Dear Eric, Christian, Fernando, Horst, Paddy, Janos, James, Chris,
John, Penelope,... and all
thanks a lot to everyone who brought some answers (on- and off-list)
to my problem (and to the others for patience). Now I would be really
grateful if you would take the time to read also this - somewhat
long - email...
It seems that the major issue is the choice of standards. Apatite
seems not to be an ideal candidate unless one analyses other apatites
and unless the standard is analysed at high angles to the c-axis.
Topaz seems to satisfy some, but others think (with serious
arguments) that "Topaz is not a good standard for anything but other
topaz". Other standards were also recommended.
Concerning the calculation of structural formulae from the EMP data
(which was my major problem initially), from your answers I
understood the reason for the high totals and for the need of the O=F
correction.
The implicit issue of my last question ("if the correction for O=F
has to be done, from what part of the formula should this number be
subtracted?"), was to understand what to do if I would like to write
an analysis in wt% so that the sum of the numbers really gives the
corrected total. In fact, our operator suggested that the O=F
correction should be subtracted from the analysed amount of fluorine!
(which I found strange)
My conclusion now is (and please correct me if I'm wrong!) that the
analysed amount of fluorine is "correct" (problems related to the
counts evolving with time etc. taken apart) and that what one could
do is to convert proportional amounts of oxides to fluorides.
Fluorine would then not be listed as a separate anion (being included
in the fluorides) and the total would correspond to the one corrected
for O=F. Is that right?
As far as I understand, if apatite is used as standard, the totals
may look "normal" because 1) there is a Cameca SX50 built-in routine
for measuring apatite which makes the O=F correction for you, BUT
also 2) because when using Durango apatite (rich in trace elements),
"the issue of "too high" total is masked by a _too low_ listing of
all elements present".
Unfortunately, I wasn't able to find out whether our probe does the
correction or not. If the Cameca software does the correction for me,
does it simply and only mean that I'll find the O=F number written at
the bottom line and the final total corrected by this number? In
fact, if my above reasoning is right, I don't understand how the
correction could be done otherwise unless listing amounts of
fluorides along with oxides...??
I put the complete Cameca output for an analysis of our apatite
standard (Durango with unknown orientation) in a file attached to
this message. Perhaps this would allow someone to tell me whether the
O=F correction is applied or not... (in any case I'm grateful to
everyone who had the patience to read till here)
Further, I didn't find it straightforward to adapt the DHZ (2nd
edition) example of calculating a structural formula in F-bearing
minerals to minerals where H2O is not directly analysed. Largely
because of the questions addressed above.
In the DHZ example, a clinohumite (16 O, 2 (OH,F)) analysis
(including analysed H2O and F) is recalculated on the basis of
18(O,OH,F), by multiplying the atomic proportions of the elements by
18/x, where x is the sum of negative charges corresponding to the
proportions of the analysed molecules ("atomic proportions of oxygen
from each molecule") _corrected_ by half the molecular/atomic
proportion of fluorine (x=2.815 in this case).
If, however, H2O is not analysed, the formula must be calculated on
the basis of 17(O) equivalents and the proportions of the elements
are multiplied by 17/x. In my understanding, in this case, however,
the fluorine negative charges must not be included in calculating "x"
(x=2.6575 in this case)! And hence the O=F correction actually plays
no role for the structural formula calculation! Am I right?? (It may
seem obvious to many, but it took me some time to find out...)
Finally, I would be glad to know your opinion on the following
analyses of (the same grains of) wagnerite and biotite. The first set
was obtained using topaz as F standard, the second using Durango
apatite (with unknown orientation), in both cases analysed with 15kV,
20nA, 10s counting time. They seem rather inconsistent as far as the
amount of fluorine concerns... The structural formulae were
calculated as outlined above, OH was calculated by difference.
001 014 001b 014b
wag-T bi-T wag-A bi-A
SiO2 0.026 37.947 0.077 37.497
TiO2 0.947 2.324 0.994 2.267
Cr2O3 0.006 0.102 0.000 0.041
Al2O3 0.004 16.625 0.000 16.555
FeO 12.343 14.388 11.771 14.932
MnO 0.102 0.000 0.000 0.000
MgO 39.626 14.838 39.414 14.722
CaO 0.080 0.000 0.071 0.000
Na2O 0.000 0.783 0.000 0.818
K2O 0.000 7.967 0.000 8.091
P2O5 42.616 0.000 42.144 0.039
F 9.277 1.347 5.907 0.243
(Tot) 105.035 96.326 100.387 95.210
O=F 3.906 0.567 2.487 0.102
Total 101.129 95.759 97.900 95.108
ox/form 4.5 11 4.5 11
Si 0.001 2.803 0.002 2.783
Ti 0.020 0.129 0.021 0.127
Cr 0.000 0.006 0.000 0.002
Al 0.000 1.448 0.000 1.449
Fe 0.288 0.889 0.278 0.927
Mn 0.002 0.000 0.000 0.000
Mg 1.648 1.634 1.657 1.629
Ca 0.002 0.000 0.002 0.000
Na 0.000 0.112 0.000 0.118
K 0.000 0.751 0.000 0.766
P 1.007 0.000 1.007 0.002
F 0.819 0.315 0.527 0.057
OH 0.181 1.685 0.473 0.943
Sumcat 2.969 7.772 2.967 7.803
XF 0.819 0.157 0.527 0.057
XFe 0.149 0.352 0.144 0.363
Although overstimation of the fluorine amount using apatite as
standard was mentioned, it looks as if in this case the topaz
standard gives overestimated results?? Or are the apatite results
underestimated?
Thanks a lot again for your attention and your contributions!
Kind regards,
Pavel
--
Pavel PITRA
Geosciences Rennes email: [log in to unmask]
Université de Rennes 1 tel: (++33) 2.23.23.65.06
Campus de Beaulieu - Bat. 15 fax: (++33) 2.23.23.56.80
F - 35 042 RENNES CEDEX
FRANCE http://www.geosciences.univ-rennes1.fr/ch/pitra/pitra.htm
Durango apatite, uknown orientation, 10s counting time, F analysed
using the TAP crystal
Elt. Peak Prec. Bkgd P/B Ix/ Sig/k Detection Beam
Acceleration
(Cps) (%) (Cps) Istd (%) limit (%) (nA)
voltage(kV)
20.0 15.0
Na 42.7 6.2 13.0 3.28 0.0245 6.3 0.0561
K 13.2 11.2 12.0 1.10 0.0007 11.3 0.0444
Fe 6.2 16.4 4.9 1.26 0.0005 16.4 0.1269
Si 75.5 4.7 31.6 2.39 0.0049 4.7 0.0242
P 1494.3 1.1 4.0 373.58 1.0095 1.1 0.0524
Mg 19.0 9.4 18.2 1.05 0.0001 9.4 0.0311
F 36.5 6.8 14.4 2.53 0.9997 6.8 1.0396
Ca 6071.8 0.5 28.8 210.81 1.1499 0.6 0.0568
Mn 4.8 18.6 6.3 0.77 0.0000 18.6 0.0000
Al 27.3 7.8 26.3 1.04 0.0001 7.8 0.0257
Cr 1.7 31.6 1.8 0.95 0.0000 31.6 0.0000
Ti 24.2 8.3 22.7 1.06 0.0002 8.3 0.0519
Analysis no. 2 within mich1
Elt. Conc. 1sigma Norm Conc. Norm Conc. Compound Concen.
(wt%) (wt%) (wt%) (at%) (wt%)
Na 0.2564 0.026379 0.2532 0.2620 Na2O 0.346
K 0.0082 0.017583 0.0081 0.0050 K2O 0.010
Fe 0.0356 0.045905 0.0352 0.0150 FeO 0.046
Si 0.1163 0.012733 0.1148 0.0973 SiO2 0.249
P 18.5325 0.220476 18.3050 14.0593 P2O5 42.466
Mg 0.0036 0.010721 0.0035 0.0035 MgO 0.006
F 3.3969 0.566482 3.3552 4.2014
Ca 39.0958 0.235194 38.6157 22.9206 CaO 54.703
Mn 0.0000 0.000000 0.0000 0.0000 MnO
Al 0.0032 0.010769 0.0032 0.0028 Al2O3 0.006
Cr 0.0000 0.000000 0.0000 0.0000 Cr2O3
Ti 0.0094 0.022284 0.0093 0.0046 TiO2 0.016
O 39.7852 39.2967 58.4287 by stoichiometry
total : 101.2432 100.0000 100.0000 101.243
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