Editing Mohs scale

{{short description|Qualitative scale characterizing scratch resistance}}
{{use dmy dates|date=August 2024}}
[[File:Mohssche-haerteskala hg.jpg|thumb|alt=Open wooden box with ten compartments, each containing a numbered mineral specimen.|Mohs hardness kit, containing one specimen of each mineral on the ten-point hardness scale]]

The '''Mohs scale''' ({{IPAc-en|m|ou|z}} {{respell|MOHZ}}) of mineral hardness is a [[Qualitative property|qualitative]] [[ordinal scale]], from 1 to 10, characterizing [[scratch hardness|scratch resistance]] of [[mineral]]s through the ability of harder material to scratch softer material.

The scale was introduced in 1812 by the German [[geologist]] and [[Mineralogy|mineralogist]] [[Friedrich Mohs]], in his book {{lang|de|Versuch einer Elementar-Methode zur naturhistorischen Bestimmung und Erkennung der Fossilien}} (English: Attempt at an elementary method for the natural-historical determination and recognition of fossils);<ref name=VonGroth1926>{{cite book | title=Entwicklungsgeschichte der Mineralogischen Wissenschaften | trans-title=History of the development of the mineralogical sciences | url=https://books.google.com/books?id=tD0LAQAAIAAJ&q=+Härteskala+1812 | publisher=Springer | last=von Groth | first=Paul Heinrich | date=1926 | publication-place=Berlin | page=250 | isbn=9783662409107 | lang=de }}</ref><ref name=Brit>{{cite encyclopedia |url=https://www.britannica.com/EBchecked/topic/387714/Mohs-hardness |title=Mohs hardness |encyclopedia=[[Encyclopædia Britannica]] |edition=online }}</ref>{{efn|
: {{lang|de|In demselben Jahre (1812) wurde MOHS als Professor am Joanneum angestellt und veröffentliche den ersten Teil seines Werkes {{noitalic|Versuch einer Elementarmethode zur naturhistorischen Bestimmung und Erkennung der Fossilien}}, in welcher die bekannte Härteskala aufgestellt wurde.}} — {{harvp|von Groth|1926}}
: &nbsp;
: In the same year (1812) MOHS was employed as a professor at the Joanneum and published the first part of his work ''Attempt at an elementary method for the natural-historical determination and recognition of fossils'', in which the well-known hardness scale was set up.<ref name=VonGroth1926/>
}} it is one of several definitions of [[hardness comparison|hardness]] in [[materials science]], some of which are more quantitative.<ref name="MinSocAm">{{cite web |title=Mohs scale of hardness |publisher=[[Mineralogical Society of America]] |url=http://www.minsocam.org/MSA/collectors_corner/article/mohs.htm |access-date=10 February 2021}}</ref>

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'', {{circa| 300 BC}}, followed by [[Pliny the Elder]] in his ''[[Natural History (Pliny)|Naturalis Historia]]'', {{circa| AD 77}}.<ref>{{cite book |url=http://www.farlang.com/gemstones/theophrastus-on-stones/page_148/view?searchterm=scratch |title=Theophrastus on Stones |author=[[Theophrastus]] |via=Farlang.com |access-date=2011-12-10}}</ref><ref>{{cite book |author=[[Pliny the Elder]] |title=Naturalis Historia |chapter=Book&nbsp;37, Chap.&nbsp;15 |chapter-url=https://www.perseus.tufts.edu/cgi-bin/ptext?lookup=Plin.+Nat.+37.15 |quote=''Adamas'': Six varieties of it. Two remedies.}}</ref><ref>{{cite book |author=[[Pliny the Elder]] |title=[[Naturalis Historia]] |chapter=Book&nbsp;37, Chap.&nbsp;76 |chapter-url=https://www.perseus.tufts.edu/cgi-bin/ptext?lookup=Plin.+Nat.+37.76 |quote=The methods of testing precious stones.}}</ref> The Mohs scale is useful for identification of minerals in the [[Field research|field]], but is not an accurate predictor of how well materials endure in an industrial setting.<ref>{{cite web |url=http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/Hardness.htm |title=Hardness |archive-url=https://web.archive.org/web/20140214185403/http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/Hardness.htm |archive-date=2014-02-14 |publisher=Non-Destructive Testing Resource Center |series=Materials Mechanical Hardness}}</ref>

==Reference minerals==
The Mohs scale of mineral hardness is based on the ability of one natural sample of mineral to visibly scratch another mineral. [[Mineral]]s are chemically pure solids found in nature. [[Rock (geology)|Rock]]s are mixtures of one or more minerals. 
:{| style="float:right;border:none;background:white;margin-left:2em"
|- style="text-align:center;"
| [[File:Mohs scale vs absolute hardness.png|frameless|center|300px]]
|-
|style="padding-left:2em;"| Mohs scale along the horizontal axis matched with <br/>one of the [[absolute hardness]] scales along the <br/>vertical. Vertical scale is logarithmic. 
|}
[[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.<ref>{{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= }}</ref>

Technically, "scratching" a material for the purposes of the Mohs scale means creating non-[[Elasticity (physics)|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.<ref>{{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 <!-- presumed --> |archive-url=https://web.archive.org/web/20160307194802/http://www.struers.com/resources/elements/12/2474/35art2.pdf |archive-date=7 March 2016 }}</ref>

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.<ref>{{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 <!-- presumed --> |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 }}</ref><ref>{{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 <!-- presumed --> |access-date= |archive-url=https://web.archive.org/web/20081017152845/http://www.inlandlapidary.com/user_area/hardness.asp |archive-date=2008-10-17 }}</ref>

:{| class="wikitable sortable" style="text-align:center"
|-
! Mohs<br/>hardness
! Reference<br/>mineral
! Chemical&nbsp;formula
! Absolute<br/>hardness<ref>{{cite book |last=Mukherjee |first=Swapna |year=2012 |title=Applied Mineralogy: Applications in industry and environment |publisher=Springer Science & Business Media |isbn=978-94-007-1162-4 |page=373 |url=https://books.google.com/books?id=mllvP7ZmWqkC&pg=PA373 |via=Google books}}</ref>
!class="unsortable"| Example image
|-
| '''1'''
| [[Talc]]
| {{chem2| Mg3Si4O10(OH)2 }} <!-- Mg<sub>3</sub>Si<sub>4</sub>O<sub>10</sub>(OH)<sub>2</sub> -->
| 1
| [[Image:Talc block.jpg|100px]]
|-
| '''2'''
| [[Gypsum]]
| {{chem2| CaSO4*2H2O }} <!-- CaSO<sub>4</sub>·2H<sub>2</sub>O -->
| 2
| [[Image:Gypse Arignac.jpg|100px]]
|-
| '''3'''
| [[Calcite]]
| {{chem2| CaCO3 }} <!-- CaCO<sub>3</sub> -->
| 14
| [[Image:Calcite-sample2.jpg|100px]]
|-
| '''4'''
| [[Fluorite]]
| {{chem2| CaF2 }} <!-- CaF<sub>2</sub> -->
| 21
| [[Image:Fluorite with Iron Pyrite.jpg|100px]]
|-
| '''5'''
| [[Apatite]]
| {{chem2| Ca5(PO4)3(OH,Cl,F) }} <!-- Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>(OH<sup>−</sup>,Cl<sup>−</sup>,F<sup>−</sup>) -->
| 48
| [[Image:Apatite Canada.jpg|100px]]
|-
| '''6'''
| [[Orthoclase|Orthoclase<br/>feldspar]]
| {{chem2| KAlSi3O8 }} <!-- KAlSi<sub>3</sub>O<sub>8</sub> -->
| 72
| [[Image:OrthoclaseBresil.jpg|100px]]
|-
| '''7'''
| [[Quartz]]
| {{chem2| SiO2 }} <!-- SiO<sub>2</sub> -->
| 100
| [[Image:Quartz Brésil.jpg|100px]]
|-
| '''8'''
| [[Topaz]]
| {{chem2| Al2SiO4(OH,F)2 }} <!-- Al<sub>2</sub>SiO<sub>4</sub>(OH<sup>−</sup>,F<sup>−</sup>)<sub>2</sub> -->
| 200
| [[Image:Topaz-120187.jpg|100px]]
|-
| '''9'''
| [[Corundum]]
| {{chem2| Al2O3 }} <!-- Al<sub>2</sub>O<sub>3</sub> -->
| 400
| [[Image:Corundum-dtn14b.jpg|100px]]
|-
| '''10'''
| [[Diamond]]
| {{chem2| C }} <!-- C -->
| 1500
| [[Image:Rough diamond.jpg|100px]]
|}

==Examples==
{{More citations needed|section|date=November 2022}}
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&nbsp;8, quartz — Mohs&nbsp;7, orthoclase — Mohs&nbsp;6, plagioclase — Mohs&nbsp;6–6.5, mica — Mohs&nbsp;2–4).
:{| class="wikitable"
|-
! Hardness
! Substance
|-
|style="text-align:center;"| 0.2–0.4
| [[Potassium]]<ref name=Samsonov68>{{cite book | editor-last=Samsonov | editor-first=G.V. | year=1968 | chapter=Mechanical properties of the elements | title=Handbook of the Physicochemical Properties of the Elements | publisher=IFI-Plenum | place=New York, NY | doi=10.1007/978-1-4684-6066-7 | isbn=978-1-4684-6068-1 | page=432 }}</ref>
|-
|style="text-align:center;"| 0.5–0.6
| [[Lithium]]<ref name=Samsonov68 />
|-
|style="text-align:center;"| 1
| [[Talc]]
|-
|style="text-align:center;"| 1.5
| [[Lead]]<ref name=Samsonov68 />
|-
|style="text-align:center;"| 2
| [[Hardwood]]<ref name="geology.com">{{Cite web|title=Mohs Hardness Scale: Testing the Resistance to Being Scratched|url=https://geology.com/minerals/mohs-hardness-scale.shtml|access-date=2021-08-09|website=geology.com}}</ref>
|-
|style="text-align:center;"| 2–2.5
| [[Plastic]]
|-
|style="text-align:center;"| 2.5
| [[Zinc]]<ref name=Samsonov68 />
|-
|style="text-align:center;"| 2.5–3
| [[Native copper|Copper]]<ref name=Samsonov68 />
|-
|style="text-align:center;"| 3
| [[Brass]]
|-
|style="text-align:center;"| 3.5
| [[Adamite]]
|-
|style="text-align:center;"| 3.5-4
| [[Sphalerite]]
|-
|style="text-align:center;"| 4
| [[Iron]]<ref name=Samsonov68 />
|-
|style="text-align:center;"| 4–4.5
| Ordinary [[steel]]
|-
|style="text-align:center;"| 4.5
| [[Colemanite]]
|-
|style="text-align:center;"| 5
| [[Apatite]]
|-
|style="text-align:center;"| 5-5.5
| [[Goethite]]
|-
|style="text-align:center;"| 5.5
| [[Glass]]
|-
|style="text-align:center;"| 5.5–6
| [[Opal]]
|-
|style="text-align:center;"| 6
| [[Rhodium]]<ref name=Samsonov68 />
|-
|style="text-align:center;"| 6-6.5
| [[Rutile]]
|-
|style="text-align:center;"| 6.5
| [[Silicon]]<ref name=Samsonov68 />
|-
|style="text-align:center;"| 6.5–7
| [[Jadeite]]
|-
|style="text-align:center;"| 7
| [[Porcelain]]
|-
|style="text-align:center;"| 7-7.5
| [[Garnet]]
|-
|style="text-align:center;"| 7.5
| [[Tungsten]]<ref name=Samsonov68 />
|-
|style="text-align:center;"| 7.5–8
| [[Emerald]]
|-
|style="text-align:center;"| 8
| [[Topaz]]
|-
|style="text-align:center;"| 8.5
| [[Chromium]]<ref name=Samsonov68 />
|-
|style="text-align:center;"| 9
| [[Sapphire]]
|-
|style="text-align:center;"| 9–9.5
| [[Moissanite]]
|-
|style="text-align:center;"| 9.5–near&nbsp;10
| [[Boron]]<ref name=Samsonov68 />
|-
|style="text-align:center;"| 10
| [[Diamond]]
|}

==Use==
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 [[Mill (grinding)|milling]]. It allows the assessment of which type of mill and grinding medium will best reduce a given product whose hardness is known.<ref name="Milling1">{{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}}</ref>

Electronic manufacturers use the scale for testing the resilience of flat panel display components (such as cover glass for [[Liquid-crystal display|LCD]]s or encapsulation for [[OLED]]s), as well as to evaluate the hardness of touch screens in consumer electronics.<ref>{{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}}</ref>

== Comparison with Vickers scale ==
Comparison between Mohs hardness and [[Vickers hardness test|Vickers hardness]]:<ref name="mindat.org">{{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}}</ref>

:{|class="wikitable" style="text-align:center"
|- 
! Mineral<br/>name
! Hardness (Mohs)
! Hardness (Vickers)<br/>(kg/mm{{sup|2}})
|-
|[[Tin]]||1.5||VHN{{sub|10}} = 7–9
|-
|[[Bismuth]]||2–2.5||VHN{{sub|100}} = 16–18
|-
|[[Native gold|Gold]]||2.5||VHN{{sub|10}} = 30–34
|-
|[[Silver]]||2.5||VHN{{sub|100}} = 61–65
|-
|[[Chalcocite]]||2.5–3||VHN{{sub|100}} = 84–87
|-
|[[Native copper|Copper]]||2.5–3||VHN{{sub|100}} = 77–99
|-
|[[Galena]]||2.5||VHN{{sub|100}} = 79–104
|-
|[[Sphalerite]]||3.5–4||VHN{{sub|100}} = 208–224
|-
|[[Heazlewoodite]]||4||VHN{{sub|100}} = 230–254
|-
|[[Goethite]]||5–5.5||VHN{{sub|100}} = 667
|-
|[[Chromite]]||5.5||VHN{{sub|100}} = 1,278–1,456
|-
|[[Anatase]]||5.5–6||VHN{{sub|100}} = 616–698
|-
|[[Rutile]]||6–6.5||VHN{{sub|100}} = 894–974
|-
|[[Pyrite]]||6–6.5||VHN{{sub|100}} = 1,505–1,520
|-
|[[Bowieite]]||7||VHN{{sub|100}} = 858–1,288
|-
|[[Euclase]]||7.5||VHN{{sub|100}} = 1,310
|-
|[[Chromium]]||8.5||VHN{{sub|100}} = 1,875–2,000
|}
==Footnotes==
{{notelist}}

==See also==
{{div col begin|colwidth=12em}}
* [[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]]
{{div col end}}

==References==
{{reflist|25em}}

==Further reading==
* {{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 }}

{{Mohs}}
{{Mineral identification}}

[[Category:Materials science]]
[[Category:Mineralogy]]
[[Category:Hardness tests]]
[[Category:1812 in science]]
[[Category:1812 in the Confederation of the Rhine]]

[[de:Härte#Härteprüfung nach Mohs]]