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Techniques for Collectorsx-ray diffraction of iron minerals

20th Feb 2010 21:40 UTCDiana Di Leonardo

I'm doing a project using x-ray diffraction to identify changes in iron oxides and hydroxides over time. Does anyone know if using a cobalt anode as opposed to a copper anode to generate the x-rays changes the pattern? I am using a cobalt anode because a copper anode gives too much noise when starting with an initially mostly amorphous iron solid (2-line ferrihydrite).

Thanks

Diana

21st Feb 2010 00:56 UTCWarren Cummings Expert

As I remember from X-ray mineralogy, 40+ years ago, the principal line of copper radiation falls just within the absroptive band of iron. In other words Cu X-rays strike iron atoms in a mineral, the iron absorbs the X-rays and re radiates X-rays effectively forming an X-ray source inside the camera, defractometer or whatever. This can range from an annoyance to a total loss. I remember seeing film srtrips come out black with virtually no lines of the mineral visible. When this happened the alternative was a tube with a molybdenum target. This cured the interference problem but shifted all the lines. There were seperate tables for moly tubes as I'm sure there are for cobalt.

21st Feb 2010 12:53 UTCDavid Von Bargen Manager

The Bragg equation does contain the factor for the X-ray wavelength. The two theta will vary between using copper and cobalt wavelengths, but you end up with the same d spacings at the end of the calculations. There might be variations in the line intensities between the different radiation sources. If you want to use peak heights to determine percentages of minerals, you will need to calibrate your equipment (something that you would have needed to do anyway) as well as using some internal standards (quartz works well for this usually).

26th Feb 2010 04:39 UTCJohn Attard Expert

Diana, I am not sure I have anything to say that Warren and David have not already said. Copper radiation causes iron to fluoresce (in the x-ray region of course) and if there is a lot of iron this rise in background can get to be annoying or it may even inhibit effective use of the method. Using Cobalt radiation solves the problem. The lines will be shifted as the Bragg equation predicts. The d-spacings will come to the same value. There is freeware to convert if needed.

A practical difference in transferring from copper to cobalt ... cobalt tubes work at considerably lower power as the thermal conductivity of cobalt is much less than that of copper. Remember that most of the electricity you are putting into the tube (about 2 kW for copper) is going into heat that must be conducted to water. If you use the such settings for cobalt you can burn a crater in the anode, new tubes are 4 grand so check the power rating and scan slower if needed. Goodluck. Tell us how it works out!

14th Dec 2010 16:50 UTCJamesT

If you only have Cu tubes, is there any way to preven this increase in background from occuring?

14th Dec 2010 22:54 UTCPeter Nancarrow 🌟 Expert

I remember one trick to get around the problem of Fe specimen fluorescence when only a Cu X-ray tube was available, was to use a thin foil screen (so-called to distinguish it from the primary Kβ filter) between the specimen and the film/detector. The screen metal is chosen to have an absorption edge such that it absorbed the main peak of the secondary fluorescent X-rays, whilst allowing a useful amount of the diffracted primary radiation to get through. (In the case of Cu radiation / Fe-rich specimen I think the screen foil used was Ni).

The obvious downside to this method is that the intensity of the diffracted beam is greatly reduced, requiring longer exposure times/slower scans and there is still a poor peak/background ratio. This can be partly remedied in the diffractometer by using a wider receiving slit, buut this does of course give poorer spectrum resolution. On "modern" diffractometers (I speak as someone out of the X-ray anlytical world for >10 years!) this problem can be moderated by using a monochromator on the primary beam, preferably in combination with energy-discrimination at the detector, which is set to the optimum for detecting diffracted primary Cu Kα whilst minimising the Fe K fluorescence.

Pete N.

16th Dec 2010 14:51 UTCJamesT

Thank you so much! I appreciate it.

1st Mar 2011 01:49 UTCBart Cannon

The Fe tube, Cu tube, Co tube is a situation that I am currently suffering through.

I do quite a bit of work on iron minerals and have tried Fe and Co tubes in my ancient GE-XRD6 diffractometer under the supposition that the background in the patterns of iron minerals would be reduced. I have never observed much improvement regardless of what filters I used.

The filamnet in my Cu tube (Coolidge CA-7) burned out a couple of years ago. Since then I have been using my Fe tube. I have noticed some peuliarties which I did not expect. The relative peak intensities are different with the Cu tube vs. the Fe tube. Also, a few rogue peaks show up.

I actually had two Cu tubes. I cut a very old one apart. The Cu target had no red metal showing. Just dark gray metallic. It would seem that for many decades the filament had been evaporating tungsten onto the Cu target. This would suggest that the radiation from the "Cu" tube is actually a combination of Cu and W ! That could really mess things up.

I am now proceeding to create an x-ray tube with it's own vacuum system so that I can open it up periodically and clean my Cu target and replace the filament when necessary.

But if anyone wants to make me a trade for cash, minerals, standards or gear, I would like to obtain a CA-7 Cu tube at a modest price..

16th Mar 2011 23:51 UTCRalph S Bottrill 🌟 Manager

We used a Ni filter with a copper tube for many years, but switched to a graphite monochromater, which gave much better results (but greater expense of course)

18th Mar 2011 08:28 UTCŁukasz Kruszewski Expert

Using Co X-ray tube with K-beta filter should work well for Fe minerals (-:

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