Want to measure the hardness of a mineral? In 1812, German mineralogist Friedrich Mohs developed a simple scale. He ranked then readily available minerals and scratched them on a scale of 1-10, 10 being the hardest.
The Mohs scale is till now one of the easiest ways to measure hardness of a mineral.
The scale includes then readily available reference minerals, ranked from 1 to 10 by their hardness. The lowest-ranked is talc, and the highest is diamond, which was the hardest mineral known in 1812.
It is not a linear scale – diamond is four times harder than corundum – but a ranking scale, comparing minerals to each other.
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How the Mohs scale works
Let’s suppose you discovered a new mineral and you want to determine its hardness. Take your newly found mineral and scratch it against any available mineral in the Mohs Scale chart. Hold the mystery mineral firmly on a flat surface, with a smooth, unscratched side facing up. Taking the testing mineral and firmly scratch a pointed bit of it across the surface of the mineral you are testing. Brush away any dust and look for scratches.
If A scratches B, A is harder. In the scenario that A does not scratch B, then B is harder. If neither one is scratched, they are about the same. If A scratches B but does not scratch C, then the hardness of A is between B and C.
Repeat the test to confirm your results. Continue through the sample kit until you find the point where one sample does not scratch, and the next one does. Your mystery sample’s hardness ranking is between these two. Harder minerals – the ones in the upper half of the scale – can require forceful pressure to test. Be careful to scratch away from yourself and your fingers.
The ten reference materials are:
- Orthoclase feldspar
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Mohs Hardness Testing Tips
- A lot of minerals in order of hardness can be a handy reference. If you determine that a specimen has a hardness of Mohs, four can quickly get a list of potential minerals.
- Practice and experience will improve your abilities when doing the test. You will become faster and more confident.
- If the hardness of the unknown specimen is around five or less, you should be able to produce a scratch without much exertion. Moreover, if the unknown specimen has a hardness of around six or greater, then producing a scratch will require some force. For those specimens, hold the unknown firmly against the table, place the standard specific against it, press firmly with the determination, then holding pressure slowly drag the standard specimen across the surface of the unknown.
- Don’t be fooled by a standard soft specimen producing a mark on a hard unknown. That mark is like what a piece of chalk produces on a blackboard. It will wipe off without leaving a scratch. Wipe your finger across the tested surface. If a scratch was produced, there will be a visible and transparent groove. If marks wipe away, then a scratch was not even produced.
- Some hard materials are also very brittle. If one of your specimens is breaking or crumbling rather than scratching, you will have to be very much careful while conducting the test. Testing tiny or granular specimens can be difficult.
- Some specimens contain impurities. If the results of your test are not visibly conclusive, or if the information from your test does not conform with other properties, do not hesitate to do the test again. It is possible that a small piece of quartz was embedded in one of your specimens.
- Don’t be wimpy! This is a very common concern. Some people even casually rub one specimen back and forth against another and then look for a mark. This is not how the test is done. It is even done with a single, determined motion with the goal of cutting a scratch.
- Be careful. When you most of the time hold the unknown specimen against the table, position it so that the known specimen will not be pulled across one of your fingers.
- This test should be done on a lab table or workbench with a durable surface or a protective covering. Don’t do this type of testing on the proper and fine furniture.
- Make sure to test tiny particles or grains by placing them between two pieces of an index mineral and scraping them together. If the grains are highly harder than the index, mineral scratches will be produced. If the grains are softer, they will smear.
Q1. What is the Mohs scale mainly used for?
Ans. It is a measure of the resistance of a smooth surface to scratching or abrasion, expressed in terms of a scale advised by the German mineralogist Friedrich Mohs. The Mohs hardness of a mineral is mainly determined by observing whether its surface is scratched by a substance of known or defined hardness.
Q2. Which is the highly hardest mineral on the Mohs scale?
Ans. Hard cannot be easily scratched by a knife but can scratch glass, Mohs 6-9; Diamond is the hardest known mineral, Mohs 10.
Q3. What are the limits scales of the Mohs scale?
Ans. The test was developed by mineralogist Friedrich Mohs in 1820. It is based on absolute scratch hardness; with talc assigned a value of 1 and diamond assigned a value of 10. The Mohs scale had two limitations; it was not linear, and most modern abrasives fall between 9 and 10.
Q4. How hard is Jade on the Mohs scale?
Ans. Jadeite is 6.5 to 7, and nephrite is 6 to 6.5 on the Mohs hardness scale, so some care should be taken to avoid scratches. Moreover, both of these gemstones have exceptional toughness and are very resistant to breaking or chipping. The heat from a jeweler torch can hard jade.