How to calculate H2O

I would like to ask for help in recalculation of data from electron probe micro-analyzer. I have problem with e.g. Ca9(Mg,Fe++)(PO4)6(PO3OH). How to calculate H2O? If somebody can help me I will be very grateful.
Best regards
Joanna

- How do you know there is PO3OH in this whitlockite-group sample?
- Can you exclude that some Fe is trivalent?

Thank you for answer.
1. Actually I don't know.
2. I have only data from microprobe so I can't tell for sure.
So what can I do? I'm starting with microprobe data so please be understanding :)
Best regards

A few seconds with a Raman device sorts out Bobdownsite from Whitlockite!!!

Rob Woodside Wrote:
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> A few seconds with a Raman device sorts out
> Bobdownsite from Whitlockite!!!

I know but now I have no possibility to use Raman :(
I have only data from microprobe and I need to interpret it.

> A few seconds with a Raman device sorts out
> Bobdownsite from Whitlockite!!!

Even if the EDS instrument is not capable of quantifying fluorine, it will not entirely miss it. Thus, it will provide information as to whether bobdownsite is a possibility or not (note that the presence of fluorine alone is no confirmation). In the fomer case, subsequent analyses may be carried out.

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Joanna,

What are your EDS results ?

My results (WDS)
P 20,1941
Ca 32,5944
S 0,0000
Cl 0,0166
K 0,0000
Fe 3,8821
Mn 2,1295
Zn 0,1197
Ba 0,0000
F 0,9386
Na 0,2838
Mg 0,6037
Al 0,0000
Si 0,0083
Sr 0,0000

This leads to the following mass balance (F, Cl included as such because they are unlikely to occur in oxo-anions):

P2O5 35.83
CaO 45.60
FeO 4.99
MnO 2.75
ZnO 0.15
MgO 0.99
Na2O 0.38
SiO2 0.02
(F) 0.94
(Cl) 0.02
----------------
sum 91.67 %(w/w)

If all iron is calculated as Fe(III), corresponding to 5.55 %(w/w) of Fe2O3, a total of 92.23 %(w/w) is obtained. That is, there is a significant gap in the mass balance (significant in this context means, that the gap is much larger than the expected measurement uncertainty; for instance, a mass balance in the order of 98-102 %(w/w) would have to be considered as conclusive).

Assuming that there are no other constituents present which you may have overlooked, it is safe to assume that there is additional oxygen (oxide, hydroxide or water) present, and the only way to obtain its content from the available data is by difference calculation from the mass balance. Before you proceed, however, it is wise to have a look at the measurement uncertainty.

Instrumental analysis has an intrinsic relative error of 2% minimum (not to be confused with repeatability precision, which can be significantly lower). This means that the true total of the constituents listed above is somewhere between 89.8 and 93.5 %(w/w) (or, between 90.4 and 94.1 %(w/w) for the calculation with trivalent iron). Thus, the water content calculated by difference to 100 %(w/w) is in the order of 6.5-10.2 %(w/w). The average has a relative error of ca. 20% (by no means unusual in this sort of calculation, but often overlooked !). There is no way to get a more precise result here. In the worst case, this value might even be inaccurate (i.e. affected by a systematic error when one (or more) of the measured element contents is false positive or false negative).

Thank you for this information but it is still too complicated for me.
Maybe I don't understand you clearly but what I have to do when sum is above 100%?

P2O5 35.83 Are you sure is it ok?

Hi again

I've also analyzed whitlockite, seems like a one having a lots of H2O. PO3OH group is evident in case of whitlockite, PO3F in bobdownsite. My whitlockite also had some F and Cl. Supposingly, S and Si would substitute for P i PO3OH group, but that was only my assumption, based mainly on stoichiometry (I was trying to compare the positive and negative charges, when using an empirical formula like (Ca...Na...)(Mg...Fe...Mn...)(PO4)12[(P...Si...S...)O3(OH...F...Cl...)].nH2O).

In my opinion, the best way to check if there is H2O not only in form of PO3OH PLUS to confirm the presence of PO3OH (and likely PO3F) is to use micro-Raman spectroscopy or, eventually, micro-IR spectroscopy.

I assume, after your name and surname , that you are in Poland? Then, I'd suggest to visit the Faculty of Chemistry of the Jagiellonian Univ. in Cracow. They do micro-Raman for minerals very well. The only problem might be the presence of epoxy in thin section, but the resolution and spot is very small.

Good luck again!

Łukasz K.