Ok just for the benefit of anyone who might encounter the same problem, it was entirely my own stupid fault.
My apatite "standards" in my Standard.MDB file already had their excess oxygen subtracted, thus when I turned on the halogen correction in a normal run my apatite standards had a double subtraction.
In the image below, on the left is without the excess oxygen corrected in my Standard.MDB file which gives me the correct halogen subtraction if I turn the halogen correction on in a run. Below right is obviously with the excess oxygen correction, resulting in a double subtraction in a run.
Hi Ben,
You absolutely are correct in your conclusions.
Many years ago when we started CalcZAF and Probe for EPMA, we decided that it was very important, for geologists at least, to account for chemical substitution of stoichiometric oxygen by elements such as halogens (and additional oxygen from say, ferric iron).
So in the standard compositional database (Standard.exe) one can enter a standard composition with all the elements in ideal stoichchiometric proportions, but we can also enter the excess *or* deficit oxygen in those situations where ideal stoichiometric assumptions are not valid, due to the presence of, for example, ferric iron or halogens.
So yes, when you've already accounted for the excess or deficit oxygen in the standard composition database, applying the halogen correction in Probe for EPMA, results in a double correction of the oxygen-halogen equivalence.
Therefore, if one is utilizing an already halogen corrected composition for a standard, in Standard, there is no need to also apply a halogen correction in Probe for EPMA. Probe for EPMA automatically notes the excess or deficit oxygen (whether it be a magnetite or a fluor-phlogopite), and compensates for the difference in the matrix effects.
However, if one specifies a standard composition in the Standard compositional database where the excess or deficit oxygen is *not* documented, then, yes, one must apply the correction for this situation in Probe for EPMA when this standard is measured as a standard (or as an unknown), whether that situation be excess oxygen from ferric iron:
https://probesoftware.com/smf/index.php?topic=92.msg8593#msg8593or for the replacement of oxygen by halogens:
https://probesoftware.com/smf/index.php?topic=92.msg8557#msg8557Of course for samples measured as unknown samples, these excess or deficit oxygen corrections must always be applied because the program knows nothing (a priori) regarding the sample.
There is one exception I must take to your post however. Your characterization of this question as being "stupid" is simply incorrect. There is nothing obvious about the way in which the ferric or halogen corrections are applied in Probe for EPMA because one would need to carefully parse the physics code on Github (see the ZAFSmp procedure in ZAF.bas) to comprehend exactly how these corrections are applied:
https://github.com/openmicroanalysis/calczafThough please note that these procedures (regardless of the naming convention) are entirely agnostic as to exactly exactly what matrix correction models are being applied, i.e., ZAF vs. phi-rho-z!
The bottom line is: thank-you for noting this quite subtle and yet interesting aspect to the matrix correction models. To be entirely honest, when we first noticed these effects, we weren't too surprised when we noticed that including the deficit oxygen correction in apatites and phlogopites made a significant difference in the results, but we were completely surprised by the effect that excess oxygen from ferric iron had, on not only the iron concentrations, but also on the other elements.
Excellent work.
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