Mohs scale
The Mohs scale (/moʊz/ MOHZ) 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 (English: Attempt at an elementary method for the natural-historical determination and recognition of fossils);[1][2][a] 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, c. 300 BC, followed by Pliny the Elder in his Naturalis Historia, c. AD 77.[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.
Mohs scale along the horizontal axis matched with
one of the absolute hardness scales along the
vertical. Diamond (Mohs 10) is 1500 (off scale).
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.[citation needed] 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]
Mohs
hardnessReference
mineralChemical formula Absolute
hardness[12]Example image 1 Talc Mg3Si4O10(OH)2 1 
2 Gypsum Ca SO4 · 2H2O 2 
3 Calcite Ca CO3 14 
4 Fluorite Ca F2 21 
5 Apatite Ca5(PO4)3 (OH−,Cl−,F−) 48 
6 Orthoclase
feldsparK Al Si3O8 72 
7 Quartz SiO2 100 
8 Topaz Al2SiO4(OH−,F−)2 200 
9 Corundum Al2O3 400 
10 Diamond C 1500 
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 that are not minerals have been assigned a hardness on the Mohs scale. However, hardness can make it difficult to determine if the substance is a mixture of other substances or if it 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 Potassium 0.5–0.6 Lithium 1 Talc 1.5 Lead 2 Hardwood[14] 2–2.5 Plastic 2.5 Zinc 2.5–3 Copper 3 Brass 3.5 Adamite 3.5-4 Sphalerite 4 Iron 4–4.5 Ordinary steel 4.5 Colemanite 5 Apatite 5-5.5 Goethite 5.5 Glass 5.5–6 Opal 6 Rhodium 6-6.5 Rutile 6.5 Silicon 6.5–7 Jadeite 7 Porcelain 7-7.5 Garnet 7.5 Tungsten 7.5–8 Emerald 8 Topaz 8.5 Chromium 9 Sapphire 9–9.5 Moissanite 9.5–near 10 Boron 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 milling. It allows the assessment of which type of mill and grinding medium will best reduce a given product whose hardness is known.[15]
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.[16]
Comparison with Vickers scale
Comparison between Mohs hardness and Vickers hardness:[17]
Mineral
nameHardness (Mohs) Hardness (Vickers)
(kg/mm2)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 Goethite 5–5.5 VHN100 = 667 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
Footnotes
- ^
- In demselben Jahre (1812) wurde MOHS als Professor am Joanneum angestellt und veröffentliche den ersten Teil seines Werkes Versuch einer Elementarmethode zur naturhistorischen Bestimmung und Erkennung der Fossilien, in welcher die bekannte Härteskala aufgestellt wurde. — von Groth (1926)
- 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.[1]
See also
References
- ^ a b von Groth, Paul Heinrich (1926). Entwicklungsgeschichte der Mineralogischen Wissenschaften [History of the development of the mineralogical sciences] (in German). Berlin: Springer. p. 250. ISBN 9783662409107.
- ^ "Mohs hardness". Encyclopædia Britannica (online ed.).
- ^ "Mohs scale of hardness". Mineralogical Society of America. Retrieved 10 February 2021.
- ^ Theophrastus. Theophrastus on Stones. Retrieved 10 December 2011 – via Farlang.com.
- ^ Pliny the Elder. "Book 37, Chap. 15". Naturalis Historia.
Adamas: Six varieties of it. Two remedies.
- ^ Pliny the Elder. "Book 37, Chap. 76". Naturalis Historia.
The methods of testing precious stones.
- ^ "Hardness". Materials Mechanical Hardness. Non-Destructive Testing Resource Center. Archived from the original on 14 February 2014.
- ^ "Mohs scale of mineral hardness". American Federation of Mineralogical Societies – via amfed.org.
- ^ Geels, Kay (26 April 2000). "The true microstructure of materials". Materialographic Preparation from Sorby to the Present (PDF). Application notes and guides (Report). The Struers metallographic library. Copenhagen, DK: Struers A/S. pp. 5–13. Archived from the original (PDF) on 7 March 2016.
- ^ "What is important about hardness?". Amethyst galleries. Mineral gallery. Archived from the original on 30 December 2006 – via galleries.com.
- ^ "Mineral hardness and hardness scales". Inland Lapidary. Archived from the original on 17 October 2008 – via inlandlapidary.com.
- ^ Mukherjee, Swapna (2012). Applied Mineralogy: Applications in industry and environment. Springer Science & Business Media. p. 373. ISBN 978-94-007-1162-4 – via Google books.
- ^ Samsonov, G.V., ed. (1968). "Mechanical properties of the elements". Handbook of the Physicochemical Properties of the Elements. New York, NY: IFI-Plenum. p. 432. doi:10.1007/978-1-4684-6066-7. ISBN 978-1-4684-6068-1.
- ^ "Mohs Hardness Scale: Testing the Resistance to Being Scratched". geology.com. Retrieved 9 August 2021.
- ^ "Size reduction, comminution". Grinding and milling. PowderProcess.net. Retrieved 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. Retrieved 16 April 2021.
- ^ Ralph, Jolyon. "Welcome to mindat.org". mindat.org. Hudson Institute of Mineralogy. Retrieved 16 April 2017.
Further reading
- Cordua, William S. (c. 1990). "The hardness of minerals and rocks". Lapidary Digest – via gemcutters.org.
- Raden, Aja (2016). Gem: The definitive visual guide. New York, NY: DK Publishing. ISBN 9781465453563.
