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Different Types of Hardness Measurements

Posted by Kyle Bayliff  
Kyle Bayliff October 11, 2018 03:56PM

I am curious whether hardness measurements other than Mohs scale measurements are used commonly for mineralogical specimens. I'm aware for engineering applications, hardness is measured by the Vickers or Brinell hardness tests, which quantify a materials resistance to indentation. Given that the Mohs scale is merely qualitative, it seems like it would be much less useful than one of these for systematic studies (for instance measuring differences in hardness of a particular mineral based on locality). Are these tests used at all in mineralogy? We could also discuss whether Brinell or Vickers hardness values can translate directly to particular values on the Mohs scale as well. I imagine there are some fundamental differences in resistance to scratching and indentation, so I'm not sure it translates easily, but I assume there is at least some correlation.
Kevin Hean October 11, 2018 04:41PM
Hi Kyle
Vickers, Brinell and Rockwell etc are all based on measuring the hardness of a pliable material, as you mention the measurement of an indentation caused by the test. The Leeb Hardness Test, a method I am not familiar with, measures how much a hardened ball rebounds when dropped from a certain height. I am sure none of these tests would be able to be applied to minerals especially in their crystalline form.
Knoops Hardness Test, although this is also an indent type test, it is designed to measure the hardness of brittle materials. I have read about it being used on glass and Quartz boules in industry, but have never witnessed such a test.
You must remember that there is a big difference between hardness and toughness, Glass is hard but easily broken, your motor car tyre is is not hard but you can't break it, a Diamond is one of the hardest things on earth but is easily crushed with a pair of pliers.
Alfredo Petrov October 11, 2018 04:44PM
Those more accurate hardness testing methods can be used on microscopic grains in polished ore mounts.
Peter Nancarrow October 11, 2018 05:45PM
Minerals do not vary in hardness with locality - it is a fundamental and invariable property of a given mineral's crystal structure.

I guess that hardness may vary with composition within some mineral series (I can't think of an example offhand) but then any variation in hardness would be due to those differences in composition, possibly with associated subtle structural differences, and therefore you would not be dealing with a "particular mineral" species.

Difference in hardness between crystal faces/directions of a particular mineral is however a very significant property. e.g. kyanite has a huge difference in scratch hardness across the width of a prism face compared to along the length of the same face, and if it wasn't for the difference between the hardness of the 111 (octahedral) direction and the 100 (cube) in diamond, it would be impossible to polish it using diamond powder, and nothing else can. (Except maybe some of the recently synthesised super-hard materials such as various nitrides and borides).

Pete N.

Edited 1 time(s). Last edit at 10/11/2018 05:55PM by Peter Nancarrow.
Kyle Bayliff October 11, 2018 09:06PM
Thanks to everyone for the responses so far. If anyone has some literature suggestions on the topic as well, I'd appreciate those as well. Just a couple of points I want to discuss:

Peter mentions that the hardness is a fundamental property associated with a mineral's crystal structure, but this is not quite correct. As least from a physics perspective, there are many other factors that contribute to hardness (as measured by Brinell and Vickers tests) such as surface electron density, composition, temperature, pressure, etc. Though the crystal structure does have a significant effect, it is not the only determining factor, so there is at least the possibility for variation at different localities based on small variations in composition (chemical doping or micro-inclusions), internal strain, etc. produced by the natural growth conditions in the area. The idea of variation based on locality was simply a suggestion, however.

Alfredo also reminds me that there are different hardness tests for different size scales as well. Some tests even use nano-sized indenters in order to deconvolute the effects of micro- and nano-structuring on the measurements. I wonder if mineralogists are interested in this sort of thing?

Just to restate, I'm simply curious if systematic studies of hardness are common in mineralogy, and what sort of quantitative approaches are used. Thanks again to everyone for the input.
David Von Bargen October 11, 2018 09:35PM

Orientation of the crystal is important in determining hardness. There are correlations between the different hardness scales, but the various techniques tend to only handle partial ranges.
Mineralogical hardnesses tend to be measured on the Mohs scale or Vicker hardness. It isn't a particularly diagnostic test, so tends not to be used much except as a general test. A lot of other factors can come in determining if something is scratched. If you make up a polished section now, it is just as easy to pop it under a microprobe as to test the hardness.

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