I have recently been contacted by a post-doc asking about accuracy issues in EPMA and how they might relate to problems they are seeing in the P/T results in pyroxene systems. These are excellent questions! The following is part of my response to these questions.
I didn't realize that the Na concentrations for these OPX/CPX thermo/barometers were so low. Knowing this now, here's the deal with EPMA accuracy (assuming no instrumental problems!):
1. For major concentrations what matters are the standard accuracy and the matrix correction accuracy. I don't think the matrix correction accuracy is a problem with modern software (see below). But the heterogeneity in the Smithsonian standards is a well known problem. In fact I think Ed Vicenzi has characterized some of these issues in the past.
2. For trace concentrations what matters are the background corrections (and interference corrections if necessary). It is interesting to note that labs using JEOL EPMA instruments (which has roughly an 80% market share) have no way to correct for spectral interferences. That is unless they utilize software from Probe Software.
Bottom line: the above two points make sense if you consider that the matrix corrections are multiplicative and scales (as does standard accuracy) with concentration (so the smaller the concentration the smaller the absolute accuracy error). That is, a 1% relative error in accuracy is 1 wt% at 100 wt%, but only .1 wt% at a 10 wt% concentration, and only 1 PPM at 100 PPM.
While the background correction is subtractive and is a constant source of error (so the smaller the concentration the larger the accuracy error). That is, a 100 PPM error in the background estimation is a 10% error at a 1000 PPM concentration, but it's a 100% error at 100 PPM and a 1000% error at 10 PPM. This is worth thinking about a bit- it's not obvious. If it was obvious I wouldn't be having to rant about this for so many years! 😁
For minor elements it's a bit of both considerations (standards/matrix and also backgrounds/interferences), but mostly background correction, with some influence from the matrix correction. And also spectral interferences depending on the elements in question. For this system I don't think spectral interferences are an issue, as the only other elements that would interference seriously with Na are Zn and a little from P (but these should elements be present in very low concentrations in most augite compositions, I think?).
Some might suggest a low Na pyroxene standard might fix this accuracy problem and that is the traditional, but wrong approach in my opinion. For highest minor/trace element accuracy in pyroxenes what we should want, is an approximately matrix matched standard with a *zero* Na concentration. A non-zero minor/trace (and homogeneous) element standard is very difficult to obtain and to characterize.
What is much easier (and better) is a "blank" standard roughly similar to ones unknown (or even a glass with similar elements) that has *zero* (say, less than 1 PPM) Na present. This we can determine easily (e.g., ICP-MS, SIMS) and then our minor/trace accuracy is as good as our measurement precision (think about this, as it's a gift from the science gods). Basically we are simply testing our background correction accuracy by using a suitable blank standard run as an unknown. This is all spelled out in the Donovan et al., 2011 Amer. Min. paper.
In the Probe for EPMA software, this "blank" correction is applied iteratively during the matrix correction for highest accuracy. It can also be subtracted out after the fact in an Excel spreadsheet manually, as long as the composition doesn't change too much from the blank correction. This is usually the case.
I know this is not what we were taught in grad school, but it's the way forward in this current mess of heterogeneous natural standards. As an example, I couldn't find a pyroxene analysis, but here's an orthoclase analysis with around .8 wt% Na showing all the various matrix corrections in the CalcZAF software:
Elemental Weight Percents:
ELEM: Na Si K Al Mg Ca Ti Mn Fe P O H Ba TOTAL
1 .768 30.520 12.862 8.625 .000 .014 .001 -.005 1.355 .000 45.798 .000 .054 99.991 Armstrong/Love Scott (default)
2 .764 30.621 12.864 8.571 .000 .014 .001 -.006 1.383 .000 45.798 .000 .054 100.064 Conventional Philibert/Duncumb-Reed
3 .768 30.419 12.861 8.623 .000 .014 .001 -.005 1.335 .000 45.798 .000 .054 99.869 Heinrich/Duncumb-Reed
4 .767 30.577 12.863 8.594 .000 .014 .001 -.006 1.356 .000 45.798 .000 .054 100.017 Love-Scott I
5 .768 30.547 12.863 8.621 .000 .014 .001 -.006 1.355 .000 45.798 .000 .054 100.015 Love-Scott II
6 .756 30.879 12.866 8.561 .000 .014 .001 -.006 1.375 .000 45.798 .000 .054 100.298 Packwood Phi(pz) (EPQ-91)
7 .763 30.668 12.862 8.497 .000 .014 .001 -.005 1.343 .000 45.798 .000 .054 99.995 Bastin (original) Phi(pz)
8 .768 30.561 12.862 8.547 .000 .014 .001 -.006 1.378 .000 45.798 .000 .054 99.978 Bastin PROZA Phi(pz) (EPQ-91)
9 .767 30.616 12.863 8.554 .000 .014 .001 -.006 1.377 .000 45.798 .000 .054 100.037 Pouchou and Pichoir-Full (PAP)
10 .764 30.630 12.863 8.570 .000 .014 .001 -.006 1.378 .000 45.798 .000 .054 100.067 Pouchou and Pichoir-Simplified (XPP)
AVER: .765 30.604 12.863 8.576 .000 .014 .001 -.006 1.364 .000 45.798 .000 .054 100.033
SDEV: .004 .119 .001 .041 .000 .000 .000 .000 .017 .000 .000 .000 .000 .109
SERR: .001 .038 .000 .013 .000 .000 .000 .000 .005 .000 .000 .000 .000
MIN: .756 30.419 12.861 8.497 .000 .014 .001 -.006 1.335 .000 45.798 .000 .054 99.869
MAX: .768 30.879 12.866 8.625 .000 .014 .001 -.005 1.383 .000 45.798 .000 .054 100.298
As we can see, the variance in the Na matrix corrections are very small (40 PPM), so the main problem with Na concentrations in the cpx/opx barometer is the heterogeneity of the Smithsonian "standard", and the background corrections, where I'm guessing many of us haven't tested our ability to measure zero Na in a roughly similar pyroxene matrix.
I did a quick search in my standard database to the Kakanui augite compositions and here is what I came up with:
(https://smf.probesoftware.com/gallery/395_18_10_21_1_10_32.png)
So this Dahlheim glass is a close match, but this was a glass produced by Dan Weill at Oregon, so yes, I have a fair amount of it (which I shared with Paul Carpenter at Washington University), but is the Na content truly zero? I don't know, but it could be characterized using ICP-MS/SIMS... anyone interested in doing this for us?
The other close match to an augite composition is the NIST K-412 glass which nominally contains no Na, but I seem to have reported some 500-600 PPM of Na in this material some decades ago. Is that accurate? I have no idea. NIST does not report it in their certificate. Has anyone out there measured trace elements in K-412 (or K-411) using ICP-MS or SIMS?
Hi John,
I agree with you, the Kakanui augite has some natural variation... We regularly use it as a secondary standard (NEVER as a primary!) for checking our regular pyroxene analyses (along with other secondary standard). We don't rely on the published value, but rather on the reproducibility over time (session after session...). Data quality is somehow ensure by looking at the totals and also the atomic proportion. From the analyses we have, it looks like there are variations of Al and Fe-Mg content. However, it **seems** that the Na-content is quite homogeneous around 1.3 wt% Na2O (or ca. 0.95 wt% Na). I should compile all the data my students and I have one day...
Regarding your second question, if you send me a chip of your glass, I can have it run for LA-ICP-MS one of these days and check the Na-content (along with other traces)... If you have a few other materials that could become great standards, I can also put them in the run! I'm not managing the LA-ICP-MS lab, but my colleague Marcel regularly has a generic setting with almost all lithophile elements. Unfortunately it doesn't seem to perform very well for Na content with detection limits around 25-30 ppm. I think this has to do with isobaric interferences... Hopefully he can improve this; I will check with Marcel when I see him.
Best,
Julien
Just to add my 2 cents to these issues...
We can try to do as precise and accurate analysis (with PfS or without it) especially that we know what to look for. However, other nonetheless important factor is that lots of these P-T calibration curves are produced on analyses of dubious quality... Or at least lacking meticulous methodology. Some discrepancies can be result of unaccounted analytical artefacts and biases. And lots of available P-T calibration curves/equations depends on analysed mineral compositions by EPMA from different kind of P-T experiments. So in the end increasing precision and accuracy of our EPMA results would not lead automagically to better and more sensible P-T recalculations.
Another cent from me is that I appallingly witnessed countless times (theoretically I am geologist BTW, specializing in igneous petrology) when obtained very logic and valid P-T of igneous system at given time-and-space is tried to be interpreted with non-applicable metamorphic petrology mind-set which leads to discarding very valid calculated values of P-T and criticizing model being not good at given range or completely wrong. The first mistreated P-T which comes to my mind is Ridolfi and Renzulli (2012) single amphibole thermo-chemo-barometer, where so far I saw all kind of critique do not recognize that P-T can increase enormously in igneous system without any changes in depth of studied mineral.
Quote from: Probeman on October 18, 2021, 01:26:34 PM
Some might suggest a low Na pyroxene standard might fix this accuracy problem and that is the traditional, but wrong approach in my opinion. For highest minor/trace element accuracy in pyroxenes what we should want, is an approximately matrix matched standard with a *zero* Na concentration. A non-zero minor/trace (and homogeneous) element standard is very difficult to obtain and to characterize.
You opinion is right in so many levels. But differently to the approach with blank standard as standard, as a non-user of PfS I don't use any analytical blind standard but many blind standards for visual setting of single off-peak with a precise slope, which fits all those blind standards of various atom masses off all possible elements in that species of mineral. Often geological minerals tend to be quite extreme zonic (particularly in volcanic environment) and thus it is not so possible to have single matrix-matched blind standard. Also my approach does not require any LA-ICP-MS.
Quote from: Julien on October 19, 2021, 12:22:53 AM
I agree with you, the Kakanui augite has some natural variation... We regularly use it as a secondary standard (NEVER as a primary!) for checking our regular pyroxene analyses (along with other secondary standard). We don't rely on the published value, but rather on the reproducibility over time (session after session...). Data quality is somehow ensure by looking at the totals and also the atomic proportion. From the analyses we have, it looks like there are variations of Al and Fe-Mg content. However, it **seems** that the Na-content is quite homogeneous around 1.3 wt% Na2O (or ca. 0.95 wt% Na). I should compile all the data my students and I have one day...
Hi Julien,
I would be very interested in data demonstrating the homogeneity/inhomogeneity of Na in the Kakanui augite. Not just within a grain, but from grain to grain (because every lab is using a different grain!).
Quote from: Julien on October 19, 2021, 12:22:53 AM
Regarding your second question, if you send me a chip of your glass, I can have it run for LA-ICP-MS one of these days and check the Na-content (along with other traces)... If you have a few other materials that could become great standards, I can also put them in the run! I'm not managing the LA-ICP-MS lab, but my colleague Marcel regularly has a generic setting with almost all lithophile elements. Unfortunately it doesn't seem to perform very well for Na content with detection limits around 25-30 ppm. I think this has to do with isobaric interferences... Hopefully he can improve this; I will check with Marcel when I see him.
I would very much appreciate this! I will look to see if I can locate the material. I think I set a small amount aside when I sent the bulk of the material to Paul Carpenter.
Hi all
Just my two cents re: LA-ICP-MS, you wont be able to get down to those Na levels required for testing a blank standard. It has very high background levels in all the tubing and interface, which is why the detection limits are quite poor.
Cheers
Quote from: BenjaminWade on October 19, 2021, 07:24:50 PM
Just my two cents re: LA-ICP-MS, you wont be able to get down to those Na levels required for testing a blank standard. It has very high background levels in all the tubing and interface, which is why the detection limits are quite poor.
Would that still be the case for normal ICP-MS do you think?
I'm beginning to suspect after talking to a number of colleagues that the main problem with quantification of Na in these thermo/barometers is the Na primary standard.
Many labs are using a natural jadeite. But natural jadeite tends to very inhomogeneous, so why use it?
Well Ed Vicenzi correctly points out that it does have a higher Na concentration of Na than say albite, but when measuring ~1 wt% Na, that shouldn't matter very much. It's still an interpolation. Our lab uses a natural nepheline as a primary Na standard and actually it has a slightly higher Na concentration than our natural jadeite:
St 336 Nepheline (partial anal.)
TakeOff = 40.0 KiloVolt = 15.0 Density = 2.600 Type = silicate Mount = alkali, carbonate
Analysis by ISE Carmichael (Na, K)
Ca = 750 PPM (EPMA by JJD)
Oxide and Elemental Composition
Average Total Oxygen: 44.418 Average Total Weight%: 100.054
Average Calculated Oxygen: 44.450 Average Atomic Number: 11.037
Average Excess Oxygen: -.032 Average Atomic Weight: 20.707
ELEM: Na2O K2O SiO2 Al2O3 FeO O CaO
XRAY: ka ka ka ka ka ka ka
OXWT: 16.920 5.610 43.491 33.761 .199 -.032 .105
ELWT: 12.552 4.657 20.329 17.868 .155 44.418 .075
KFAC: .0735 .0409 .1500 .1333 .0013 .2112 .0007
ZCOR: 1.7068 1.1395 1.3548 1.3408 1.1894 2.1027 1.1090
AT% : 11.299 2.465 14.980 13.705 .057 57.454 .039
24 O: 4.720 1.030 6.257 5.725 .024 24.000 .016
St 316 Jadeite (san benito)
TakeOff = 40.0 KiloVolt = 15.0 Density = 3.300 Type = pyroxene Mount = hydrous
Contains interstitial albite?
(Na2O = 14.02%, K2O=0.06%, by Flame Photometry, by J. Hampel)
Oxide and Elemental Composition
Average Total Oxygen: 47.486 Average Total Weight%: 100.001
Average Calculated Oxygen: 47.489 Average Atomic Number: 10.684
Average Excess Oxygen: -.003 Average Atomic Weight: 20.221
ELEM: SiO2 Al2O3 TiO2 FeO MnO MgO CaO Na2O K2O O
XRAY: ka ka ka ka ka ka ka ka ka ka
OXWT: 59.453 25.221 .000 .021 .000 .010 .020 15.220 .060 -.003
ELWT: 27.790 13.348 .000 .016 .000 .006 .014 11.291 .050 47.486
KFAC: .2124 .1007 .0000 .0001 .0000 .0000 .0001 .0660 .0004 .2517
ZCOR: 1.3083 1.3257 1.1884 1.1951 1.2136 1.5263 1.1065 1.7101 1.1524 1.8864
AT% : 20.008 10.003 .000 .006 .000 .005 .007 9.931 .026 60.014
24 O: 8.001 4.000 .000 .002 .000 .002 .003 3.972 .010 24.000
And our nepheline is *much* more homogeneous than our jadeite. What primary standard does your lab use for Na?
Another possible reason Ed mentioned is the beam sensitivity of say, albite compared to jadeite. No question jadeite is a less sensitive Na standard. Of course the TDI correction can help in this case, but how much of an effect is this beam sensitivity? Well of course it's going to depend on the beam current and the beam size.
Next we will take a look at some TDI data on these standards if I can find any!
Very interesting discussion and the problem of interlaboratory bias, either stemming from unreliable standards and/or the variability in -let's call it - analytical 'rigor" between labs has been a longtime problem.
The problem with MIT data in particular has been occasionally mentioned before. I think I saw it first in a 1996 paper by Yang et al. that states: "An interlaboratory comparison has been made (Reynolds 1995) including MIT, the Smithsonian Institution in Washington, Lamont Doherty and University of Hawaii. It is the practice in our laboratory to correct microprobe data obtained elsewhere to an MIT reference before making thermobarometric or modeling analyses (see Table 1). Although Grove et al. (1992) neglected to discuss this issue, the Smithsonian data discussed in that paper was corrected before plotting and estimation of crystallization pressure. Failure to do so can result in significant errors, and is most commonly evident as a discrepancy in the pressures estimated from the different equations."
I attached the paper.
I think there are also some small remarks in earlier papers by the Langmuir group at Harvard. We ran into this problem as well for a study from one of my MSc students (Voigt et al., 2017, Lithos) and had to correct the literature data to be able to compare various geobarometers.
I would also point you to the large study by Allison Gale (Gale et al., 2013 The mean composition of ocean ridge basalts - Gale - 2013 - Geochemistry, Geophysics, Geosystems - Wiley Online Library) that touches upon interlaboratory bias if you haven't seen it yet (see attached table).
Here is the table of interlaboratory bias factors, showing only the values for EPMA labs.
(https://smf.probesoftware.com/gallery/17_20_10_21_8_14_40.png)
Regarding the Kakanui Augite:
Huebner and Woodruff (U. S. Geological Survey Open File Report 85-718, Chemical Compositions and Critical Evaluation of Microprobe Standards Available in the Reston Microprobe Facility) state:
"The classical or wet chemical analyses of the Kakanui augite have sums that are slightly high, particularly when the trace elements are included. The preferred analysis (3) incorporates revised values of Al203, Fe203, and FeO. The revised Al203 and total iron are substantiated by Wiggins 1 microprobe values. The preferred analysis can be recalculated to a perfectly stoichiometric pyroxene without any adjustment of the ferrous/ferric ratio!
(Na,Ca,Mn,Sr,Co,Ni,Fe,Mg)1.000(Mg,V,Ti,Fc,Cr,Al)1.000(Al,Si)2.000O6 The augite from Kakanui is homogeneous with respect to all its major elements PXKA should be an excellent standard and superior known-unknown for major elements in pyroxenes. It has not been used as much as it deserves."
I attached the relevant pages.
John Fournelle looked more recently also into the variability of Kakanui Augite and three other Smithsonian pyroxene standards. He looked at 54 grains in total.
https://ui.adsabs.harvard.edu/abs/2012AGUFM.V23C2827F/abstract (https://ui.adsabs.harvard.edu/abs/2012AGUFM.V23C2827F/abstract)
I am sure he can weigh in on the variability of sodium in particular. However, the Kakanui augite should not be used as a primary standard for sodium anyway because of its low abundances.
I would be interested to hear if anyone knows of a homogeneous Jadeite. I recently mounted a Jadeite from Japan (#108468, no further information at hand) and it is god awful.
I am overall happy with my Amelia Albite but I have to re-polish every 6-9 months in my experience to remain happy.
Has anyone experience or thoughts on Sodalite?
Quote from: Anette von der Handt on October 20, 2021, 08:16:11 PM
Here is the table of interlaboratory bias factors, showing only the values for EPMA labs.
(https://smf.probesoftware.com/gallery/17_20_10_21_8_14_40.png)
I note that MIT had the 2nd highest Al, the highest Ca and the 4th lowest Na values. So in this table 1.000 indicates no bias from the average of reported results or from the "accepted" values?
Quote from: Anette von der Handt on October 20, 2021, 08:35:10 PM
I would be interested to hear if anyone knows of a homogeneous Jadeite. I recently mounted a Jadeite from Japan (#108468, no further information at hand) and it is god awful.
My experience as well.
Quote from: Anette von der Handt on October 20, 2021, 08:35:10 PM
I am overall happy with my Amelia Albite but I have to re-polish every 6-9 months in my experience to remain happy.
I think it would be better than jadeite, but I think nepheline is a better Na primary standard if one can obtain it. Wondering if pure end member nepheline can be synthesized?
Quote from: Anette von der Handt on October 20, 2021, 08:35:10 PM
Has anyone experience or thoughts on Sodalite?
Very beam sensitive and inhomogeneous in my experience.
Quote from: Anette von der Handt on October 20, 2021, 08:10:20 PM
Very interesting discussion and the problem of interlaboratory bias, either stemming from unreliable standards and/or the variability in -let's call it - analytical 'rigor" between labs has been a longtime problem.
The problem with MIT data in particular has been occasionally mentioned before. I think I saw it first in a 1996 paper by Yang et al. that states: "An interlaboratory comparison has been made (Reynolds 1995) including MIT, the Smithsonian Institution in Washington, Lamont Doherty and University of Hawaii. It is the practice in our laboratory to correct microprobe data obtained elsewhere to an MIT reference before making thermobarometric or modeling analyses (see Table 1). Although Grove et al. (1992) neglected to discuss this issue, the Smithsonian data discussed in that paper was corrected before plotting and estimation of crystallization pressure. Failure to do so can result in significant errors, and is most commonly evident as a discrepancy in the pressures estimated from the different equations."
I attached the paper.
I think there are also some small remarks in earlier papers by the Langmuir group at Harvard. We ran into this problem as well for a study from one of my MSc students (Voigt et al., 2017, Lithos) and had to correct the literature data to be able to compare various geobarometers.
I would also point you to the large study by Allison Gale (Gale et al., 2013 The mean composition of ocean ridge basalts - Gale - 2013 - Geochemistry, Geophysics, Geosystems - Wiley Online Library) that touches upon interlaboratory bias if you haven't seen it yet (see attached table).
At least one issue with the MIT EPMA lab is their "procedure" for standardization. Apparently they:
1. Acquire a single point on their primary standard (already I'm starting to worry!).
2. Acquire a single point on their secondary standard (now I'm really worried!)
3. Analyze that single point on their secondary standard, and if they do not obtain the expected composition of the secondary standard...
4. They then edit the primary standard intensity raw data until they do (OK, now the alarm bells in my head are ringing!).
Yes, you read that correctly: they *edit* the raw data intensity of the primary standard...
I'm sure everyone can understand why this "procedure" might be problematic. First we need more than a single point per sampling, to obtain an average and observe the variance. A single point has a zero variance and we won't know if that single point is simply an outlier due to some instrumental or sampling effects.
Second, no one should be editing their standard (or unknown for that matter) raw intensities to obtain an "expected" result! We should modify our models to fit our data, not modify our data to fit our models! >:(
Third, by "nulling out" the check on accuracy by using a secondary standard, they are now essentially removing their ability to ascertain their accuracy levels. Why even bother using a primary standard?
Consider the following: let's assume they are lucky and the single point measurement of their primary intensity is good, now what if the secondary standard (kakanui augite?) is somewhat inhomogeneous? Or that single grain does not represent the wet chemistry as reported by Gene Jarosewich? By editing the primary standard intensities, they are normalizing the calibration to that single point on that single grain, whether or not it's representative of the average wet chemistry.
No wonder things are problematic at "Boston Tech".
Quote from: Probeman on October 19, 2021, 08:11:29 PM
Quote from: BenjaminWade on October 19, 2021, 07:24:50 PM
Just my two cents re: LA-ICP-MS, you wont be able to get down to those Na levels required for testing a blank standard. It has very high background levels in all the tubing and interface, which is why the detection limits are quite poor.
Would that still be the case for normal ICP-MS do you think?
Hey sorry for the delay. Traditional solution ICP-MS will give you better DL than laser, but it requires a clean instrument and clean acids. Most of the sodium sits in the tubing and on the back of the interface cones and lenses. What kind of Na level are you needing to characterize a "blank" sample? I am assuming something along the lines of single digit ppm, ideally sub ppb like the Ti in quartz.
To give you an idea if our instrument is really clean (and hasn't been sucking seawater through it any time recently), we can do single digit ppb measurements. However this is on a diluted sample (perhaps 500x to 1000x diluted), which would correct back up to a sample that has ~0.5-1ppm Na. The samples need to be diluted otherwise the major components of the solution cause plasma loading and signal suppression. For better counting statistics you may get away with a smaller dilution if you wanted to get as precise a measurement as you can. However it would depend on how much signal suppression (plasma loading) you get from the major elements in a pyroxene matrix (ie mostly Fe,Mg,Si), and thus the required internal standard correction back to 100% might be a large correction introducing more uncertainty.
I guess one thing to bear in mind is that of course this would be a bulk analysis as well, so would be impossible to determine microscale heterogeneity using solution ICP-MS.
Cheers
This sounds very useful.
The ultimate detection limit for EPMA has got to be around ~1 to 2 PPM (or worse).
For example when we did our trace element study (Donovan et al., 2011, Amer. Min.) we tuned up 5 spectrometers for an aggregate Ti analysis and got a t-test detection limit of around 2 to 3 PPM at high beam currents and long counting times.
And Ti (Ka) is pretty easy compared to many emitters and SiO2 is a relatively low Z matrix, so anything else under 2 PPM is probably going to essentially be a big fat zero (blank) for EPMA.
If only someone somewhere could grow high purity synthetic (end member) nepheline (NaAlSiO4) and/or leucite (KAlSi2O6) synthetic crystals! The web seems to disagree on the exact formulas, so I guess I should ask, are synthetic end members of Na and K aluminum silicates possible?
Quote from: Anette von der Handt on October 20, 2021, 08:35:10 PM
I am overall happy with my Amelia Albite but I have to re-polish every 6-9 months in my experience to remain happy.
I also have the best success with the Amelia Albite
Quote from: Anette von der Handt on October 20, 2021, 08:35:10 PM
Has anyone experience or thoughts on Sodalite?
I have a very good Brazilian sodalite that is homogeneous for Cl but not so great for Na or Si. Also to Probeman's point about beam sensitivity in sodalite, I have several grains mounted in my standard block some of which show very noticeable beam damage/sensitivity and others that are very robust and resilient under the same beam conditions. I suspect sodalite exhibits similar anisotropic sensitivity to the e-beam as apatite, but haven't tested that yet as I don't have access to EBSD where I am currently employed to check crystallographic orientations. :-\
I don't know what origin sodalite we have, but I would not compare Na instability with F migration inside Apatite. In apatite the F migrates inside and is a reversible process (the F which initially migrates to deeper, partially homogenize/diffuses back to surface in a few weeks/moths). In Sodalite the Na escapes to Vacuum and is irreversible. The speeds of changes during probing are also complete different. Apatite is slow, where Sodalite is extremely fast (just enough of few nA·s for Na content to be halved, where Apatite-F can take hundreds of nA·s). In sense of Na-to-vacuum leak comparable to Sodalite is Gagarinite. Na in albite is clearly much much better binned and compared to those is extremely stable (at low current, and low beam density).
Where could I buy such Amelia Albite (Europe)?