An Updated And Extended Mohs Mineral Hardness Scale printable pdf download

An Updated and Extended Mohs Mineral Hardness Scale

Friedrich Mohs (1773-1839), an Austrian/German mineralogist, developed a scale of hardness for

minerals in 1812 (

). This hardness scale classifies minerals on a scale from

although some sources reference 1822

one (the softest) to ten (the hardest) and has been used by mineralogists since its inception. The minerals

Friedrich Mohs selected for his scale were natural, pure substances which were commonly or readily

available at the time. Basically – it tests to see how easily one substance can scratch another. For example;

if your material to be tested can be scratched by Fluorite (hardness of 4) but not scratched by Calcite

(hardness of three), then your test material would have a hardness of 3.5 on the Mohs scale.

Although the Mohs Mineral Hardness scale appears to be Linear (one to ten), it is not – it’s a Relative

scale. Which is why the “absolute scale” was developed. If you look at the Mohs scale, it would appear that

Corundum (9) is one-half as soft as Diamond (10), when in actuality, Corundum is four-times as soft. That

is why scientists who required a more accurate measurement of hardness developed a “proportional

measurement” known as the “absolute mineral hardness” to account for this (non-linear) discrepancy in the

Mohs traditional scale by using a very sensitive piece of equipment called a “sclerometer. The sclerometer

accounts for the large relative difference in hardness as one approaches the higher end of the Mohs scale.

It should be noted that Friedrich Mohs was not the first person to use this method of comparing

hardness by observing which mineral scratches another. Indeed – some references suggest that Friedrich

Mohs obtained the idea from the miners of his time. Plus, comparing mineral hardness has some roots in

antiquity. It was first mentioned by Theophrastus in his treatise On Stone, circa 300 BC and later followed

by Pliny the Elder in his Naturalis Historica, circa 77 AD.

Additionally, the traditional Mohs scale did not account for half-number hardness values. For example;

Dolomite will scratch Calcite (hardness of 3) but not Fluorite (hardness of 4). Therefore, how would you

classify Dolomite on the traditional Mohs scale since it is neither a 3 or a 4? The answer was to add the

half-number values to the scale. Now, Dolomite would be classified as a 3.5 on the hardness scale.

Lastly, the hardness of some minerals vary based upon how the hardness test is performed – with the

grain or against the grain. A good example of this is Kyanite, which usually is a long bladed crystal. When

you scratch Kyanite across the grain (the short side of the blade) it has a hardness of 7. However, when you

scratch it with the grain (longways down the blade) it has a hardness of 4. Muscovite is another good

example of this dual-hardness property. Muscovite has a hardness of 2.5 when taken across the flat surface

of a cleavage sheet, but has a hardness of 4 when taken across the grain. Basically this dual-hardness

phenomenon depends upon the strength of the molecular bond that holds the mineral together.

NOTES:

1. Data taken from Discover Magazine, July/August 2009, Page 12, Physics Beat by Adam Hadhazy. The

content of that article is contained within the box that follows…

HOW TO SCRATCH A DIAMOND

The reputation of diamond as the hardest material around is under threat. Researchers in China and

the United States recently determined that two naturally occurring substances surpass diamond’s

resistance to scratching and indentation. They calculated that the mineral Ionsdaleite – made of carbon,

like diamond – is 58 percent harder than its famous cousin. And, Wurtzite boron nitride beats

diamond’s hardness by about 18 percent after being subjected to pressure, which alters its atomic

bonds.

Still, in the short term diamond will continue to dominate in practical applications such as saws,

drill bits and industrial abrasives, since the newly studied materials are extremely rare. Ionsdaleite

forms only under the extreme pressure and heat accompanying meteorite impacts, while Wurtzite

boron nitride is a by-product of intense volcanic eruption. But scientists can create both substances in

the lab, says physicist John Janik at the Carnegie Institution for Science. Although producing the

conditions required to grow the substances in bulk remains a challenge, Janik and others are working

on synthesizing Wurtzite boron nitride and Ionsdaleite to pave the way for commercial use.

Garry W. Kappel, President – Sedona Gem & Mineral Club, Inc. (2011)

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An Updated And Extended Mohs Mineral Hardness Scale

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