September 25, 1995
Hi Paul ,
I was discussing, with Paulo Vasconcelos, some of his recent work on color in quartz that he has collaborated with George Rossman. I had done some of the preliminary microprobe trace analyses with him when he was a graduate student here. During our discussion he happened to mention some work that you or John were involved in, regarding a problem in microanalysis with obtaining apparently high totals on minerals in the Si-Mg-Fe system.
I am aware that you and John have some expertise in microanalysis, so I am sure that the effect that you are seeing is at least partially due to other more interesting causes, but I feel compelled to point out a discrepancy, that I had noticed several years ago, in the tabulated mass absorption coefficient data for the Mg ka absorbed Fe used by Berkeley (Henke) and Cal Tech (Heinrich).
I should mention that I also had originally experienced difficulties with high totals myself when using pure Mg2SiO4 and Fe2SiO4 as standards and analyzing intermediate olivine compositions. That is, until I obtained a set of mass absorption coefficients (Henke, et. al., 1982) from Burt Henke, who at that time was at the Soft X-ray Optics Laboratory at LBL. After incorporating this new table into my analysis software, I found that Henke's MAC data set gave somewhat better results for analysis with many soft x-rays, even when using Armstrong's own CITZAF algorithms! The problems with high totals in the Si-Mg-Fe system seemed to stem from a large difference in the mass absorption coefficient of Mg Ka absorbed by Fe. This use of Heinrich's tabulation was apparently the cause of high totals (when extrapolating from pure oxide or end-member compositions) for many minerals, especially those with high Mg-Fe concentrations, such olivine and garnets.
Please note the following values (soft x-ray) quoted from Heinrich and Henke :
| Heinrich (CITZAF) | Henke (1982) | |
| Mg Ka in Si | 802 | 859 |
| Mg Ka in Fe | 6121 | 5250 |
| Si Ka in Mg | 2825 | 2902 |
| Si Ka in Fe | 2502 | 2305 |
As you can see there is about 20% difference in the mass absorption coefficients for Mg ka in Fe, although the others are reasonably close. This difference will have a significant effect on the quantitative analysis (about 1 % or so). To illustrate the magnitude of the problem I recently analyzed a couple of USNM olivines and using pure oxide and synthetic end-member olivines standards.
The standards were analyzed on our SEMQ (52.5), 15 KeV, 30 nA on MgO, Mg (ADP, FPC), Fe (LiF, Xenon), Si (PET, Xenon) using off-peak backgrounds.
The separate output is enclosed, however a summary of the results (average of 8 points each) shown here, reveals that the use of the Heinrich tabulated mass absorption coefficients will cause significantly higher than 100% totals when extrapolating from pure oxide or end-member compositions.
| Heinrich (Mg ka in Fe MAC = 6121) | Henke (Mg ka in Fe MAC = 5250) | ||||||||||
| standard | Mg2SiO4 | Fe2SiO4 | SiO2 | Mg2SiO4 | Fe2SiO4 | SiO2 | |||||
| USNM 111312/444 (866) | Mg | Fe | Si | Total | Mg | Fe | Si | Total |
| measured | 30.15 | 7.64 | 19.09 | 100.89 | 29.65 | 7.63 | 19.03 | 100.32 |
| published | 29.80 | 7.42 | 19.08 | 100.30 | 29.80 | 7.42 | 19.08 | 100.30 |
| % variance | 1.15 | 2.99 | .08 | -.52 | 2.83 | -.22 | ||
| Oxygen specified : 43.60 | ||||||||
| USNM 2566 (867) | Mg | Fe | Si | Total | Mg | Fe | Si | Total |
| measured | 26.97 | 13.33 | 18.83 | 101.18 | 26.37 | 13.34 | 18.74 | 100.50 |
| published | 26.28 | 12.92 | 18.21 | 99.46 | 26.28 | 12.92 | 18.21 | 99.46 |
| % variance | 2.63 | 3.16 | 3.45 | .33 | 3.27 | 2.91 | ||
| Oxygen specified : 41.82 | ||||||||
It may be that there are some other anomalous effects that occur in the analysis of Mg-Fe-Si systems, but at least some attention must be paid to the discrepancy between the Heinrich and Henke mass absorption coefficients for Mg ka in Fe. I hope you will find these results intriguing and please do not hesitate to contact me if you have anything to discuss. Thanks for your time.
Sincerely yours,
John Donovan
Henke, B. L., Lee, P., Tanaka, T. J., Shimabukuro, R. L., Fujikawa, B. K., Atomic Data and Nuclear Data Tables, Low-Energy X-ray Interaction Coefficients : Photoabsorption, Scattering and Reflection, University of Hawaii, Honolulu, Hawaii, Academic Press, New York, Vol. 27, Number 1, 1982
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