Mohs scale of mineral hardness

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Mohs hardness kit, containing one specimen of each mineral on the ten-point hardness scale Mohssche-haerteskala hg.jpg
Mohs hardness kit, containing one specimen of each mineral on the ten-point hardness scale

The Mohs scale of mineral hardness ( /mz/ ) is a qualitative ordinal scale characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. Created in 1812 by German geologist and mineralogist Friedrich Mohs, it is one of several definitions of hardness in materials science, some of which are more quantitative. [1] 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. [2] [3] [4] While greatly facilitating the identification of minerals in the field, the Mohs scale does not show how well hard materials perform in an industrial setting. [5]

Contents

Usage

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 kind of mill will best reduce a given product whose hardness is known. [6] The scale is used at electronic manufacturers for testing the resilience of flat panel display components (such as cover glass for LCDs or encapsulation for OLEDs).

The Mohs scale has been used to evaluate the hardness of smartphone screens. Most modern smartphone displays use Gorilla Glass that scratches at level 6 with deeper grooves at level 7 on the Mohs scale of hardness. [7]

Minerals

The Mohs scale of mineral hardness is based on the ability of one natural sample of mineral to scratch another mineral visibly. The samples of matter used by Mohs are all different minerals. Minerals are chemically pure solids found in nature. Rocks are made up of one or more minerals. As the hardest known naturally occurring substance when the scale was designed, diamonds are at the top of the scale. 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 fall between 4 and 5. [8] "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]

The Mohs scale is a purely 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 pictorial examples. [10] [11]

Mohs hardnessMineralChemical formulaAbsolute hardness [12] Image
1 Talc Mg3Si4O10(OH)21 Talc block.jpg
2 Gypsum CaSO4·2H2O2 Gypse Arignac.jpg
3 Calcite CaCO314 Calcite-sample2.jpg
4 Fluorite CaF221 Fluorite with Iron Pyrite.jpg
5 Apatite Ca5(PO4)3(OH,Cl,F)48 Apatite Canada.jpg
6 Orthoclase feldspar KAlSi3O872 OrthoclaseBresil.jpg
7 Quartz SiO2100 Quartz Bresil.jpg
8 Topaz Al2SiO4(OH,F)2200 Topaz cut.jpg
9 Corundum Al2O3400 Cut Ruby.jpg
10 Diamond C1500 Rough diamond.jpg

On the Mohs scale, a streak plate (unglazed porcelain) has a hardness of approximately 7.0. Using these ordinary materials of known hardness can be a simple way to approximate the position of a mineral on the scale. [1]

Intermediate hardness

The table below incorporates additional substances that may fall between levels: [13]

HardnessSubstance or mineral
0.2–0.3 caesium, rubidium
0.5–0.6 lithium, sodium, potassium
1 talc
1.5 gallium, strontium, indium, tin, barium, thallium, lead, graphite, ice [14]
2hexagonal boron nitride, [15] calcium, selenium, cadmium, sulfur, tellurium, bismuth, gypsum
2–2.5 halite (rock salt), fingernail [16]
2.5–3 gold, silver, aluminium, zinc, lanthanum, cerium, jet
3 calcite, copper, arsenic, antimony, thorium, dentin
3.5 platinum
4 fluorite, iron, nickel
4–4.5ordinary steel
5 apatite (tooth enamel), zirconium, palladium, obsidian (volcanic glass)
5.5 beryllium, molybdenum, hafnium, glass, cobalt
6 orthoclase, titanium, manganese, germanium, niobium, uranium
6–7 fused quartz, iron pyrite, silicon, ruthenium, iridium, tantalum, opal, peridot, tanzanite, rhodium, jade
7 osmium, quartz, rhenium, vanadium
7.5–8 emerald, beryl, zircon, tungsten, spinel
8 topaz, cubic zirconia, enhanced hardened steel such as REX 121 steel
8.5 chrysoberyl, chromium, silicon nitride, tantalum carbide
9 corundum (includes sapphire and ruby), tungsten carbide, titanium nitride
9–9.5 silicon carbide (carborundum), tungsten carbide, tantalum carbide, zirconium carbide, alumina, beryllium carbide, titanium carbide, aluminum boride, boron carbide. [note 1] [17] [18]
9.5–near 10 boron, boron nitride, rhenium diboride (a-axis), [19] stishovite, titanium diboride, moissanite (crystal form of silicon carbide)
10 diamond, carbonado

Comparison with Vickers scale

Comparison between Mohs hardness and Vickers hardness: [20]

Mineral
name
Hardness (Mohs)Hardness (Vickers)
(kg/mm2)
Graphite 1–2VHN10 = 7–11
Tin 1.5VHN10 = 7–9
Bismuth 2–2.5VHN100 = 16–18
Gold 2.5VHN10 = 30–34
Silver 2.5VHN100 = 61–65
Chalcocite 2.5–3VHN100 = 84–87
Copper 2.5–3VHN100 = 77–99
Galena 2.5VHN100 = 79–104
Sphalerite 3.5–4VHN100 = 208–224
Heazlewoodite 4VHN100 = 230–254
Carrollite 4.5–5.5VHN100 = 507–586
Goethite 5–5.5VHN100 = 667
Hematite 5–6VHN100 = 1,000–1,100
Chromite 5.5VHN100 = 1,278–1,456
Anatase 5.5–6VHN100 = 616–698
Rutile 6–6.5VHN100 = 894–974
Pyrite 6–6.5VHN100 = 1,505–1,520
Bowieite 7VHN100 = 858–1,288
Euclase 7.5VHN100 = 1,310
Chromium 8.5VHN100 = 1,875–2,000

See also

Notes

  1. The hardness level of the carbides of the following elements falls between 9 and 10: [17] [18] W, Ta, Zr, Be, Ti, Si, B.

Related Research Articles

Corundum Oxide mineral

Corundum is a crystalline form of aluminium oxide typically containing traces of iron, titanium, vanadium and chromium. It is a rock-forming mineral. It is also a naturally transparent material, but can have different colors depending on the presence of transition metal impurities in its crystalline structure. Corundum has two primary gem varieties: ruby and sapphire. Rubies are red due to the presence of chromium, and sapphires exhibit a range of colors depending on what transition metal is present. A rare type of sapphire, padparadscha sapphire, is pink-orange.

Mineral Element or chemical compound that is normally crystalline, formed as a result of geological processes

In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure, that occurs naturally in pure form..

Mineralogy Scientific study of minerals and mineralised artifacts

Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical properties of minerals and mineralized artifacts. Specific studies within mineralogy include the processes of mineral origin and formation, classification of minerals, their geographical distribution, as well as their utilization.

Muscovite true mica, phyllosilicate mineral; polytypes: 1M, 2A, 2M2, 3T

Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula KAl2(AlSi3O10)(F,OH)2, or (KF)2(Al2O3)3(SiO2)6(H2O). It has a highly perfect basal cleavage yielding remarkably thin laminae (sheets) which are often highly elastic. Sheets of muscovite 5 meters × 3 meters (16.5 feet × 10 feet) have been found in Nellore, India.

Topaz Silicate mineral

Topaz is a silicate mineral of aluminium and fluorine with the chemical formula Al2SiO4(F, OH)2. Topaz crystallizes in the orthorhombic system, and its crystals are mostly prismatic terminated by pyramidal and other faces. It is one of the hardest naturally occurring minerals (Mohs hardness of 8) and is the hardest of any silicate mineral. This hardness combined with its usual transparency and variety of colors means that it has acquired wide use in jewellery as a cut gemstone as well as for intaglios and other gemstone carvings.

Fluorite

Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF2. It belongs to the halide minerals. It crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon.

<i>Natural History</i> (Pliny) Encyclopedia published c. AD 77–79 by Pliny the Elder

The Natural History is a work by Pliny the Elder. It is one of the largest single works to have survived from the Roman Empire to the modern day and purports to cover all ancient knowledge. The work's subject area is thus not limited to what is today understood by natural history; Pliny himself defines his scope as "the natural world, or life". It is encyclopedic in scope, but its structure is not like that of a modern encyclopedia. It is the only work by Pliny to have survived, and the last that he published. He published the first 10 books in AD 77, but had not made a final revision of the remainder at the time of his death during the AD 79 eruption of Vesuvius. The rest was published posthumously by Pliny's nephew, Pliny the Younger.

Arsenopyrite

Arsenopyrite is an iron arsenic sulfide (FeAsS). It is a hard metallic, opaque, steel grey to silver white mineral with a relatively high specific gravity of 6.1. When dissolved in nitric acid, it releases elemental sulfur. When arsenopyrite is heated, it produces poisonous sulfur and arsenic fumes which can be fatal if inhaled in large quantities. With 46% arsenic content, arsenopyrite, along with orpiment, is a principal ore of arsenic. When deposits of arsenopyrite become exposed to the atmosphere, the mineral will slowly oxidize, converting the arsenopyrite into an iron arsenate, a relatively stable compound. Arsenopyrite is generally an acid consuming sulfide mineral unlike iron pyrite which can lead to acid mine drainage.

Chalcopyrite

Chalcopyrite ( KAL-ko-PY-ryt) is a copper iron sulfide mineral that crystallizes in the tetragonal system. It has the chemical formula CuFeS2. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mohs scale. Its streak is diagnostic as green tinged black.

Gadolinite nesosilicate mineral

Gadolinite, sometimes known as ytterbite, is a silicate mineral consisting principally of the silicates of cerium, lanthanum, neodymium, yttrium, beryllium, and iron with the formula (Ce,La,Nd,Y)
2
FeBe
2
Si
2
O
10
. It is called gadolinite-(Ce) or gadolinite-(Y), depending on the prominent composing element. It may contain 35.5% yttria sub-group rare earths, 2.2% ceria earths, as much as to 11.6% BeO, and traces of thorium. It is found in Sweden, Norway, and the US.

Friedrich Mohs German geologist and mineralogist

Carl Friedrich Christian Mohs was a German geologist and mineralogist. He was the creator of the Mohs scale of mineral hardness. Mohs also introduced a classification of the crystal forms in crystal systems independently of Christian Samuel Weiss.

An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away by friction. While finishing a material often means polishing it to gain a smooth, reflective surface, the process can also involve roughening as in satin, matte or beaded finishes. In short, the ceramics which are used to cut, grind and polish other softer materials are known as abrasives.

Chrysocolla

Chrysocolla is a hydrated copper phyllosilicate mineral with formula Cu
2–x
Al
x
{H
2–x
Si
2
O
5
)(OH)
4
•nH
2
O
(x<1) or (Cu,Al)
2
H
2
Si
2
O
5
(OH)
4
•nH
2
O)
. The structure of the mineral has been questioned, as spectrographic studies suggest material identified as chrysocolla may be a mixture of the copper hydroxide spertiniite and chalcedony.

Diopside Pyroxene mineral

Diopside is a monoclinic pyroxene mineral with composition MgCaSi
2
O
6
. It forms complete solid solution series with hedenbergite and augite, and partial solid solutions with orthopyroxene and pigeonite. It forms variably colored, but typically dull green crystals in the monoclinic prismatic class. It has two distinct prismatic cleavages at 87 and 93° typical of the pyroxene series. It has a Mohs hardness of six, a Vickers hardness of 7.7 GPa at a load of 0.98 N, and a specific gravity of 3.25 to 3.55. It is transparent to translucent with indices of refraction of nα=1.663–1.699, nβ=1.671–1.705, and nγ=1.693–1.728. The optic angle is 58° to 63°.

Aggregated diamond nanorod Nanocrystalline form of diamond

Aggregated diamond nanorods, or ADNRs, are a nanocrystalline form of diamond, also known as nanodiamond or hyperdiamond.

Hardness is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. Some materials are harder than others. Macroscopic hardness is generally characterized by strong intermolecular bonds, but the behavior of solid materials under force is complex; therefore, there are different measurements of hardness: scratch hardness, indentation hardness, and rebound hardness.

Scratch hardness tests are used to determine the hardness of a material to scratches and abrasion. The earliest test was developed by mineralogist Friedrich Mohs in 1820. It is based on relative scratch hardness, with talc assigned a value of 1 and diamond assigned a value of 10. Mohs' scale had two limitations; it was not linear, and most modern abrasives fall between 9 and 10.

The Rosiwal scale is a hardness scale in mineralogy, with its name given in memory of the Austrian geologist August Karl Rosiwal. The Rosiwal scale bases its measure on absolute values, unlike the Mohs scale whose values are relative values, its interest is relegated to the amateur or an approach that makes it useful in the research field.

References

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  2. Theophrastus on Stones. Farlang.com. Retrieved on 2011-12-10.
  3. Pliny the Elder. Naturalis Historia. Book 37. Chap. 15. ADamas: six varieties of it. Two remedies.
  4. Pliny the Elder. Naturalis Historia. Book 37. Chap. 76. The methods of testing precious stones.
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  7. https://www.youtube.com/watch?v=7jD5Gkh4K34&feature=youtu.be&t=101
  8. American Federation of Mineralogical Societies. "Mohs Scale of Mineral Hardness". amfed.org
  9. Geels, Kay. "The True Microstructure of Materials", pp. 5–13 in Materialographic Preparation from Sorby to the Present. Struers A/S, Copenhagen, Denmark - archived Mar 7 2016
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  13. Samsonov, G.V., ed. (1968). "Mechanical Properties of the Elements". Handbook of the Physicochemical Properties of the Elements. New York: IFI-Plenum. p. 432. doi:10.1007/978-1-4684-6066-7. ISBN   978-1-4684-6068-1.
  14. "Ice is a mineral" Archived 2015-10-30 at the Wayback Machine in Exploring Ice in the Solar System. messenger-education.org
  15. Berger, Lev I. (1996). Semiconductor Materials (First ed.). Boca Raton, FL: CRC Press. p.  126. ISBN   978-0849389122.
  16. "Mohs Hardness Scale: Testing the Resistance to Being Scratched". geology.com.
  17. 1 2 "Material Hardness Tables, Ted Pella, Inc". www.tedpella.com. Retrieved 2019-05-09.
  18. 1 2 "Hardness table" (PDF). Retrieved 2019-05-09.
  19. Levine, Jonathan B.; Tolbert, Sarah H.; Kaner, Richard B. (2009). "Advancements in the Search for Superhard Ultra-Incompressible Metal Borides" (PDF). Advanced Functional Materials. pp. 3526–3527. doi:10.1002/adfm.200901257. Archived from the original (PDF) on 2016-03-04. Retrieved 2015-12-08.
  20. Ralph, Jolyon. "Welcome to mindat.org". mindat.org. Hudson Institute of Mineralogy. Retrieved April 16, 2017.

Further reading