Author Topic: Low totals in oxides  (Read 526 times)

theo_nt

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Low totals in oxides
« on: April 04, 2018, 01:19:52 am »
We have a problem in analyzing titanomagnetites. We always getting low totals, even if we assume that 2/3 of the Fe is Fe3+.

Our microprobe is a Cameca SXFive FE and the analytical conditions are 15 kV, 20 nA and the beam beam size 1 micron.

In the attachement you will find the detailed analytical conditions (.doc file) and some analyses (.xlsx file) to demonstrate the problem.

Also we observed a negative correlation between Ti and totals: The higher the TiO2 contain the lower the totals. We tried using different standards to calibrate Ti and Fe. Also we are using as a secondary standard a synthetic ilmenite. Calibrating Fe in FeO or even on almandine and Ti on rutile we always get a total of 100 wt% on this imenite.

Also we thought about peak shift. Calibrating Fe on EDS we are getting the same results. Low totals for titanomagnetite and good totals for ilmenite.

The only thing that we have not yet checked is the ABS coefficients. Here is the question:

Is it possible that the Cameca ABS coeficients (in SXFive FE) have a specific problem in the system Fe-Ti for the specific range of concentration applying to titanomagnetite?

I would appreciate any suggestion/comment

Theo Ntaflos
« Last Edit: April 04, 2018, 05:26:51 pm by John Donovan »

jon_wade

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Re: Low totals in oxides
« Reply #1 on: April 04, 2018, 07:46:16 am »
From memory alone, it looks like your backgrounds are too tight, especially for Ti (-550, +600)
« Last Edit: April 04, 2018, 05:27:05 pm by John Donovan »

Probeman

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Re: Low totals in oxides
« Reply #2 on: April 04, 2018, 08:59:16 am »
Hi Theo,
Would it be possible to plot up and post here some WDS scans showing the background positions of the elements?
john
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Mike Jercinovic

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Re: Low totals in oxides
« Reply #3 on: April 04, 2018, 03:48:43 pm »
Hi Theo!
A couple of comments... First, just looking over the data table you attached.  First, seems like the TiO2 contents of points 10 and 11 are high even for end member ulvospinel, so must be some exsolution with ilmenite or rutile (?) if this is supposed to be titanomagnetite.  Boundary fluorescence of Ti in fine Fe-Ti oxide mixtures can be problematic too. 

Second point, maybe a little out there, but magnetite-titanomagnetite can be magnetized, having picked up permanent magnetization especially if there is maghemite or other impurities that might increase the magnetic retentivity.  If they have been polished on a system using magnetic disks, that could do it too.  We don't do magnetite very often, but we have noted beam deflection from time to time compromising x-ray focus, and we wonder if the current flow from the beam could actually induce some of this effect under the right circumstances.  We have analyzed tetrataenite which is even more problematic, with some very dramatic magnetic field effects, but we could compensate by storing a beam deflection (in the setup) to get the beam back into correspondence with the position during calibration.  Very tedious because the fields would vary grain to grain.  So, anytime we think about probing ferromagnetic minerals, we consider beam shift as a possible variable.  As you note, your ilmenites seem fine, so appealing to MAC tables is unlikely.  Anyway, maybe beam deflection is a possibility?

theo_nt

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Re: Low totals in oxides
« Reply #4 on: April 05, 2018, 05:16:52 am »
I think the Ti backgrounds are not too narrow. Maybe the lower Fe background is little bit narrow.

John I attached the WDS scans for Fe and Ti.

Mike, you are write about the points 10 and 11.

As to the beam deflection seems not to be the issue. I attached the optical and the BSE images where the beam contamination is visible on the standard ilmenite and on unknown titanomagnetite. Both kind of images show the same point.

If there was a background problem why the EDS is giving as you see in the excle file I sent identical results?

ciao
Theo
« Last Edit: April 05, 2018, 08:40:56 am by John Donovan »

Mike Jercinovic

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Re: Low totals in oxides
« Reply #5 on: April 05, 2018, 06:28:21 am »
Theo, it would be interesting to compare the beam position between your Fe standard and some of the points with lowest totals, like 8, 9, or 42.  Not sure how to evaluate point 10 other than it is getting closer to an ilmenite composition anyway.

Probeman

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Re: Low totals in oxides
« Reply #6 on: April 05, 2018, 08:48:48 am »
The Fe backgrounds shown will likely cause low totals on unknowns assuming that the backgrounds on your Fe standard are wider.  Could that be the main problem?

Here's something else to check: what is the database composition of your ilmenite standard?  Have you entered the excess oxygen properly? What do you see if you analyze your ilmenite standard as an unknown?  Can we see the entered composition for your ilmenite standard and an analysis of the ilmenite standard as an unknown?

Or maybe even better, since you see an issue with increasing Ti content, can you try analyzing a TiO2 standard as an unknown using the ilmenite standard as the primary standard or perhaps the other way around?
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jon_wade

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Re: Low totals in oxides
« Reply #7 on: April 06, 2018, 02:00:02 pm »
Our LPETs is definitely 'flabbier' than the small PET (your set up has Ti on LPET?)
Fe backgrounds are too narrow for my taste.

The pictures though....the one on the left looks like the analysis crosses a boundary and the one one the right has cracks(?) or  something else (?)  under the spot. 

theo_nt

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Re: Low totals in oxides
« Reply #8 on: April 16, 2018, 04:24:12 am »
Sorry for not responding. The probe is for the whole month booked. So I will be able to do some tests according to your suggestions in May.
Theo