Analysis of Fluorine in Fe-bearing phases: Problems and solutions John Fournelle Eugene Cameron Electron Microprobe Lab Department of Geology & Geophysics University of Wisconsin-Madison

Earlier this year I reviewed a manuscript on this subject. It made me rethink my procedure for measuring Fluorine in Fe-bearing phases. The paper will be appearing next January in American Mineralogist (authors: Witter and Kuehner, from “the other UW”)

Problems:  Several Fe L lines fall near both the Fluorine Ka peak position and possible background positions  TAP crystal has good resolution (discrimination), but low count rate and P/B is low  With TAP also there is a significant peak shift effect that must be compensated for, unless the same species used for standard as unknown  Layered synthetic diffractors give high count rates and large P/B, and small peak shift  F being low energy (“light element”), one needs to have some concern for absorption correction, and perhaps use lower accelerating voltage  Ingrained habits are hard to break….

Problems: Lots of elements can interfere with Fluorine Ka, e.g. in biotite, hornblende, apatite, volcanic glass … Consider here a phase with 0.1 wt% F in the presence of significant amounts of other first and higher order interfering elements (above: capture of Virtual WDS, a Windows PC program/ database)

Or great count rates with the synthetic W/Si 60Å (PC1), but interferences (peak & background) and impossible to evaluate peak shifts Here are the choices I thought I had: no interference if I used the TAP-- but lousy count rate, and peak shifts

Crystal/ LSM Peak Cts Bkg Cts Peak/ Bkg TAP W/Si 60 Å W/Si 45 Å And then I realized that my underused 45Å diffractor (PC0) might offer a better opportunity for EPMA of Fluorine With good counts and Nice P/B

All scans with 200 channels Crystal -LSM Peak Cts Bkg Cts Peak/ Bkg TAP W/Si 60 Å W/Si 45 Å Comparison of all 3 diffractors (on Wards Biotite) Integral PHA mode, for “worst” case scenario for n=2 lines wt% 5.9 wt% 4.1 wt% 13.1 wt% 8.5 wt%

Same with Log Scale

While there still is some overlap from the Fe La onto the F Ka, the 45Å diffractor has better resolution, and requires a smaller interference correction than the 60Å diffractor, as shown in this comparison of “interference counts” at the F Ka peak position on 2 F-free phases (andradite and magnetite). … and therefore,a much smaller correction

Conclusion If Fluorine is to be measured in Fe-rich phases, the best option is the W/Si 45Å if at all possible, and certainly if the amount of Fluorine is low.