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Techniques for CollectorsEDS / WDS - Polished vs. unpolished samples?

23rd Mar 2016 05:44 UTCJoel Dyer

Hi Mindaters,

I was prompted to open this thread by a mention in a thread by Reiner. Pavel Kartashov mentioned in passing that for instance comparison of spectra can be effective only for polished and carbon-coated samples. http://www.mindat.org/mesg-11-377799.html

I know that I shouldn't stick to a few words, but I use this as an example only.

I have repeatedly read in EDS / WDS operating manuals, laboratory standards instructions, college literature and some articles that polished samples give often better quality results. There certainly seems to be valid reasons for the use of polished samples. Or am I half-blind or stupid, and misinterpreting information..?

...

Now, I've started to produce polished sections myself & am in the process of making my first "SEM button", to be followed by first trials of creating thin sections. Ore mineral samples are pretty much useless for inspecting with my reflected light'scope without preparation, so that's one reason I'm doing a lot of polishing work, even for pegmatite samples.

Like Donald Peck stated in one thread, it is a challenge & delight to try and identify some minerals oneself, instead of just sending them to a lab for an XRD or EDS / WDS. The important point is, using "old methods" one is forced to study and work really hard to learn new things. I am using microscopes plus microhardness testing and some chemical tests myself currently.

I spoke with a prominent ore geologist & he stated that the old methods were certainly used a lot before, but nowadays very little. It is so much more efficient, quicker and more reliable / more exact to use modern technology.

My problem is, I simply cannot afford the 100-150€ fee for SEM-EDS here, and don't always want to send samples to the other side of the world, though of course I will when possible. A Finnish company I've tested for XRD charges separately 60-70€ for polishing, then 20€ for carbon coating, required by the older equipment in use. Why should I pay - if I could - for polishing, if I can do it myself, down to at least 0,25 microns?

...

Now, I would like the opinion on 2 questions from experienced experts who have carried out hundreds, thousand or even tens of thousand of EDS / WDS tests.

1) Will it help if I send, or possibly get sent by another "more connected person", samples that are polished?

2) Should I try & make a "SEM button", if the material I have turns out suitable, of a specific size to make things easier?

Thanks in advace for any comments and recommendations. I take this hobby seriously, and will go to almost no end of trouble to develop my skills and better my understanding, with the help of experienced professionals ad amateurs.

Cheers,

23rd Mar 2016 06:11 UTCPhil M. Belley Expert

Yes, polishing and carbon coating will greatly improve the accuracy of results in EDS in the majority of cases.

You refer to XRD but mention polishing and carbon coating. This is not XRD (powder or single crystal, no polishing or coating). Do you mean WDS?

When I didn't have access to EDS, I used Kerry Day's service based in Ottawa, Canada. I think the price is $10 now, per grain. Samples are unpolished, uncoated, but you can get a rough idea of what you are dealing with and Kerry is good at IDing even unusual minerals.

Unless the accuracy is very important to you (i.e. identification of rare species, complex minerals, fine grained mixtures, or where a mineral sits relative to different end members), qualitative EDS should be fine. If you have more sample (say the size of a rice grain), you can also have it analyzed with X-ray powder diffraction. I think one XRD service is advertised here on mindat. Of course, there are also limitations to what both techniques can tell you.

23rd Mar 2016 08:28 UTCJoel Dyer

Hi Phil,

Well, your answer certainly came quickly, thanks for the help :-). It is imperitive for myself to hear comments from the practical side of issues, not just thoughts based on theory.

I only mentioned XRD in connection with EDS, because the same company here offers both services, and others as well. I'm well familiar of the way XRD works.

Yes, I've been thinking of sending some of my samples in for EDS in CAnada, partly because I was born and lived there on 3 separate occasions ;-) . As for XRD, Attard has kindly analyzed some of my samples in California & surely I'll use his services again. I just need to line op some useful samples, instead of metamict REE's ;-)

Cheers,

23rd Mar 2016 10:08 UTCFrank Craig

Hello:

The simple answer is to your question is yes - a polished surface is desirable, coated with carbon, gold, paladium, or other conductive material (I prefer carbon). The EDS/WDS is designed around x-rays being generated by the beam hitting a flat surface, at an angle of 90 degrees. This will give you the most accurate quantitative results. But, meaningfull EDS results can also be obtained by hitting a particle with a beam - just be aware of the pitfalls/issues accompanying the spectra.

Hope this helps

Regards,

Frank

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