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, from 1 to 10, characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material.

Contents

The scale was created in 1822 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] [2]

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. [3] [4] [5] 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 – toughness. [6]

Use

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. [7] 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. [8] [9]

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. [10]

"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. [11]

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. [12] [13]

Mohs hardnessMineralChemical formulaAbsolute hardness [14] 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-120187.jpg
9 Corundum Al2O3400 Corundum-dtn14b.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. [15]

Intermediate hardness

The table below incorporates additional substances that may fall between levels: [16]

HardnessSubstance or mineral
0.2–0.3 caesium, rubidium
0.5–0.6 lithium, sodium, potassium, candle wax
1 talc
1.5 gallium, strontium, indium, tin, barium, thallium, lead, graphite, ice [17]
2hexagonal boron nitride, [18] calcium, selenium, cadmium, sulfur, tellurium, bismuth, gypsum
2–2.5 halite (rock salt), fingernail, [19] mica [20]
2.5–3 gold, silver, aluminium, zinc, lanthanum, cerium, jet
3 calcite, copper, arsenic, antimony, thorium, dentin, chalk [21]
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, garnet, [21] pyrite [21]
7 osmium, quartz, rhenium, vanadium
7.5–8 emerald, beryl, zircon, tungsten, spinel
8 topaz, cubic zirconia, hardened steel, spinel [22]
8.5 chrysoberyl, chromium, silicon nitride, tantalum carbide
9 corundum (includes sapphire and ruby), tungsten carbide, titanium nitride, aluminium oxide, [21] bedrock (hardest)
9–9.5 silicon carbide (carborundum), tantalum carbide, zirconium carbide, alumina, beryllium carbide, titanium carbide, aluminum boride, boron carbide. [lower-alpha 1] [23] [24]
9.5–near 10 boron, boron nitride, rhenium diboride (a-axis), [25] stishovite, titanium diboride, moissanite (crystal form of silicon carbide), boron carbide [21]
10 diamond, carbonado

Comparison with Vickers scale

Comparison between Mohs hardness and Vickers hardness: [26]

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. Carbides of some metals and semi-metals are quite hard (as well as brittle); carbides of tungsten (WC), tantalum (TaC), zirconium (ZrC), beryllium (Be
    2
    C
    ), titanium (TiC), silicon (SiC), and boron (B
    4
    C
    ) all have Mohs hardness levels between 9 and 10. [23] [24]

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 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.

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.

Lapidary Shaping of gemstones etc. for jewellery etc.

Lapidary is the practice of shaping stone, minerals, or gemstones into decorative items such as cabochons, engraved gems, and faceted designs. A person who practices lapidary is known as a lapidarist. A lapidarist uses the lapidary techniques of cutting, grinding, and polishing. Hardstone carving requires specialized carving techniques.

<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.

Chalcopyrite Copper iron sulfide mineral

Chalcopyrite ( KAL-kə-PY-ryte, -⁠koh-) is a copper iron sulfide mineral and the most abundant copper ore mineral. It has the chemical formula CuFeS2 and crystallizes in the tetragonal system. 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.

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.

Moissanite Silicon carbide mineral

Moissanite is naturally occurring silicon carbide and its various crystalline polymorphs. It has the chemical formula SiC and is a rare mineral, discovered by the French chemist Henri Moissan in 1893. Silicon carbide is useful for commercial and industrial applications due to its hardness, optical properties and thermal conductivity.

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.

Amateur geology

Amateur geology or rock collecting is the non-professional study and hobby of collecting rocks and minerals or fossil specimens from the natural environment. In Australia, New Zealand and Cornwall, the activities of amateur geologists are called fossicking. The first amateur geologists were prospectors looking for valuable minerals and gemstones for commercial purposes. Eventually, however, more people have been drawn to amateur geology for recreational purposes, mainly for the beauty that rocks and minerals provide.

Superhard material Material with Vickers hardness exceeding 40 gigapascals

A superhard material is a material with a hardness value exceeding 40 gigapascals (GPa) when measured by the Vickers hardness test. They are virtually incompressible solids with high electron density and high bond covalency. As a result of their unique properties, these materials are of great interest in many industrial areas including, but not limited to, abrasives, polishing and cutting tools, disc brakes, and wear-resistant and protective coatings.

Grossular Garnet, nesosilicate mineral

Grossular is a calcium-aluminium species of the garnet group of minerals. It has the chemical formula of Ca3Al2(SiO4)3 but the calcium may, in part, be replaced by ferrous iron and the aluminium by ferric iron. The name grossular is derived from the botanical name for the gooseberry, grossularia, in reference to the green garnet of this composition that is found in Siberia. Other shades include cinnamon brown (cinnamon stone variety), red, and yellow. Grossular is a gemstone.

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. In general, different materials differ in their hardness; for example hard metals such as titanium and beryllium are harder than soft metals such as sodium and metallic tin, or wood and common plastics. 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.

Rhenium diboride Chemical compound

Rhenium diboride (ReB2) is a synthetic superhard material. It was first synthesized in 1962  and re-emerged recently due to hopes of achieving high hardness comparable to that of diamond. The reported ultrahigh hardness has been questioned, although this is a matter of definition as in the initial test rhenium diboride was able to scratch diamond.

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's scale had two limitations: it was not linear, and most modern abrasives fall between 9 and 10.

Early writing on mineralogy, especially on gemstones, comes from ancient Babylonia, the ancient Greco-Roman world, ancient and medieval China, and Sanskrit texts from ancient India. Books on the subject included the Naturalis Historia of Pliny the Elder which not only described many different minerals but also explained many of their properties. The German Renaissance specialist Georgius Agricola wrote works such as De re metallica and De Natura Fossilium which began the scientific approach to the subject. Systematic scientific studies of minerals and rocks developed in post-Renaissance Europe. The modern study of mineralogy was founded on the principles of crystallography and microscopic study of rock sections with the invention of the microscope in the 17th century.

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 attempts to give more quantitative values of scratch hardness, unlike the Mohs scale which is a qualitative measurement with relative values.

References

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Further reading