Mohs scale

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

The Mohs scale (Template:IPAc-en Template:Respell) 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 {{#invoke:Lang|lang}} (English: Attempt at an elementary method for the natural-historical determination and recognition of fossils);<ref name=VonGroth1926>Template:Cite book</ref><ref name=Brit>Template:Cite encyclopedia</ref>Template:Efn it is one of several definitions of hardness in materials science, some of which are more quantitative.<ref name="MinSocAm">{{#invoke:citation/CS1|citation |CitationClass=web }}</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, Template:Circa, followed by Pliny the Elder in his Naturalis Historia, Template:Circa.<ref>Template:Cite book</ref><ref>Template:Cite book</ref><ref>Template:Cite book</ref> 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Reference mineralsEdit

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.

File:Mohs scale vs absolute hardness.png

Mohs scale along the horizontal axis matched with
one of the absolute hardness scales along the
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>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

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.<ref>Template:Cite report</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.Template:Fact 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>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Mohs hardness	Reference mineral	Chemical formula	Absolute hardness<ref>Template:Cite book</ref>	Example image
1	Talc	Template:Chem2	1	File:Talc block.jpg
2	Gypsum	Template:Chem2	2	File:Gypse Arignac.jpg
3	Calcite	Template:Chem2	14	File:Calcite-sample2.jpg
4	Fluorite	Template:Chem2	21	File:Fluorite with Iron Pyrite.jpg
5	Apatite	Template:Chem2	48	File:Apatite Canada.jpg
6	Orthoclase feldspar	Template:Chem2	72	File:OrthoclaseBresil.jpg
7	Quartz	Template:Chem2	100	File:Quartz Brésil.jpg
8	Topaz	Template:Chem2	200	File:Topaz-120187.jpg
9	Corundum	Template:Chem2	400	File:Corundum-dtn14b.jpg
10	Diamond	Template:Chem2	1500	File:Rough diamond.jpg

ExamplesEdit

Template:More citations needed 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).

Hardness	Substance
0.2–0.4	Potassium<ref name=Samsonov68>Template:Cite book</ref>
0.5–0.6	Lithium<ref name=Samsonov68 />
1	Talc
1.5	Lead<ref name=Samsonov68 />
2	Hardwood<ref name="geology.com">{{#invoke:citation/CS1\|citation	CitationClass=web }}</ref>
2–2.5	Plastic
2.5	Zinc<ref name=Samsonov68 />
2.5–3	Copper<ref name=Samsonov68 />
3	Brass
3.5	Adamite
3.5-4	Sphalerite
4	Iron<ref name=Samsonov68 />
4–4.5	Ordinary steel
4.5	Colemanite
5	Apatite
5-5.5	Goethite
5.5	Glass
5.5–6	Opal
6	Rhodium<ref name=Samsonov68 />
6-6.5	Rutile
6.5	Silicon<ref name=Samsonov68 />
6.5–7	Jadeite
7	Porcelain
7-7.5	Garnet
7.5	Tungsten<ref name=Samsonov68 />
7.5–8	Emerald
8	Topaz
8.5	Chromium<ref name=Samsonov68 />
9	Sapphire
9–9.5	Moissanite
9.5–near 10	Boron<ref name=Samsonov68 />
10	Diamond

UseEdit

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.<ref name="Milling1">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

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.<ref>Template:Cite magazine</ref>

Comparison with Vickers scaleEdit

Comparison between Mohs hardness and Vickers hardness:<ref name="mindat.org">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Mineral name	Hardness (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

FootnotesEdit

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ReferencesEdit

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