mohs scale

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.

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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.{{cite web |title=Mohs scale of mineral hardness |publisher=American Federation of Mineralogical Societies |url=https://www.amfed.org/mohs-scale |via=amfed.org |access-date= }}

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.{{cite report |last=Geels |first=Kay |publication-date=26 April 2000 |section=The true microstructure of materials |pages=5–13 |title=Materialographic Preparation from Sorby to the Present |department=Application notes and guides |series=The Struers metallographic library |publisher=Struers A/S |place=Copenhagen, DK |url=http://www.struers.com/resources/elements/12/2474/35art2.pdf |url-status=dead |archive-url=https://web.archive.org/web/20160307194802/http://www.struers.com/resources/elements/12/2474/35art2.pdf |archive-date=7 March 2016 }}

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.{{fact|date=October 2024}} The table below shows the comparison with the absolute hardness measured by a sclerometer, with images of the reference minerals in the rightmost column.{{cite web |title=What is important about hardness? |department=Amethyst galleries |series=Mineral gallery |url=http://www.galleries.com/minerals/hardness.htm |url-status=dead |access-date= |via=galleries.com |archive-url=https://web.archive.org/web/20061230174242/http://www.galleries.com/minerals/hardness.htm |archive-date=30 December 2006 }}{{cite web |title=Mineral hardness and hardness scales |publisher=Inland Lapidary |url=http://www.inlandlapidary.com/user_area/hardness.asp |via=inlandlapidary.com |url-status=live |access-date= |archive-url=https://web.archive.org/web/20081017152845/http://www.inlandlapidary.com/user_area/hardness.asp |archive-date=2008-10-17 }}

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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 that are not minerals have been assigned a hardness on the Mohs scale. Hardness may be difficult to determine, or may be misleading or meaningless, if a material is a mixture of two or more substances; 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).

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Despite its lack of precision, the Mohs scale is relevant for field geologists, who use it 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 the assessment of which type of mill and grinding medium will best reduce a given product whose hardness is known.{{cite web |title=Size reduction, comminution |series=Grinding and milling |publisher=PowderProcess.net |url=http://www.powderprocess.net/Grinding_Milling.html |access-date=27 October 2017}}

Electronic manufacturers use the scale 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.{{cite magazine |last=Purdy |first=Kevin |date=16 May 2014 |title=Hardness is not toughness: Why your phone's screen may not scratch, but will shatter |magazine=Computerworld |publisher=IDG Communications Inc. |url=https://www.computerworld.com/article/2833434/hardness-is-not-toughness--why-your-phone-s-screen-may-not-scratch--but-will-shatter.html |access-date=16 April 2021}}

Comparison between Mohs hardness and Vickers hardness:{{cite web |last=Ralph |first=Jolyon |title=Welcome to mindat.org |website=mindat.org |publisher=Hudson Institute of Mineralogy |url=https://www.mindat.org/ |access-date=April 16, 2017}}

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• Brinell scale
• Geological Strength Index
• Hardnesses of the elements (data page)
• Knoop hardness test
• Meyer hardness test
• Pencil hardness
• Rockwell scale
• Rosiwal scale
• Scratch hardness
• Superhard material

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• {{cite magazine |last=Cordua |first=William S. |year=c. 1990 |title=The hardness of minerals and rocks |magazine=Lapidary Digest |url=http://www.gemcutters.org/LDA/hardness.htm |via=gemcutters.org }}
• {{Cite book |last=Raden |first=Aja |year=2016 |title=Gem: The definitive visual guide |publisher=DK Publishing |isbn=9781465453563 |location=New York, NY |lang=en }}

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{{Mineral identification}}

Category:Materials science

Category:Mineralogy

Category:Hardness tests

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