Why is it sometimes so hard to get a “good” electron probe analysis for _________ (mineral)

1. Why is it sometimes so hard to get a “good” electron probe analysis for _________ (mineral) John H. Fournelle Eugene Cameron Electron Microprobe Lab Department of Geoscience University of Wisconsin Madison, Wisconsin

2. Some Points of this Talk: • Peak centering for Si, Al and Mg is not always trivial • Chemical shifts for Si, Al and Mg may create issues • Standards should be treated with a critical eye • Area Peak Factors may relate to elements other than “light elements” • Probe for EPMA provides may useful tools

3. A veteran prober wasn’t happy with the numbers coming out of the new $1M fancy schmancy electron probe …. • He was using the same standards he’d been using for 4 decades, • but now getting totals on pyroxene and garnet ~98.0-98.9 wt% (no OH, no Fe2O3), • whereas in past with older less automated probes, with old school Bence-Albee, he was getting “good results”.

4. After much troubleshooting, we determined The rapid Cameca ROM peaking was returning slightly off peak positions in some (critical) cases … but before that was concluded, we found that We had chemical shifts in Al, Mg and Si Ka peaks

5. Results of Automated Al Ka Peaking Options ROM very reproducible: 10 measurements, s.d. of 1.2, range 32374-8 …and sometimes very wrong Cameca says they use “barycentric” routine to find peak center

6. We requested a modification of the peaking procedure in PfW (now PfE) … Here is a post-scan on Al Ka, showing that the peak center returned by ROM was several units off the true peak center. The operator now has final say over peak center determination

7. Chemical Peak Shifts Have been recognized since the origins of x-ray spectroscopy in the 1920s, e.g., Cl and S Kb peaks (M-L transitions: M shell electrons = valence electrons)

8. Al Ka Chemical Shifts … have been recognized for ~50 years in geological materials White, McKinstry & Bates, 1959, Advan. X-ray Analysis Al Ka Shift vs coordination relative to Al metal: Feldspar (IV): -0.07; Sillimanite (IV+VI): -0.11; Kyanite (VI): -0.12 Also Day, 1963, Nature; Wardle and Brindley, 1971, American Mineralogist

9. 7 units or 0.6 eV Al metal TAP sp1 Al Ka Peak Shifts on UW SX51

10. NaAlSi O Al metal 3 8 KAlSi O 3 8 82%(NaAlSi O ) - 18%(CaAl Si O ) 3 8 2 2 8 (Na,K)AlSi O 3 8 51%(NaAlSi O ) 3 8 - BaAl Si O 49%(CaAl Si O ) 2 2 8 2 2 8 CaAl Si O 2 2 8 Al Ka Peak Shifts - Feldspars Only White and Gibbs, 1969, Am. Min., noted that K-feldspar had the greatest Al K peak shift b relative to Al metal (and sanidine more than microcline).

11. Coord 4 6 4 4 6 4 6 6 6 6 6+4 4 4 4 4 Coord 4 4 6 6 6 6 6 4 4 6 6+4 4 4 4 4 Al Ka Peak Shifts Two independent measurements, very similar trends … and not a simple function of Al coordination (e.g., consider the range in feldspars)

12. Al Ka Peak Shifts in Garnet Note: on my 2 TAPs, Al Ka ‘peak plateau” is ~5 sin theta units wide, so a poorly centered peak is deadly…

13. Si Kb, Ka Chemical Shifts: Historical White, McKinstry and Roy, 1962, GSA Abstract Measured majorSi Kb shifts in SiO2 relative to Si metal: Stishovite (IV): -0.010 Å; Quartz, cristobalite (VI): -0.015Å ˙though no Si Ka Shift between IV and VI seen Kaufman and Moll, 1966, Advances X-ray Analysis Examined Si Ka1, Ka3, Ka4 and Kb for Si metal and 10 common silicate minerals; found differences between silicates for all K lines but NOT Ka1

14. Si Ka Peak Shifts - UW SX51 - 2004

15. Translating the above data as Ka shifts for quartz: Spectro1 = 0.5 ±0.1 eV; Spectro4 =0.6±0.1 eV A check Compare Above With: • Si Ka shifts of Quartz by HRXFS (high resolution x-ray fluorescence spectroscopy) • Okura et al (1990 Spectrochimica Acta) a-quartz 0.655 eV • Liu et al (2004 Physical Review B) “SiO2” 0.62 eV Implication: there is a 0.7 - 0.9 eV shift for microcline Si Ka relative to Si metal, -- And one should NOT peak Si on K-feldspar for plagioclase.

16. Mg Ka Peak Shifts:

17. Mg Ka Peak Shifts:

18. Na Ka Peak Shifts in Silicates: • Preliminary results: • There are chemical peak shifts (albite vs jadeite, ~10 unit peak shift) • Peaks are very wide (albite ~18 units wide) • Other issues muddy the waters (element migration, lower counts -> poor statistics)

19. Si, Al and Mg Ka Peak Shifts: • Al: need pay special attention to which specific minerals are being analyzed, and use appropriate standard for peaking/counting (feldspars especially!) • Si: special attention to K, Na feldspars • Mg: MgO is not necessarily a good standard for all silicates; use like phases for standards

20. Al Ka Peak Positions (on TAP) are very sensitive to stage Z position A misfocus of 5 microns in Z equals a peak shift of 3-4 sin theta units, not a trivial difference.

21. Q: Do all electron probes have issues with Al, Si, Mg ka peak shifts? A: I have no idea. I spent a couple of hours on the Barcelona SX50 a couple of years ago trying to reproduce Al Ka shifts that I see on my probe and I couldn’t. On the other hand, a guy with a JEOL HyperProbe volunteered that he too was seeing Mg peak shifts. I know people have talked about different batches of crystals with different count rate efficencies (e.g. during sign-off evaluation). Having “too sharp” (which is what this is about) a TAP crystal can be a pain and require added toil in the probe lab, for “routine” silicate mineral EPMA. If I ever am involved in a probe purchase I would want TAP crystals with less sharp peaks… or could there be orientation issues?

22. Standards: Need to be critical evaluated Most probe labs have standards passed down from one person to the next… Many are Smithsonian USNM standards originally developed by Gene Jarosewich. Others come from a variety of natural sources (e.g. mineral collections) or supply houses. Recently I was helping a postdoc to EPMA her olivines which were to be ion probed; they had a reference “San Carlos olivine” grain for sims oxygen std in them…and we analyzed it…and did not get the USNM published values. At the same time, another researcher wanted EPMA on a large number of “San Carlos” olivines purchased from gem dealers.

23. What is the actual variability of the USNM supplied San Carlos olivine I wondered… Plan: evaluate a large number of grains from this suite which was the one for which wet chemical measurements made, and for which compositions were published for EPMA standards.

24. SiO2 FeO MgO SiO2 FeO MgO Amelia Logan of the Smithsonian supplied 2 small vials of the USNM111312/44 material. Jarosewich et al (1980) calculated "Boyd homogeneity indices" for USNM standards, with 100 measurements on 10 grains of each standard, and values <3 were OK.

25. Evaluation of 25 grains of USNM San Carlos olivine I made 236 measurements on 25 small (200-300um) grains of USNM 111312/44 in 1 vial, which show a wider range of heterogeneity in Si and Mg than originally found in the 1980 paper: Si 2.32 (vs 0.81), Mg 3.05 (vs 1.00), and Fe 1.2 (vs 0.9). These "Boyd" numbers are less easy to comprehend than a simple k-ratio criteria, which I use here: take the average x-ray count of a large number of points (which will represent the wet chemical analysis), and then take the lowest and the highest counts, to represent the worst case scenario. Here the results were: Si ranged from 0.947 to 1.017 (5% max difference), Mg 0.969 to 1.043 (4% max) and Fe 0.946 to 1.055 (6% max). However, 82% fell within the Fo90.1 (±0.2) window.

26. Precision of counting stats for 25 grains of USNM SC olivine Ave ± n  Ave which actually fall in this range Looking at count rate statistics, we can compare that concept of count rate precision to variation around a measured value. The above numbers suggest that there is a 1 in 10 possibility that the Si counts will be at least 1.5% different from the assumed value, and 1.1% different for the Mg counts. This demonstrates that the range of observed Mg and Si X-ray intensities are outside that expected only from limited counting statistics. e.g. Si example

27. There is a small but not unreasonable probability that EPMA users who assume that any ONE grain of USNM Carlos olivine is exactly the published composition could be making an error of ~5% in the measured K-ratio for Mg or Si. EPMAers need to acquire "a reasonably large number of counts on a reasonably large number of grains" (Jarosewich et al, 1980).

28. …reading the fine print… “Occasional grains of the reference sample will differ in composition because of heterogeneity. These problems can never be eliminated... The overall homogeneity of each sample was determined using the criterion given by Boyd el al whereby the sample is considered to be homogeneous if the ratio (homogeneity index) of observed standard deviation to the standard deviation predicted by counting statistics alone does not exceed 3. The ratios were obtained by taking ten 10-second counts on each of ten randomly selected grains... When the criteria of these ratios are used as a measure of homogeneity, all the reference samples are very homogeneous provided a reasonably large number of counts are taken on a reasonably large number of grains. In practice, however, fewer counts and grains are normally used for standardization, and under these circumstances a grain having a slightly different composition may influence the microprobe results adversely. For this reason, grains showing some discrepancy in composition should be avoided. The percentages of these ”impurities” in the whole samples are minimal and the effects on the bulk analyses of the samples are negligible.” --Jarosewich, Nelen and Norberg, 1980 (emphasis added)

29. Large “gem” San Carlos olivines from dealers … two populations, ~Fo91 and ~Fo89 Caveat emptor….

30. Problems with Garnet EPMA At AGU in Dec 2008 and again in 2009, C. Geiger (U. Kiel) came to the EPMA session I co-chaired and spoke about the problems he had “getting good EPMA results on garnets” that he had grown. XRD and other techniques indicated they were garnets, but he “could never get very good garnet stoichiometries in nearly all cases”. I also had records from emails x years ago with Paul Carpenter and John Donovan talking about problems with garnets, with maybe MACs being the problem. And veteran prober Gordon Medaris spoke about decades old discussions about issues with garnet EPMA. I agreed to look at some of Geiger’s garnets and see if maybe peak shifts were involved.

31. Troubleshooting Garnet EPMA • But first • What standards were they using? • Al: corundum • Si: wollastonite • Fe: “fayalite-rich olivine”…which? …”Rockport” :( • Mg: Springwater olivine (Fo83) • Conditions? 15 kV, 15 nA; using both Cameca and JEOL probes in 3 locations • Matrix correction? Armstrong (1991) phi-rho Z; Henke MACs

32. Troubleshooting Garnet EPMA Keep it simple: focus on his pure synthetic pyrope Mg3Al2Si3O12 Use very similar standards to his: Al2O3, wollastonite, Fo90 olivine; use 15 kV Looked for peak shifts between the pyrope and standards -- nothing significant found… Acquired a series of 17 measurements on one pyrope crystal, using the above standards. Totals not too bad (average 99.06) but stoichometries off….

33. Troubleshooting Garnet EPMA Run of 6/5/10 Al is most off, ~5%, Si and Mg ~1-2% off…

34. Troubleshooting Garnet EPMA …initial guess…compare whole peak waveforms for pyrope vs stds … start with Al Sk’ Sk3 Sk4

35. Troubleshooting Garnet EPMA …initial guess…compare whole peak waveforms for pyrope vs stds … start with Al Sk’ Sk3 Sk4 So what? This seems like a very small difference in the total peak area…

36. Troubleshooting Garnet EPMA Set up detailed wavescans: every 10-12 sin theta units, 5 sec per channel. With PfE, easy to integrate and find see if Area Peak Factors ≠ 1.00. Run the standard (here, corundum, Al2O3)

37. Troubleshooting Garnet EPMA …and then run the unknown, and take the appropriate values, multiply, and see if there is a difference between results using single channel peak intensity and results integrating the total x-ray generation summed under the curve…

38. Relative to: wollastonite corundum Fo90 olivine Troubleshooting Garnet EPMA Calculated APFs for 2 synthetic garnets runs of 7/4-7/5/10 Al APF is interesting…not sure anyone has ever mentioned this before

39. Troubleshooting Garnet EPMA Consider different matrix corrections And select one by one and view the results one by one….or

40. Troubleshooting Garnet EPMA -> Check the box and “analyze”

41. Troubleshooting Garnet EPMA Recall the nominal composition: 20.90 13.38 18.09 47.62 100.00 This suggests that the the problem exists regardless of which matrix correction is used…

42. Troubleshooting Garnet EPMA 20.90 13.38 18.09 47.62 100.00 This suggests that the the problem exists regardless of which matrix correction is used…

43. Troubleshooting Garnet EPMA Consider using different MACs: in general MacMaster has some of the highest values, whereas FFAST has some of the lowest. Regardless, the problems still persist for Al and Mg

44. Troubleshooting Garnet EPMA The solution would seem to be to use garnet standards for garnet unknowns. Using wollastonite, corundum and olivine, the ZAF values are: Si: 1.45 Al: 1.52 Mg: 1.38 This does not address the whole problem though; the evidence does suggest that Al area:peak discrepancy between garnet and Al2O3 is one issue, but there seem to be other issues. Next approach is to use Kyanite Al2SiO5 which would seem to have less difference in matrix correction.

45. EPMA of Calcite Doing EPMA of carbonate… how to correctly include the Carbon without actually measuring it? And being included within the matrix correction? Without “fudging” by forcing total to 100 wt%?? Easy with PfE.

46. Carbonate analysis…with piece of mind…. Good analytical totals, good stoichometry

47. Ca Ka in Calcite….who would have thunk? July 2009 Normally I use low current (1 nA) and wide beam to measure calcite which is “easily damaged”. This time, we needed a tight beam to x-ray map this small forams…so since we have the TDI tool in PfE, I ran it…and to my surprise, Ca counts increased with time, not decreased… hmmmm….. So “low counts” aren’t necessarily due to “damage” but may be a sign elements are migrating …would be interesting to measure O and C and see what they are doing also….

49. Electronegativity ----> On the basis for chemical shift in Al and Si Ka Precision of EPMA peak measurements is much less than that possible using XPS and AES, and those fields’ literature provide a basis for understanding the EPMA observations. Streubel et al (1991 J. Electron Spectro & Related Phenom): Data on Si and P chemical shifts using XPS and AES Figure 2 (top) plots relative Pauling Electronegativity vs relative binding energy of L shell (2p) DE(Ka) = DE(1s) - DE(2p) Ka peak shift = Difference (vs Si metal) in K binding energy minus Difference in L binding energy

50. Suggested reason for Al Ka shifts in Ca vs K-Na feldspars DE(Ka) = DE(1s) - DE(2p) Ka peak shift = Difference (vs Si metal) in K binding energy minus Difference in L binding energy E (2p) (eV) K-Na-feldspar= KAlSi3O8 NaAlSi3O8 K-Na-feldspar 0.6 eV Ca-feldspar Ca-feldspar= CaAl2Si2O8 Si metal 2p Bonding Energy