Mohs Scale – Hardness and Other Scales.

Topics :

  • Mohs Scale? (What is it and Who was invented?)
  • The Hardness of Gems. (scale of Gems)
  • Other scales of Hardness. (information of Rosiwal Scale and Knoop Scale )
  • Mohs Scale of other Minerals.
  • Mohs Hardness Assessing Procedure. (Hardness testing)
  • Breaking Myths

We will study the hardness scale of Mohs, very important to know the hardness of the materials. The hardness of a rock or mineral is determined by its resistance to scratches, cuts, penetrated or different deformations caused. This data allows to identify and classify them to know their different uses in everyday life

To regulate the hardness of materials various scales are used. Out of them, three are here.

  1. Mohs Scale
  2. Knoop Scale
  3. Rosiwal Scale

 1. Mohs Scale :

Mohs deliberated physics, mathematics, and chemistry. He began to classify minerals by their corporal attributes, rather than by their chemical constitution, as had been done before. 

Friedrich_Mohs

The Mohs hardness scale is one of the oldest forms of hardness measurement. And that was delivered by the German mineralogist Friedrich Mohs in 1812 with the intention of establishing differences between rocks and minerals, throughout history.

It is not a scientific mechanism, although the simplicity it provides in terms of memorization and handling of data, has facilitated the fieldwork of many geologists. Friedrich Mohs was the best geologist and mineralogist.

It is a qualitative and relative scale consisting of 10 reference minerals arranged from 1 to 10. In position 1 talc is the softest mineral, and in 10 there is the diamond, the hardest mineral.

Below table gives you the Mohs hardness scale, Absolute hardness of the minerals. Its also gives the other minerals and Daily equivalents and observation or test report of minerals.

MineralsMohs
Hardness
Scale
Absolute
Hardness
Other
Minerals
Daily
Equivalent
Observation
Talc11talcum powderhas greasy fill and
scratched by fingernail
Gypsum23~2.2 Fingernailfingernail scratched by fingernail
Calcite39~3.2 Copper Pennybronze coinscratched by copper
coin and knife easily
Fluorite421iron naileasily scratched by a knife
but not easily with calcite
Apatite548~5.1 Pocketknife
~5.5 Glass Plate
glassdifficult to scratched by a knife
Orthoclase/Feldspar672~6.5 Steel needlepenknife difficult to scratch on glass,
cannot be scratched with a knife
quarts7100~7.0 Streak Platesteel knifescratches glass
topaz8200abrasive papereasily scratches glass
corundum9400Rubycuts glass
Diamond101600Synthetic Diamondyou can make a glass cutter

Scratch Test for Minerals:

The scale proceeds from 1 to 10. The diamond is at the peak state on the scale, along with a hardness of 10, Talc is the squashy or pulpy, along with a hardness of 1. You can utilize the minerals of which you know its hardness to regulate the hardness of any other mineral. 

A mineral of a certain hardness will grate to another mineral of lower hardness. For example with the nail of your finger (2) you can grate a talcum mineral (1) or with smashed glass (5), you can grate a mineral of calcite (3) or fluorite (4).

scratch

To apply the hardness scale, attempt to grate the facet of a sample of the unknown mineral with a sample of a mineral of known hardness that is on the scale (well-known samples). If the unrevealed representative cannot be grated with a piece of calcite (3) however it can be grated with a piece of fluorite (4), then its hardness is betwixt 3 and 4.

An example of minerals with a hardness betwixt 3 and 4 are barite, celestite, and cerussite (3 to 3.5). This test could be used to discriminate betwixt calcite and barite or barite and fluorite. 

If you want to know the hardness of an unrevealed mineral remember that minerals can be defaced and lose value if they are not grated with the property.

Below table gives you the information about Mohs Scale with the minerals chemical formula and images of minerals. 

1.Talc Mg3Si4O10(OH)2 Talc
2.Gypsum CaSO4•2H2O Gypsum
3.Calcite CaCO3 calcite
4.Fluorite CaF2 flourite
5.Apatite Ca5(PO4)3(OH-,Cl-,F-) apatite
6.Orthoclase/Feldspar KAlSi3O8
7.quartz SiO2 quartz
8.Topaz Al2SiO4(OH-,F-)2 topaz
9.corundum Al2O3 corundum
10.DiamondCdiamond

Although to measure the hardness of a rock or mineral, through the Mohs Scale, it is necessary to have some of the others identified in the table, in the absence of them you can use different objects, comparable in terms of resistance. For Example:

  • Fingernails: Mohs hardness approximately equal to 2
  • copper coin : 3
  • knife: 5
  • Crystal: 5.5
  • steel file: 6.5

When trying to make a mark, it is important to verify the direction in which the force is printed. This is because hardness is a vector property. Also, the granularity or exploitability of the rock or mineral can increase its propensity to rupture, even when the material used is softer than the one that receives the load.

The hardness of Gems :

Most gems have a hardness of seven or more. however, there are some gems that have a lower hardness such as Jade (6.5), Lapislazuli (5 – 5.5), Opal (5.5 – 6.5), Turquoise (5 – 6) or Feldspar (6 – 6.5) how Sunstone, Labradorite, Moonstone, or Amazonite.

GemsHardness
Diamond10
Safiro, Rubi9
Chrysoberyl, Cat’s Eye, Alexandrite 8.5
Beryl, Emerald, Aquamarine 7.5 – 8
Zircon 7.5
Tourmaline (Elbaite, Dravita) 7 – 7.5
Quartz (Ametic, Citrine) 7
Garnet (Hesonite, Rhodolite, Espesartite) 6.5 – 7.5

Other Scales of Hardness :

To date, different ways of measuring hardness have been devised, much more accurate than the Mohs scale. You can see various tests such as Brinell, Vickers, Shore, Rosiwal and Knoop, these last two being the most popular today because the values ​​established by each are absolute.

Rosiwal Scale :

The Rosiwal scale is based on laboratory tests and expresses the abrasion resistance of the mineral, taking as reference the corundum which is given to hardness of 1000.

Mohs Scale Vs Rosiwal Scale :

MineralMohs ScaleRosiwal Hardness
Talc10.03
Gypsum21.25
Calcite34.5
Flourite45
Apatite55.5
Felspar637
Quartz7120
Topaz8175
Corundum91000
Diamond10140000

Knoop Scale :

The Knoop scale determines microhardness and is used mainly in thin sheets or very fragile materials. The material to be tested is subjected to pressure with a diamond indenter and the gap is measured with the aid of a microscope. The hardness of Knoop is then calculated by dividing the load or force applied between the printing area.

Mohs Scale Vs Knoop Scale :

Knoop-and_Mohs-_scale_

Hardness tests are carried out of the field of mineralogy for many types of materials, for various purposes and with various laboratory tests. For example, in mechanical engineering, the Vickers test and the Rockwell test are used, both based on the use of diamond penetrators or other hard materials, such as hardened steel, and a scale to measure the cracks they cause.

The test of hardness derived by Friedrich Mohs was the first familiar test to evaluate the defiance of material to scraping. It is a very easy but imprecise relative trial. Maybe its clarity has allowed it to enhance the test of hardness that is most used.

As the scale of Mohs was derived in 1812, from that year until now, much non-identical hardness trials have been generated in order to improve the results. These involve tests by Brinell, Knoop, Rockwell, Vickers, Rosiwal, and Shore. For each of these trials, a small “penetrator” is used that is petitioned to the material being trialed with a conservative dignified quantity of power. Next, the size or depth of the mark and the quantity of power used in the test allows us to calculate the value of the hardness of the material.

Mohs Scale of Other Minerals :

Mohs ScaleMineral
10Diamond
9Corundum
8.5Chrysoberyl
8Topaz
7.5 - 8Beryl
7.5 - 8Spinel
7.5Zircon
7 - 7.5Cordierite
7 - 7.5Staurolite
7 - 7.5Tourmaline
7Quartz
6.5 - 7.5Garnet
6.5 - 7Jadeite
6.5 - 7.5Sillimanite
6.5 - 7Olivine
6.5 - 7Spodumene
6 - 7.5Marcasite
6 - 7Cassiterite
6 - 7Epidote
6 - 7Zoisite
6 - 6.5Orthoclase
6 - 6.5Plagioclase
6 - 6.5Prehnite
6 - 6.5Pyrite
6 - 6.5Rutile
5.5 - 6.5Diopside
5.5 - 6.5Rhodonite
5.5 - 6Arsenopyrite
5.5 - 6Augite
5.5 - 6Chromite
5.5 - 6.5Hematite
5.5 - 6Nepheline
5.5 - 6Sodalite
5 - 6.5Magnetite
5 - 6Enstatite
5 - 6Hornblende
5 - 6Ilmenite
5 - 6Nephrite
5 - 6Turquoise
5 - 6Uraninite
5 - 5.5Monazite
5 - 5.5Titanite
5Apatite
4.5 - 5.5Wollastonite
44.5 - 5 or 7Kyanite
4Fluorite
3.5 - 5Magnesite
3.5 - 4.5Pyrrhotite
3.5 - 4.5Siderite
3.5 - 4Azurite
3.5 - 4Chalcopyrite
3.5 - 4Cuprite
3.5 - 4Dolomite
3.5 - 4Malachite
3.5 - 4Rhodochrosite
3.5 - 4Sphalerite
3 - 5Serpentine
3 - 3.5Anhydrite
3 - 3.5Witherite
3 - 3.25Bornite
3Calcite
2.5 - 3.5Barite
2.5 - 3
Biotite
2.5 - 3
Chalcocite
2.5 - 3
Copper
2.5 - 3
Gold
2.5 - 3
Silver
2.5 - 2.75Galena
2 - 3Muscovite
2 - 2.5Chlorite
2 - 2.5Cinnabar
2 - 2.5Halite
2Glauconite
2Sylvite
1.5 - 2.5Sulfur
1.5 - 2Gypsum
1 - 5Limonite
1 - 3Bauxite
1 - 2Graphite
1 - 2Molybdenite
1 - 2Pyrophyllite
1Talc

Mohs Hardness Assessing Procedure :

mohs-hardness-test

Mohs Hardness Test: When the test is carried out, place the unknown template on the tabletop and keep it from one place to another. Then place the point of reference template opposite the flat, unmarked facet of an unknown sample. Press the reference template against strangers, and firmly press and drag it to the flat facet. To avoid injury, remove the known sample from your body and parallel the fingers holding an unknown sample.

  • Start by looking for a simple, unhealthy surface for testing.
  • With one hand, hold the template of unknown hardness against the tabletop so that the facet tested is open and accessible. The table supports the top sample and helps you keep it stable for testing.
  • On the other hand, hold the standard hardness samples and place the sample point in front of an unknown sample’s selected flat surface.
  • Firmly press the standard sample point against the unknown template, and drag the standard sample point on an unknown sample facet.
  • Check out an unrevealed sample facet. Clean finger, any mineral pieces or powder produced by finger. What started with the test? Be careful not to melt the mineral powder or residue with scraping. On a scraping mineral facet, separate acne will be cut, not a facet impression that is cleaned.
  • Take a second test to confirm your results.

Some of the Mohs Hardness Testing Tips :

  • The list of minerals can be an easy allusion on the basis of hardness. If you decide that Mohs 4 acquire a hardness in one sample, you can rapidly obtain a list of feasible minerals.
  • While trialing this implementation and occurrence will enhance your wherewithal. You’ll get a quick and additional conviction.
  • If the strangeness of an unknown sample is 5 or fewer then you should be accomplished to generate it from scrape without ample effort. However, if there is the hardness of 6 or more in an unrevealed template, then some force will need to be created from the beginning. For those samples, hold the strings across the surface, put a standard template across it, press dynamically with the determination, then slowly push the pressure onto the facet of the unrevealed.
  • Do not be deceived by a soft quality template that targets strangers. The mark is that what a segment of the box constructs on a blackboard. It will clean off without abanding scraping. Clear your finger on the trialed facet. If the Scrape has been constructed, there will be an ability to be seen in the trench. If the marks were removed, a scrape was not created.
  • Some hard kinds of stuff are too extremely fragile. If your owned samples have a break or breakdown rather than scraping, then you need to be careful while managing the exam. Examining a small or rounded sample can be laborious.
  • Some samples include degradation. If the output of your trial is not obviously undeniable, or if your trialing information is not compatible with other belongings, do not temporize to re-examine. It is feasible that one little segment of quartz ( impurities) has been inserted in your templates.
  • Do not sink! This is the usual issue. A few people put a template against each other and then find a sign. It is not wherein it is tested. It is concluded with the aim of bitting from start to finish, with a fixed speed.
  • Be cautious. albeit you grip an unrevealed template across the table, transfer it to prevent the revealed template be on the fence be dragged into your fingers.
  • This trail should be concluded in a lab or a workbench with a long-lasting facet or a preservative cover. Do not do this kind of trailing on excellent furniture.
  • Examine the small fragments or cereals by positioning them betwixt two segments of intimation mineral and sliding them corporative. If the grain index is solid than the mineral, then the scrapes will occur. If the cereals are soft then they will be fragrant.

Here is the video of the Mohs Hardness Test.

BREAKING MYTHS :

Although the diamond is recognized worldwide as the hardest mineral, it is not really that way. Over time, more resistant materials have been discovered, but they are difficult to find. Scientific research has revealed that boron nitride, lonsdaleite, and even graphene, can be much harder than brilliant.

The same thing does not happen with talcum, because until now it is the softest mineral that exists. However, it is possible that some metals equal their weakness, such is the case of cesium, potassium, rubidium and also lithium, although it is unlikely that an evaluation of their hardness will be necessary, which determines the accuracy in each case.

Diamond Informations Other Resources :

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