Quantitative Apatite Analysis

[EricKelly]

Hi Everyone,

I plan to analyze many (~50) apatite crystals spanning the full range of F and Cl proportions, and I am hoping for advice on an efficient quantitative protocol that includes several trace elements as well (e.g., La, Ce, Mn).

I have read Goldoff et al (2012, Am Min v97 p1103-1115) and Henderson (2011, U Michigan MS thesis), and I have spoken with several people on the topic.  Most agree on these:

• c-axis parallel to beam
• large spot diameter (I'll try 10 microns)
• low current (I'll try 5-10 nA)
• 15 kV to get the REEs
• fresh surfaces (not damaged by previous analyses) on both standards and unknowns

However, it sounds like there are two camps on the topic of counting time:

• long counting times on the order of 10 minutes are required for quality results, especially for F-rich grains
• short counting times (30 or 60 seconds per element) with TDI correction give quality results even with F-rich compositions

I am mostly interested in advice on counting time (do you agree with 1 or 2?) but other thoughts are certainly welcome.  Thanks!
(Instrument: JEOL 8200)

[Probeman]

As long as you use a Time Dependent Intensity Correction (TDI), and perform that correction on *both* your unknowns and standards, you shouldn't be required to orient the crystals for quantification.

http://probesoftware.com/smf/index.php?topic=11.0

[Anette von der Handt]

Hi,

I just finished a big batch of apatite analyses, the following is my routine for apatites in carbonatites and phonolites (so you may want/need other elements) on a JEOL 8900. Overall, I go with approach 2.

Analytical conditions: 15kV, 10 nA and a beam diameter of 15 µm

Element setup:
LDE1: F
PET: Ca, P, S, La, Ce, Cl
TAP: Si, Mg, Na, Sr
LIF: Fe, Mn

Counting times: 60/30 (Pk/bkgr) for F and Cl, 40/20 for Fe, all other 30/15

PFE specific settings:
Measurement options: TDI (but I only use it for F in the end; TDI also on the standards), Asynchronous mode, pick background position and models from a good wavescan
Analytical options: Halogen correction

I usually re-calibrate every six hours at least the apatite standard (Durango) to track any drift. I also carbon coat the standards together with the samples if possible.

Another good paper to read is also: Marks et al. 2012: The volatile inventory (F, Cl, Br, S, C) of magmatic apatite: An integrated analytical approach. Chemical Geology 291, 241 - 255.

I hope this helps. Enjoy your apatites!

[EricKelly]

Thanks for both of your replies.  I'll let the forum know if I find anything else to add.

[chenderson]

1) Durango apatite is heterogeneous and is thus not recommended as a standard.  Instead, use F-topaz or MgF2 for the F standard.  These two materials are much more robust than any fluorapatite and show very limited F decay with beam exposure.  Synthetic MgF2 should be easy to find.

2) Performing TDI corrections on both the standard and unknown does not assure good results, since the shape of the TDI curve changes with sample crystallographic orientation.  The TDI curve shapes are also complex and not conducive to fitting.  Eliminate at least one source of error by using a robust standard (see above).

3) As Cl concentration increases, you have a better chance of getting good analyses, but I am skeptical of any results on fluorapatites, unless I know that the sample was analyzed with the beam perpendicular to the c-axis.  If the sample is hexagonal, move on. 

4) In general, there will be a systematic over-estimation of F with TDI back-calculation.  The literature is full of EPMA apatite analyses with higher F concentrations than are possible in end-member fluorapatite.

Regards,
Carl

[John Donovan]

2) Performing TDI corrections on both the standard and unknown does not assure good results, since the shape of the TDI curve changes with sample crystallographic orientation.  The TDI curve shapes are also complex and not conducive to fitting.  Eliminate at least one source of error by using a robust standard (see above).

Hi Carl,
The reason for performing TDI curves on both the unknown and standard is *exactly* because the crystallographic orientation will generally be different between the unknown and standard!

Now the TDI extrapolation may not always be "conducive" to fitting, but compared to trying to orient one's unknowns to the same crystallographic orientation as the standard, it is quite easy. Also, PFE has an enormous number of TDI fitting options so one can usually do a very nice TDI extrapolation to zero time and Eric is using PFE.

4) In general, there will be a systematic over-estimation of F with TDI back-calculation.  The literature is full of EPMA apatite analyses with higher F concentrations than are possible in end-member fluorapatite.

Yes, true. But this is generally *not* due to the TDI overfitting, instead it is due to the matrix correction being miscalculated by assuming stoichiometric oxygen when the fluorine content is replacing some of that stoichiometric oxygen in the mineral.

How could this matter? Well F Ka is highly absorbed by oxygen, so if the matrix totals say, 106% due to excess stoichiometric oxygen, then the F concentration is overestimated by around 15%.

See here for details on this interesting (and unpublished!) method to automatically subtract the oxygen equivalence of all measured (or specified!) halogens during the matrix iteration:

http://probesoftware.com/smf/index.php?topic=81.msg292#msg292

For the above post, scroll all the way to the bottom for the halogen example.

http://probesoftware.com/smf/index.php?topic=8.msg1127#msg1127

[EricKelly]

1) Durango apatite is heterogeneous and is thus not recommended as a standard.  Instead, use F-topaz or MgF2 for the F standard.  These two materials are much more robust than any fluorapatite and show very limited F decay with beam exposure.  Synthetic MgF2 should be easy to find.

Thanks for the input, Carl.  I have both Durango Apatite and F-topaz.  A previous user in our lab said that F in our F-topaz standard migrates much faster than Durango Apatite.  I did not see the data myself, so I will do my own testing to be sure of the better standard.  The heterogeneity in Durango Apatite is certainly a problem.  Perhaps I will map our grains with hopes of finding a good portion of the standard (and then repolish).

... I am skeptical of any results on fluorapatites, unless I know that the sample was analyzed with the beam perpendicular to the c-axis.  If the sample is hexagonal, move on. 

The orientation debate is difficult to assess given that several studies seem to report different conclusions.  However, many of my samples are already oriented – they come from a previous collection – so I don’t have much choice anyway.

[EricKelly]

In Anette’s post, she suggests asynchronous mode but in Probewin.pdf (e.g., p. 216, 218, 252) synchronous is the suggested mode when using the TDI correction.  Has anyone noticed a significant difference between modes?  I would like to keep the analysis fast so asynchronous is my preference, but not if it degrades the TDI correction significantly.

Thanks

[Probeman]

In Anette’s post, she suggests asynchronous mode but in Probewin.pdf (e.g., p. 216, 218, 252) synchronous is the suggested mode when using the TDI correction.  Has anyone noticed a significant difference between modes?  I would like to keep the analysis fast so asynchronous is my preference, but not if it degrades the TDI correction significantly.

My word! Someone is actually reading the documentation! 

Actually I have to apologize because the documentation is slightly out of date on that point. It used to be that to get the interval between removing the faraday cup and starting the counting as short as possible, it was necessary to utilize synchronous mode. But not anymore.

It might also be true that this interval is now still slightly shorter using synchronous mode, but I believe that the asynchronous mode is almost as fast if not as fast.

Try turning on the "Debug Mode" and "Time Stamp Mode" in the Output menu and see exactly what the timing differences are- would be worth as post.

I should also note that some investigators such as Stuart Kearns rightfully warn of a non-linearity in the first few seconds of counting in some alkali glasses because the Na and K ion migration doesn't really get starting until the sample heats up from beam exposure. See here for more discussion:

http://probesoftware.com/smf/index.php?topic=116.msg454#msg454

Not sure if that applies to F in apatites though...

[BenjaminWade]

Hi all
Quick question with regards to MgF2. Where have people sourced this from? I can easily find MgF2 on chemical companies like Sigma Aldrich, but should I be concerned about the purity? Should I be trying to source a single crystal of MgF2?
In addition, has anyone had any success using the Uni of Edin REEFluoride glasses as F standards for apatite analyses?

Cheers

[Jeremy Wykes]

MTI corp has substrates that would be good for standard material:

http://www.mtixtl.com/crystalssubstratesa-z.aspx

Single crystal, oriented, high purity (99.99%).

Unfortunately, many of the substrates must be purchased in multiples of 4 or more. Are any other labs interested in this material? Our standard collection is so dire that I am thinking of getting most of the simple oxide substrates.

[BenjaminWade]

Interesting link. Thanks Jeremy, I will have a peruse of that website.

Also to all, with regard to my earlier question about the Uni of Edin REE Fluoride glasses....ignore that, I had a brain fade, they are silicate glasses. However I know Astimex has some LREE Fluoride glasses (which we have). Apart from purity/stoichiometry issues, does anyone have a feeling for F migration in REE fluorides?

Cheers

[BenjaminWade]

Thanks Owen. Unfortunately we don't have MgF2 on our Taylor block, but we do have PbF2, SrF2, and BaF2 on it. Sounds like I will have to do some TDI tests on these and the Astimex REE F2 to see if any of them are suitable.

[BenjaminWade]

Ah yeah, didn't think of that. Looking at the MACS it lookes like BaF2 and possibly the REEF2 might be the best of a bad bunch out of that lot. But yes, if you are listening Jeremy, I would be interested in getting some MgF2 from that company if you wanted to go in together. I will have a proper look at the website and see if there is anything else we might need.
Thanks all.

[Jeremy Wykes]

I have contacted them about getting raw material that is not wafered, oriented, polished or clean room packed.

Let me know what phases you are interested in and I will add that to my list.