Assaying, Microscopy, Mineralogy & XRF/XRD

Assaying, Microscopy, Mineralogy & XRF/XRD 2017-03-23T09:37:54+00:00
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Amorphization in Grinding (6 replies)

John Koenig
1 year ago
John Koenig 1 year ago

I had an interesting discussion with an applications specialist in XRD who was asked a question regarding 'amorphization' by inappropriate grinding. Their response was that this is a red herring, stating the age-old argument that peak-broadening is all that happens. I do not agree and would very much like to hear some opinion from users out there. Of course, 'amorphization' may be poorly posed/defined here. The question of a phase being rendered amorphous does not necessarily imply it is totally destroyed, just 'non-diffracting' under the conditions, not so? What else is germane to this discussion/issue? Am I being pedantic or does non-diffracting really mean amorphous? 

Bill Fraser
1 year ago
Bill Fraser 1 year ago

As someone who spent a lot of years aiming to amorphise minerals and assorted other materials by grinding I have a fair idea of what is going on. It is possible to amorphise using grinding, however the time and energy required are extremely high and well above the conditions used in a classic ball mill. The presence of a liquid, most commonly water, but also organic liquids will greatly reduce the energy of ball impacts and seems to essentially prevent amorphisation. It is also true that softer materials grind more quickly than hard materials. As a consequence of all these factors, it seems most unlikely that any amorphisation occurs in commercial scale grinding mills as the residence time is much too short.

As to the XRD side of things. As the crystallite size (as opposed to particle size - most particles are not single crystals) decreases the unit cell size of the phase varies around the standard d-spacing and as the particles get smaller the variation increases and the peaks become weaker and broader. Eventually the peaks are so broad and weak they become essentially indistinguishable from background. This is often combined with an elevated background due to fluorescence with some metal x-ray combinations so it can be hard to separate the two effect unless the radiation is changed and the scan rerun. For copper radiation broad peaks (>3-5deg wide) around 30 and 50deg 2-theta are taken to indicate the presence of amorphous material, but these can be hard to distinguish.

True amorphisation is actually quite unusual, especially in natural mineral systems, what is generally called amorphous is simply a phase which with small crystallites which do not show the classic peaks from the database. This can sometimes be overcome by running the scan with longer count times as the signal to noise increases with the square root of time, i.e. to double S/N requires 4x count time. So, a short count time will reduce both minor and low crystallinity phases into the background.

I hope this is clear, and answers at least some of your questions.

John Koenig
1 year ago
John Koenig 1 year ago

You touch on the real issue which is peak attenuation/weakening AND broadening, which is more pronounced in softer minerals. This is why the Micronising Mill aims, at a scaled down analogue to ball milling, to reduce particle size to under 10 microns, while not ultra-pulverizing passing 1 to 3 micron material into the angstrom range.

Victor Bergman
1 year ago
Victor Bergman 1 year ago

Just for clarification: as stated before, the term amorphisation is probably not a good choice. We use X-ray scattering amorphous to indicate that the material's coherent scattering domain size has decreased to a level where it will not produce 'peaks', no matter how long one measures, as the statistical noise form the instrument and all components is larger than any coherent scattering one might observe.

And contrary to some comments, I have seen the diffraction pattern of a phase 'disappear' with grinding, but these were two special cases. one where a a combination of sulphides where ground together, producing an alloy phase of different composition and on where a metastable mineral phase was reduced to the level where the transformation energy to the stable phase was achieved in the grinding circuit. From my observations, I will always have some concerns if a sample containing 'hard' and 'soft' minerals and possibly some with strong preferred cleavage, has been 'overground', as semi quantitative XRD analysis in this case can no longer provide accurate quantities of the mineral phases present in the original sample.

John Koenig
1 year ago
John Koenig 1 year ago

You point out nicely the relationship between coherent scattering domain size, crystallite size & grinding: thanks

Alan Carter
1 year ago
Alan Carter 1 year ago

When we first received a miconizing mill we ran a number of tests grinding pyrrhotite for various time increments. At 10 minutes, the diffraction peaks for the pyrrhotite completely disappeared - nothing to see but background. At 8 minutes, there were weak very broad peaks. I was convinced we could destroy the crystallinity by grinding. I was thinking that this effect is analogous to the formation of pseudotachylite in fault gouge.

John Koenig
1 year ago
John Koenig 1 year ago

I have seen this effect when grinding dry versus grinding with an appropriate fluid (ethanol). It falls within the 'common pitfalls and myths' learning experiences. Nice share.

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