5–6 | Opal, turquoise, anatase, arsenopyrite |
| 6 | Orthoclase feldspar, titanium, uranium, rhodium |
| 6-6.5 | Rutile, pyrite |
| 6.5 | Silicon, iridium, baddeleyite, chloritoid, berlinite, cuprospinel |
| 6.5–7 | Peridot, jadeite |
| 7 | Quartz (including amethyst and citrine), porcelain, bowieite |
| 7-7.5 | Garnet, Tourmaline |
| 7.5 | Tungsten, zircon, euclase, hambergite, grandidierite |
| 7.5–8 | Beryl (including emerald and aquamarine) |
| 8 | Topaz, cubic zirconia, spinel, hardened steel[16] |
| 8.5 | Chromium, silicon nitride, tantalum carbide, chrysoberyl, tongbaite |
| 9 | Corundum (including ruby and sapphire), tungsten carbide, titanium nitride |
| 9–9.5 | Moissanite, silicon carbide (carborundum), tantalum carbide, zirconium carbide, beryllium carbide, titanium carbide, aluminium boride, boron carbide.[17] [18] |
| 9.5–near 10 | Boron, boron nitride, rhenium diboride (a-axis),[19] titanium diboride, boron carbide |
| 10 | Diamond | |
Use
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 type of mill and grinding medium will best reduce a given product whose hardness is known.[20]
The scale is used by electronic manufacturers for testing the resilience of flat panel display components (such as cover glass for LCDs or encapsulation for OLEDs), as well as to evaluate the hardness of touch screens in consumer electronics.[21]
Comparison with Vickers scale
Comparison between Mohs hardness and Vickers hardness:[22]
See also
Further reading
- Cordua . William S. . c. 1990 . The hardness of minerals and rocks . . gemcutters.org .
- Book: Raden, Aja . 2016 . Gem: The definitive visual guide . . 9781465453563 . New York, NY . en .
Notes and References
- Book: von Groth, Paul Heinrich . Entwicklungsgeschichte der Mineralogischen Wissenschaften . History of the development of the mineralogical sciences . Springer . 1926 . Berlin . 250 . 9783662409107 . de .
- Encyclopedia: Mohs hardness . . online .
- Web site: Mohs scale of hardness . . 10 February 2021.
- Book: Theophrastus on Stones . . Farlang.com . 2011-12-10.
- Book: . Naturalis Historia . Book 37, Chap. 15 . https://www.perseus.tufts.edu/cgi-bin/ptext?lookup=Plin.+Nat.+37.15 . Adamas: Six varieties of it. Two remedies..
- Book: . . Book 37, Chap. 76 . https://www.perseus.tufts.edu/cgi-bin/ptext?lookup=Plin.+Nat.+37.76 . The methods of testing precious stones..
- Web site: Hardness . https://web.archive.org/web/20140214185403/http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/Hardness.htm . 2014-02-14 . Non-Destructive Testing Resource Center . Materials Mechanical Hardness.
- Web site: Mohs scale of mineral hardness . . amfed.org .
- Geels . Kay . 26 April 2000 . The true microstructure of materials . 5–13 . Materialographic Preparation from Sorby to the Present . Application notes and guides . The Struers metallographic library . Struers A/S . Copenhagen, DK . dead . https://web.archive.org/web/20160307194802/http://www.struers.com/resources/elements/12/2474/35art2.pdf . 7 March 2016 .
- Web site: What is important about hardness? . Amethyst galleries . Mineral gallery . dead . galleries.com . https://web.archive.org/web/20061230174242/http://www.galleries.com/minerals/hardness.htm . 30 December 2006 .
- Web site: Mineral hardness and hardness scales . Inland Lapidary . inlandlapidary.com . live . https://web.archive.org/web/20081017152845/http://www.inlandlapidary.com/user_area/hardness.asp . 2008-10-17 .
- Book: Mukherjee, Swapna . 2012 . Applied Mineralogy: Applications in industry and environment . Springer Science & Business Media . 978-94-007-1162-4 . 373 . Google books.
- Book: Samsonov . G.V. . 1968 . Mechanical properties of the elements . Handbook of the Physicochemical Properties of the Elements . IFI-Plenum . New York, NY . 10.1007/978-1-4684-6066-7 . 978-1-4684-6068-1 . 432 .
- Web site: Mohs Hardness Scale . . Fundamental Geologic Principles . 18 November 2022.
- Web site: Reade Advanced Materials – Mohs' Hardness (Typical) of Abrasives. 2021-08-09. www.reade.com.
- Web site: Mohs Hardness Scale: Testing the Resistance to Being Scratched. 2021-08-09. geology.com.
- Web site: Material hardness tables . www.tedpella.com . 2019-05-09 . 2015-09-12 . https://web.archive.org/web/20150912034606/https://www.tedpella.com/company_html/hardness.htm . dead .
- Web site: Hardness table . 2019-05-09.
- Levine . Jonathan B. . Tolbert . Sarah H. . Kaner . Richard B. . 2009 . Advancements in the search for superhard ultra-incompressible metal borides . Advanced Functional Materials . 19 . 22 . 3526–3527 . 10.1002/adfm.200901257 . 98675890 . 2015-12-08 . dead . https://web.archive.org/web/20160304102359/http://tolbert.chem.ucla.edu/Publication/AdvFuncMater-19-p3519.pdf . 2016-03-04.
- Web site: Size reduction, comminution . Grinding and milling . PowderProcess.net . 27 October 2017.
- Purdy . Kevin . 16 May 2014 . Hardness is not toughness: Why your phone's screen may not scratch, but will shatter . Computerworld . IDG Communications Inc. . 16 April 2021.
- Web site: Ralph . Jolyon . Welcome to mindat.org . mindat.org . Hudson Institute of Mineralogy . April 16, 2017.
