Hi John,
Here is an example of an analysis (PAP model, accelerating potential = 15 kV, takeoff angle = 40 deg.) of pure magnetite using a pure magnetite standard. I've assumed that all Fe in the unknown is present as FeO. Under "Calculation Options," I've selected "Calculate with Stoichiometric Oxygen."
(https://smf.probesoftware.com/gallery/381_06_03_16_10_57_45.jpeg)
If I then recalculate the result assuming 3 cations and 4 oxygens per formula unit, I obtain 67.996 wt% Fe2O3, 30.591 wt% FeO, and oxide total = 98.586 wt%. (I've used molar mass Fe = 55.845 g/mol and molar mass O = 15.9994 g/mol.)
The standard and unknown both contain 72.359 wt% Fe and 27.641 wt% O, and Fe k-raw is precisely one. If it is assumed that Fe is present in the unknown as FeO, then the calculated weight per-cent of oxygen is 20.731, producing an oxide total of 93.090 wt%. However, during the matrix correction iterations, compositions are normalized, and, during the first iteration, the Fe and O contents of the unknown become, respectively, 77.730 and 22.270 wt%. If matrix corrections are determined based on this composition, then, relative to the standard, the Fe atomic number correction (PAP model) is 0.9851 (=1.0659/1.0820) when in truth it should be precisely one. This mostly accounts for the anomalously low wt% Fe in the result since the absorption correction is close to unity.
Why does CalcZAF not include a provision to account for mixed oxidation state of a chosen element during the matrix correction iterations? Or is this somehow possible in CalcZAF, and I just don't see it?
Quote from: Brian Joy on March 06, 2016, 12:05:49 PM
Why does CalcZAF not include a provision to account for mixed oxidation state of a chosen element during the matrix correction iterations? Or is this somehow possible in CalcZAF, and I just don't see it?
Hi Brian,
CalcZAF mostly certainly does account for different valences in the matrix correction for oxygen. In fact I've spent considerable effort to make sure that excess oxygen is handled properly even in cases where one has halogen replacement of stoichiometric oxygen (e.g., biotites/phlogopites).
There is an example of magnetite in the CalcZAF.dat sample data file (from the File | Open CalcZAF Input data File menu), but it's assuming 6.9 % or so of excess oxygen and FeO. Here is the CalcZAF.dat example magnetite with excess oxygen specified:
SAMPLE: 6, TOA: 40, ITERATIONS: 3, Z-BAR: 20.96875
ELEMENT ABSCOR FLUCOR ZEDCOR ZAFCOR STP-POW BKS-COR F(x)u Ec Eo/Ec MACs
Si ka 1.5172 1.0000 .9336 1.4164 .8910 1.0478 .5969 1.8390 8.1566 1955.60
Fe ka .9969 1.0000 1.0666 1.0633 1.0944 .9746 .9877 7.1120 2.1091 55.6969
Mg ka 2.4698 .9999 .9272 2.2899 .8716 1.0638 .3508 1.3050 11.4943 4477.22
ELEMENT K-RAW K-VALUE ELEMWT% OXIDWT% ATOMIC% FORMULA KILOVOL
Si ka -.00008 -.00003 -.005 -.010 -.005 -.001 15.00
Fe ka 1.35236 .67675 71.956 92.571 42.452 7.929 15.00
Mg ka .00069 .00033 .075 .124 .101 .019 15.00
Mn .054 .070 .032 .006
Ca .000 .000 .000 .000
Ni .000 .000 .000 .000
Al .201 .380 .245 .046
O 6.899 6.899 14.207 2.653
Ti .012 .020 .008 .002
O 20.861 ----- 42.959 8.023
TOTAL: 100.053 100.053 100.000 18.677
You might find it helpful to run through all the examples in this data file, as it demonstrates many of the CalcZAF modes.
But if you set the excess oxygen to zero in the magnetite example, and then specify Fe as Fe3O4, then you will get this:
SAMPLE: 6, TOA: 40, ITERATIONS: 2, Z-BAR: 20.97229
ELEMENT ABSCOR FLUCOR ZEDCOR ZAFCOR STP-POW BKS-COR F(x)u Ec Eo/Ec MACs
Si ka 1.5173 1.0000 .9335 1.4164 .8909 1.0478 .5969 1.8390 8.1566 1955.23
Fe ka .9969 1.0000 1.0665 1.0632 1.0943 .9746 .9877 7.1120 2.1091 55.6884
Mg ka 2.4701 .9999 .9272 2.2900 .8716 1.0638 .3508 1.3050 11.4943 4476.34
ELEMENT K-RAW K-VALUE ELEMWT% OXIDWT% ATOMIC% FORMULA KILOVOL
Si ka -.00008 -.00003 -.005 -.010 -.005 -.001 15.00
Fe ka 1.35236 .67675 71.953 99.438 42.476 7.929 15.00
Mg ka .00069 .00033 .075 .124 .101 .019 15.00
Mn .054 .070 .032 .006
Ca .000 .000 .000 .000
Ni .000 .000 .000 .000
Al .201 .380 .246 .046
O .000 .000 .000 .000
Ti .012 .020 .008 .002
O 27.732 ----- 57.142 10.666
TOTAL: 100.022 100.022 100.000 18.666
The ZAF window should look like this:
(https://smf.probesoftware.com/gallery/395_06_03_16_12_55_05.png)
One can also simply enter Fe3O4 using the "Enter Composition As Formula String" button and you will get this after clicking the calculate button:
ELEMENT ABSFAC ZEDFAC FINFAC STP-POW BKS-COR F(x)e
Fe ka 1.0157 4.3900 4.4588 .2087 .9161 .9846
O ka 1.4270 3.9154 5.5873 .2438 .9546 .7008
SAMPLE: 32767, TOA: 40, ITERATIONS: 0, Z-BAR: 21.02464
ELEMENT ABSCOR FLUCOR ZEDCOR ZAFCOR STP-POW BKS-COR F(x)u Ec Eo/Ec MACs
Fe ka .9969 1.0000 1.0660 1.0626 1.0935 .9748 .9877 7.1120 2.1091 55.6319
O ka 1.6478 .9937 .8648 1.4160 .7989 1.0826 .4253 .5317 28.2114 3227.13
ELEMENT K-RAW K-VALUE ELEMWT% OXIDWT% ATOMIC% FORMULA KILOVOL
Fe ka .00000 .68095 72.359 ----- 42.857 1.000 15.00
O ka .00000 .19520 27.641 ----- 57.143 1.333 15.00
TOTAL: 100.000 ----- 100.000 2.333
Or just enter pure Fe with a concentration of 72.359 wt%, and 3 cations and 4 oxygens and and then click the Calculate with Stoichiometric Oxygen checkbox and you will get this:
ELEMENT ABSCOR FLUCOR ZEDCOR ZAFCOR STP-POW BKS-COR F(x)u Ec Eo/Ec MACs
Fe ka .9969 1.0000 1.0660 1.0626 1.0935 .9748 .9877 7.1120 2.1091 55.6318
ELEMENT K-RAW K-VALUE ELEMWT% OXIDWT% ATOMIC% FORMULA KILOVOL
Fe ka .00000 .68095 72.359 100.000 42.857 1.000 15.00
O .000 .000 .000 .000
O 27.641 ----- 57.143 1.333
TOTAL: 100.000 100.000 100.000 2.333
Then I thought to myself: Brian Joy is a very smart guy, so what am I missing? Maybe there's a problem with the PAP corrections, so I used the magnetite example in CalcZAF.dat with PAP and I get this;
SAMPLE: 6, TOA: 40, ITERATIONS: 3, Z-BAR: 20.96556
ELEMENT ABSCOR FLUCOR ZEDCOR ZAFCOR STP-POW BKS-COR F(x)u Ec Eo/Ec MACs
Si ka 1.5129 1.0000 .9264 1.4016 .8896 1.0415 .5956 1.8390 8.1566 1948.07
Fe ka .9966 1.0000 1.0828 1.0791 1.1103 .9752 .9871 7.1120 2.1091 55.4812
Mg ka 2.4533 .9999 .9148 2.2442 .8644 1.0583 .3504 1.3050 11.4943 4460.17
ELEMENT K-RAW K-VALUE ELEMWT% OXIDWT% ATOMIC% FORMULA KILOVOL
Si ka -.00008 -.00003 -.005 -.010 -.005 -.001 15.00
Fe ka 1.35236 .66415 71.671 92.204 42.430 7.929 15.00
Mg ka .00069 .00032 .071 .118 .097 .018 15.00
Mn .054 .070 .032 .006
Ca .000 .000 .000 .000
Ni .000 .000 .000 .000
Al .201 .380 .246 .046
O 6.899 6.899 14.256 2.664
Ti .012 .020 .008 .002
O 20.778 ----- 42.935 8.024
TOTAL: 99.682 99.682 100.000 18.688
Which is a slightly lower total (using Fe2SiO4 as an Fe standard), but it's still within normal accuracy.
And if I set the excess oxygen to zero and specify Fe3O4 for the cations/oxygens I get this with PAP:
ELEMENT ABSCOR FLUCOR ZEDCOR ZAFCOR STP-POW BKS-COR F(x)u Ec Eo/Ec MACs
Si ka 1.5130 1.0000 .9264 1.4016 .8894 1.0415 .5955 1.8390 8.1566 1947.30
Fe ka .9966 1.0000 1.0827 1.0790 1.1102 .9753 .9871 7.1120 2.1091 55.4632
Mg ka 2.4537 .9999 .9147 2.2443 .8643 1.0584 .3503 1.3050 11.4943 4458.32
ELEMENT K-RAW K-VALUE ELEMWT% OXIDWT% ATOMIC% FORMULA KILOVOL
Si ka -.00008 -.00003 -.005 -.010 -.005 -.001 15.00
Fe ka 1.35236 .66415 71.663 99.039 42.478 7.929 15.00
Mg ka .00069 .00032 .071 .118 .097 .018 15.00
Mn .054 .070 .033 .006
Ca .000 .000 .000 .000
Ni .000 .000 .000 .000
Al .201 .380 .247 .046
O .000 .000 .000 .000
Ti .012 .020 .008 .002
O 27.619 ----- 57.143 10.667
TOTAL: 99.617 99.617 100.000 18.667
All seems to be good.
john
Hi John,
I overlooked some of the options in CalcZAF. What I was looking for was a way to specify the total number of cations (3) along with the total anion charge (-8) during the matrix correction iterations (which is how I've approached it). Note that specifying molar Fe:O ratio = 3:4 in magnetite does not necessarily work because other cations can substitute for either Fe2+ or Fe3+ so that the molar ratio of FeO to Fe2O3 is not always 1:1. Let me dig up an example.
Brian
Quote from: Brian Joy on March 06, 2016, 01:39:42 PM
Hi John,
I overlooked some of the options in CalcZAF. What I was looking for was a way to specify the total number of cations (3) along with the total anion charge (-8) during the matrix correction iterations (which is how I've approached it). Note that specifying molar Fe:O ratio = 3:4 in magnetite does not necessarily work because other cations can substitute for either Fe2+ or Fe3+ so that the molar ratio of FeO to Fe2O3 is not always 1:1. Let me dig up an example.
Brian
It sounds like you are wanting a mineral normalization for magnetite. Probe for EPMA has a number of mineral normalization methods based on site occupancy, but not CalcZAF. You might want to try Julien Allaz's mineral re-normalization web page which is for many minerals:
http://cub.geoloweb.ch/index.php?page=mineral_formula
I've re-coded his php code in VB, but haven't implemented it into PFE yet.
Andrew Locock also posted an oxide normalization spreadsheet here:
http://smf.probesoftware.com/index.php?topic=92.msg3926#msg3926
john
Quote from: Probeman on March 06, 2016, 01:52:51 PM
It sounds like you are wanting a mineral normalization for magnetite. Probe for EPMA has a number of mineral normalization methods based on site occupancy, but not CalcZAF. You might want to try Julien Allaz's mineral re-normalization web page which is for many minerals:
http://cub.geoloweb.ch/index.php?page=mineral_formula
I've re-coded his php code in VB, but haven't implemented it into PFE yet.
Andrew Locock also posted an oxide normalization spreadsheet here:
http://smf.probesoftware.com/index.php?topic=92.msg3926#msg3926
john
I understand what you're suggesting, but that's not what I'm looking for. What I'm looking for is a means of adjusting the oxygen content of the analyzed mineral
during the matrix correction iterations. I wanted to compare my solution to the problem with possible other methods. Let me post an example later. Perhaps this is dealt with in Probe for EPMA (which of course I don't have).
Quote from: Brian Joy on March 06, 2016, 02:17:01 PM
Quote from: Probeman on March 06, 2016, 01:52:51 PM
It sounds like you are wanting a mineral normalization for magnetite. Probe for EPMA has a number of mineral normalization methods based on site occupancy, but not CalcZAF. You might want to try Julien Allaz's mineral re-normalization web page which is for many minerals:
By
http://cub.geoloweb.ch/index.php?page=mineral_formula
I've re-coded his php code in VB, but haven't implemented it into PFE yet.
Andrew Locock also posted an oxide normalization spreadsheet here:
http://smf.probesoftware.com/index.php?topic=92.msg3926#msg3926
john
I understand what you're suggesting, but that's not what I'm looking for. What I'm looking for is a means of adjusting the oxygen content of the analyzed mineral during the matrix correction iterations. I wanted to compare my solution to the problem with possible other methods. Let me post an example later. Perhaps this is dealt with in Probe for EPMA (which of course I don't have).
Hi Brian,
Probe for EPMA does adjust the oxygen concentration during the matrix correction iteration, for example when performing the halogen equivalence correction as I mentioned previously. PFE also adjusts the concentrations of interfering elements during the spectral interference correction (which again, is iterated in the matrix correction), and for a number of other corrections which are also compositionally dependent.
So this is something I am very interested in.
Below are a couple of magnetite analyses that I've processed (PAP/MAC30) by placing constraints on the total number of cations (3) and total anion charge (-8) per formula unit within the matrix correction iterations; I've incorporated these constraints within the subroutine that I use to normalize composition during the iterations. Of course the JEOL software contains no provision for calculating Fe3+/Fe2+ while processing data, and so otherwise I get stuck recalculating after the fact and getting inaccurate results. Usually I check my work against CalcZAF, but, like I noted, I couldn't find a way to apply the constraints in the same manner. In this case the Fe standard is pure Fe3O4 synthesized by Peter Roeder. The output is a little rough; for Z=26, wt% FeO is listed first and Fe2O3 second. In analysis 137, which is close to end-member magnetite, note that Fe Ka Z-std is very close to unity.
(https://smf.probesoftware.com/gallery/381_06_03_16_7_31_28.jpeg)
Quote from: Brian Joy on March 06, 2016, 08:01:03 PM
Below are a couple of magnetite analyses that I've processed (PAP/MAC30) by placing constraints on the total number of cations (3) and total anion charge (-8) per formula unit within the matrix correction iterations; I've incorporated these constraints within the subroutine that I use to normalize composition during the iterations. Of course the JEOL software contains no provision for calculating Fe3+/Fe2+ while processing data, and so otherwise I get stuck recalculating after the fact and getting inaccurate results. Usually I check my work against CalcZAF, but, like I noted, I couldn't find a way to apply the constraints in the same manner. In this case the Fe standard is pure Fe3O4 synthesized by Peter Roeder. The output is a little rough; for Z=26, wt% FeO is listed first and Fe2O3 second. In analysis 137, which is close to end-member magnetite, note that Fe Ka Z-std is very close to unity.
Hi Brian,
Let me see if I understand what you are doing.
So you are essentially calculating the excess oxygen (over FeO) by adjusting the oxygen concentration so that the total oxygens for all cations is exactly 4 atoms (-8 charge). And that this is because you have some Cr, Ti, Al, Mn and Mg replacing some Fe?
Have I got it right so far?
john
Quote from: Probeman on March 07, 2016, 12:10:03 PM
Hi Brian,
Let me see if I understand what you are doing.
So you are essentially calculating the excess oxygen (over FeO) by adjusting the oxygen concentration so that the total oxygens for all cations is exactly 4 atoms (-8 charge). And that this is because you have some Cr, Ti, Al, Mn and Mg replacing some Fe?
Have I got it right so far?
john
Hi John,
Specifically what I'm doing is normalizing first to a given number of cations specified by the user in a settings file. I then calculate the total charge on the cations assuming, for instance, that all Fe is divalent. For this case, the number of Fe3+ per formula unit is equal to the anion charge (corresponding to the number of oxygen ions specified by the user in the settings file) minus the total charge on the normalized cations assuming all Fe is divalent. I then calculate the mass of oxygen required to balance the charge on the cations (whatever they may be) during the iteration while using an "average" Fe charge determined by the above procedure. It's essentially the same procedure used in after-the-fact recalculations, but I've simply built it into the matrix correction iterations. I was afraid that it might hinder convergence, but I haven't run into problems with this.
The scheme that I've implemented is the simplest possible one, and one could envision much more elaborate schemes in which cations are assigned to specific sites during the normalization, as is necessary in the amphiboles, for instance.
Hi Brian,
Ok, so you're essentially performing a mineral formula normalization during the matrix iteration. As I said before I do this in some cases- e.g., fluor-phlogopites where by not including the change in oxygen from the halogen equivalence, there is about a 15% effect on the fluorine concentration. But that's a 5 or 6% absolute change in oxygen concentration, and fluorine Ka is strongly absorbed by oxygen...
Can you post an example composition where you don't iterate the matrix correction during the mineral normalization and the same composition where you do re-iterate the matrix correction after the oxygen has been modified by the mineral re-normalization?
I'd be curious to see the magnitude of the effect.
john
So I tried a test myself. Here is a typical chromite analysis:
ELEMENT ABSCOR FLUCOR ZEDCOR ZAFCOR STP-POW BKS-COR F(x)u Ec Eo/Ec MACs
Fe ka 1.0235 1.0000 1.1012 1.1271 1.1454 .9614 .9620 7.1120 2.1091 175.945
Cr ka .9993 .9530 1.0946 1.0424 1.1351 .9643 .9806 5.9900 2.5042 82.3851
Ti ka 1.0058 .8822 1.0858 .9634 1.1191 .9703 .9665 4.9670 3.0199 135.744
Al ka 1.6721 .9995 .9812 1.6398 .9537 1.0288 .5307 1.5600 9.6154 2436.91
Mn ka .9973 .9958 1.1182 1.1104 1.1617 .9625 .9853 6.5390 2.2939 64.2572
ELEMENT K-RAW K-VALUE ELEMWT% OXIDWT% ATOMIC% FORMULA KILOVOL
Fe ka .27180 .18431 20.774 26.725 10.317 1.931 15.00
Cr ka .47690 .30522 31.817 46.503 16.972 3.177 15.00
Ti ka .00550 .00305 .294 .490 .170 .032 15.00
Al ka .10830 .04715 7.731 14.608 7.947 1.488 15.00
Mn ka .00200 .00147 .163 .211 .082 .015 15.00
Mg 6.290 10.431 7.178 1.344
V .122 .179 .066 .012
O 1.080 1.080 1.872 .350
O 31.956 ----- 55.395 10.371
TOTAL: 100.228 100.228 100.000 18.721
Note the 1.08 wt% excess oxygen. Now I edit the excess oxygen number to 0 wt% and you can see the changes:
ELEMENT ABSCOR FLUCOR ZEDCOR ZAFCOR STP-POW BKS-COR F(x)u Ec Eo/Ec MACs
Fe ka 1.0239 1.0000 1.0997 1.1259 1.1433 .9618 .9616 7.1120 2.1091 175.497
Cr ka .9994 .9529 1.0932 1.0410 1.1330 .9648 .9805 5.9900 2.5042 81.9200
Ti ka 1.0060 .8817 1.0845 .9618 1.1170 .9708 .9663 4.9670 3.0199 134.939
Al ka 1.6755 .9995 .9801 1.6413 .9519 1.0297 .5296 1.5600 9.6154 2417.43
Mn ka .9974 .9957 1.1167 1.1090 1.1596 .9629 .9852 6.5390 2.2939 63.9071
ELEMENT K-RAW K-VALUE ELEMWT% OXIDWT% ATOMIC% FORMULA KILOVOL
Fe ka .27180 .18431 20.752 26.698 10.510 1.931 15.00
Cr ka .47690 .30522 31.774 46.441 17.283 3.175 15.00
Ti ka .00550 .00305 .293 .489 .173 .032 15.00
Al ka .10830 .04715 7.738 14.621 8.111 1.490 15.00
Mn ka .00200 .00147 .163 .210 .084 .015 15.00
Mg 6.290 10.431 7.319 1.345
V .122 .179 .068 .012
O .000 .000 .000 .000
O 31.936 ----- 56.452 10.371
TOTAL: 99.069 99.069 100.000 18.371
Obviously the total is now low by some 1% or so, but the absolute changes in the measured elements due to the reduced excess oxygen are pretty small. On the order of a few hundred PPM for each element.
That is generally smaller than the measurement precision, so is it worth doing? I have to ask myself...
I've attached a CalcZAF input file for this below. Note that there are two samples in it. The first calculating everything elemental and the second calculating everything as oxides (and excess oxygen). The 2nd sample in the file is the example I used above.
Hi John,
Yes, I agree that the effect is small unless Fe2O3 is present in large quantity. This is why I chose magnetite as an example. As far as silicates, possible problematic cases would be minerals such as aegirine-augite and riebeckite, especially if the latter is fluorine-rich.
Below are the same two magnetite analyses as before with all Fe assumed to be present as FeO during the matrix corrections. If I recalculate for Fe2O3 after the fact, then for the first analysis I get 30.62 wt% FeO, 67.14 wt% Fe2O3, and total = 98.71 wt%. For the second analysis, I get 31.68 wt% FeO, 26.16 wt% Fe2O3, and total = 100.04 wt%; wt% Cr2O3 is 31.43 instead of 31.57.
(https://smf.probesoftware.com/gallery/381_07_03_16_2_46_38.jpeg)
Quote from: Brian Joy on March 07, 2016, 03:04:02 PM
Yes, I agree that the effect is small unless Fe2O3 is present in large quantity. This is why I chose magnetite as an example. As far as silicates, possible problematic cases would be minerals such as aegirine-augite and riebeckite, especially if the latter is fluorine-rich.
Hi Brian,
You're showing a magnetite and a chromite in the image of the FORTRAN output. Did you post the wrong image?
If I run a magnetite both ways I get this for magnetite with excess oxygen of 6.881 wt%:
ELEMENT ABSCOR FLUCOR ZEDCOR ZAFCOR STP-POW BKS-COR F(x)u Ec Eo/Ec MACs
Fe ka .9970 1.0000 1.0666 1.0634 1.0944 .9746 .9875 7.1120 2.1091 56.5165
Cr ka 1.0008 .7464 1.0623 .7935 1.0840 .9800 .9791 5.9900 2.5042 88.8413
Ti ka 1.0088 .8934 1.0558 .9516 1.0680 .9885 .9635 4.9670 3.0199 148.135
Al ka 1.8553 .9996 .9587 1.7779 .9077 1.0561 .4783 1.5600 9.6154 2935.69
Mn ka .9982 .9707 1.0841 1.0505 1.1097 .9769 .9844 6.5390 2.2939 68.3900
ELEMENT K-RAW K-VALUE ELEMWT% OXIDWT% ATOMIC% FORMULA KILOVOL
Fe ka 1.00000 .67813 72.110 92.770 42.354 7.907 15.00
Cr ka .00305 .00195 .155 .226 .098 .018 15.00
Ti ka .00013 .00007 .007 .011 .005 .001 15.00
Al ka .00279 .00121 .216 .408 .263 .049 15.00
Mn ka .00082 .00060 .063 .082 .038 .007 15.00
Mg .072 .119 .097 .018
O 6.881 6.881 14.107 2.633
O 20.993 ----- 43.039 8.035
TOTAL: 100.498 100.498 100.000 18.668And if I reduce the excess oxygen to 5.881 wt% (which I think is larger than any change you will see in the oxygen concentration from your charge balancing), the results are:
ELEMENT ABSCOR FLUCOR ZEDCOR ZAFCOR STP-POW BKS-COR F(x)u Ec Eo/Ec MACs
Fe ka .9971 1.0000 1.0648 1.0617 1.0920 .9752 .9875 7.1120 2.1091 56.2059
Cr ka 1.0009 .7455 1.0607 .7915 1.0815 .9807 .9790 5.9900 2.5042 88.3326
Ti ka 1.0090 .8929 1.0543 .9499 1.0656 .9893 .9633 4.9670 3.0199 147.256
Al ka 1.8604 .9996 .9575 1.7806 .9056 1.0573 .4770 1.5600 9.6154 2915.34
Mn ka .9983 .9706 1.0824 1.0488 1.1073 .9775 .9843 6.5390 2.2939 68.0066
ELEMENT K-RAW K-VALUE ELEMWT% OXIDWT% ATOMIC% FORMULA KILOVOL
Fe ka 1.00000 .67813 71.999 92.626 43.232 7.907 15.00
Cr ka .00305 .00195 .154 .226 .100 .018 15.00
Ti ka .00013 .00007 .007 .011 .005 .001 15.00
Al ka .00279 .00121 .216 .409 .269 .049 15.00
Mn ka .00082 .00060 .063 .082 .039 .007 15.00
Mg .072 .119 .099 .018
O 5.881 5.881 12.326 2.254
O 20.961 ----- 43.932 8.035
TOTAL: 99.354 99.354 100.000 18.289
And again, it's a change on the order of a few hundred PPM for Fe. If you leave all the excess oxygen out (6.9 wt%), of course you will get a significantly different matrix correction. But you should be comparing the difference (as I'm trying to show above) between including the mineral normalization in the matrix correction or not.
My point being that whether or not we are performing a mineral normalization for charge balance in the matrix correction, we should *always* be including the oxygen from Fe2O3 in the matrix correction.
I've attached my magnetite example file for CalcZAF below.
No, I didn't post the wrong image. The rock (the locality escapes me) contains a spinel-group mineral that ranges in composition (apparently continuously) from essentially pure magnetite to a mineral that contains in excess of 30 wt% Cr2O3. I don't know if the Cr-rich material has the "inverse" or "normal" spinel structure or something intermediate. I may have used the term "magnetite" too loosely. Regardless, the cation:oxygen ratio is still 3:4.
The only point I'm really trying to make is that there is some convenience in specifying the above ratio within the matrix correction iterations, as then the excess oxygen mass is determined automatically.
Quote from: Brian Joy on March 07, 2016, 07:24:48 PM
No, I didn't post the wrong image. The rock (the locality escapes me) contains a spinel-group mineral that ranges in composition (apparently continuously) from essentially pure magnetite to a mineral that contains in excess of 30 wt% Cr2O3. I don't know if the Cr-rich material has the "inverse" or "normal" structure. I may have used the term "magnetite" too loosely. Regardless, the cation:oxygen ratio is still 3:4.
The only point I'm really trying to make is that there is some convenience in specifying the above ratio within the matrix correction iterations, as then the excess oxygen mass is determined automatically.
Hi Brian,
I appreciate that the composition varies. It's just that I asked you for an example of the *same* composition both with and without the mineral normalization adjustment of oxygen in the matrix iteration. The reason being that I suspect the effect on the matrix correction, with the expected range of change in excess oxygen, is only a few hundred PPM and therefore "in the noise".
I also appreciate that the method you propose can calculate the excess oxygen automatically for a given stoichiometry. That is very cool. But in this particular range of compositions, it would make as much sense to perform a specific mineral normalization *after* specifying oxygen by difference, by fixed composition or by fixed stoichiometry in the matrix iteration. I suspect the change in the matrix effect in all these cases will be relatively insignificant from the mineral normalization.
But I'll grant you it's a clever idea to perform the mineral re-normalization in the matrix iteration. Though besides the apparent minimal effect of the matrix correction from this iterated mineral normalization, my other difficulty would be figuring out how to make this method of yours "universal" for all compositions and stochiometries.
It still seems like a lot of work for a small benefit, but maybe there other examples with larger matrix corrections (such as the halogen equivalence correction for F Ka in fluor-phlogopite example I have already implemented in the CalcZAF/Probe for EPMA code), that would benefit more from this treatment? Let's think about it.
Thanks for posting your thoughts and questions. I am very interested in pursuing these ideas further.
Here's an example of a modification of a calculated oxygen concentration during the matrix correction due to the halogen equivalence correction.
If we analyze a fluor-phlogopite with default oxygen stochiometry we obtain something like this:
Un 4 fluor-phlogopite, Results in Oxide Weight Percents
ELEM: MgO F SiO2 O Al2O3 K2O SUM
121 29.467 9.261 42.990 .000 12.100 11.180 104.998
122 29.329 9.508 43.069 .000 12.100 11.180 105.185
123 29.270 9.348 42.982 .000 12.100 11.180 104.879
124 28.759 8.999 43.073 .000 12.100 11.180 104.111
125 28.779 9.208 42.038 .000 12.100 11.180 103.305
126 28.974 9.253 42.571 .000 12.100 11.180 104.078
AVER: 29.096 9.263 42.787 .000 12.100 11.180 104.426
SDEV: .300 .167 .412 .000 .000 .000 .719
SERR: .123 .068 .168 .000 .000 .000
%RSD: 1.03 1.81 .96 -154.92 .00 .00
STDS: 273 835 273 --- --- ---
ZCOR: 1.4193 3.6139 1.3345 --- --- ---
Note that the total is high and that the fluorine concentration is around 9.2 wt% when it should be 9.0 wt%. Why is this? Because the matrix correction for F ka is too high from the extra 3 or 4 % oxygen being added in by stoichiometry. Note the ZCOR (matrix correction) is about 3.6.
But because fluorine is replacing some of the stoichiometric oxygen, we need to apply the halogen correction in PFE, which not only subtracts the halogen equivalent of oxygen, but re-calculates the matrix correction for fluorine as seen here iteratively:
Un 4 fluor-phlogopite, Results in Oxide Weight Percents
ELEM: MgO F SiO2 O Al2O3 K2O SUM
121 29.203 9.036 43.035 -3.805 12.100 11.180 100.750
122 29.060 9.270 43.115 -3.903 12.100 11.180 100.821
123 29.005 9.118 43.027 -3.839 12.100 11.180 100.591
124 28.507 8.785 43.115 -3.699 12.100 11.180 99.988
125 28.520 8.982 42.083 -3.782 12.100 11.180 99.082
126 28.713 9.026 42.616 -3.801 12.100 11.180 99.834
AVER: 28.835 9.036 42.832 -3.805 12.100 11.180 100.178
SDEV: .296 .159 .412 .067 .000 .000 .673
SERR: .121 .065 .168 .027 .000 .000
%RSD: 1.03 1.76 .96 -1.76 .00 .00
STDS: 273 835 273 --- --- ---
ZCOR: 1.4065 3.5254 1.3359 --- --- ---
Now the fluorine concentration is correct and note that the matrix correction is significantly less at 3.5. This is because F Ka is strongly absorbed by oxygen and therefore the matrix correction needs to be calculated again.
The CalcZAF.dat example input file contains a similar sample.
Quote from: Brian Joy on March 07, 2016, 07:24:48 PM
No, I didn't post the wrong image. The rock (the locality escapes me) contains a spinel-group mineral that ranges in composition (apparently continuously) from essentially pure magnetite to a mineral that contains in excess of 30 wt% Cr2O3. I don't know if the Cr-rich material has the "inverse" or "normal" structure. I may have used the term "magnetite" too loosely. Regardless, the cation:oxygen ratio is still 3:4.
The only point I'm really trying to make is that there is some convenience in specifying the above ratio within the matrix correction iterations, as then the excess oxygen mass is determined automatically.
Hi Brian,
I do agree with you on the "determined automatically" aspect of this.
Even if the difference in the matrix correction between say, oxygen by difference and oxygen by 3:4 stoichiometry is insignificant in these oxides, this method of yours does allow for an independent determination of Fe charge states.
That is to say, this method doesn't have to be in the matrix correction iteration, but it certainly doesn't hurt to have it there... I'm trying to think of how to implement this for all oxide chemistries.
Can someone explain something to me (I'm no geologist!)? If chromite is FeCr2O4 to MgCr2O4 (all iron is Fe+2), why do the Smithsonian chromite compositions contain some excess oxygen?
St 396 Chromite (UC # 523-9)
Average Total Oxygen: 33.042 Average Total Weight%: 100.194
Average Calculated Oxygen: 31.942 Average Atomic Number: 17.533
Average Excess Oxygen: 1.100 Average Atomic Weight: 27.796
ELEM: Cr2O3 TiO2 Al2O3 FeO MnO MgO V2O3 O SiO2
XRAY: ka ka ka ka ka ka ka ka ka
OXWT: 46.632 .500 14.530 26.620 .191 10.431 .179 1.100 .011
ELWT: 31.905 .300 7.690 20.692 .148 6.290 .122 33.042 .005
KFAC: .3060 .0031 .0469 .1836 .0013 .0330 .0011 .2290 .0000
ZCOR: 1.0427 .9632 1.6398 1.1272 1.1104 1.9040 1.0698 1.4431 1.4097
AT% : 17.023 .174 7.907 10.279 .075 7.180 .066 57.292 .005
24 O: 7.131 .073 3.312 4.306 .031 3.008 .028 24.000 .002
St 455 Chromite USNM 117075
Average Total Oxygen: 33.208 Average Total Weight%: 99.640
Average Calculated Oxygen: 32.909 Average Atomic Number: 17.187
Average Excess Oxygen: .299 Average Atomic Weight: 27.414
ELEM: Cr2O3 FeO Al2O3 MgO CaO MnO TiO2 NiO SiO2 O
XRAY: ka ka ka ka ka ka ka ka ka ka
OXWT: 60.502 13.040 9.920 15.200 .120 .230 .120 .160 .049 .299
ELWT: 41.395 10.136 5.250 9.166 .086 .178 .072 .126 .023 33.208
KFAC: .3859 .0890 .0317 .0493 .0009 .0016 .0008 .0011 .0002 .2384
ZCOR: 1.0727 1.1388 1.6541 1.8607 .9619 1.1152 .9396 1.1224 1.3950 1.3931
AT% : 21.903 4.993 5.353 10.376 .059 .089 .041 .059 .023 57.103
24 O: 9.206 2.099 2.250 4.361 .025 .037 .017 .025 .009 24.000
If Al2O3 replaces some Cr2O3, that won't change the cation stoichiometry, right?
If measured oxygen is < the calculated, how is that an excess?
Quote from: qEd on March 09, 2016, 10:28:50 AM
If measured oxygen is < the calculated, how is that an excess?
It's a "negative" excess! ;D
What? You can't handle negative results? ;)
In any case, we're not measuring oxygen, we're just calculating it by cation stoichiometry and then adjusting that oxygen concentration for various reasons, e.g., halogen equivalence, FeO/Fe2O3 ratio, etc... and then re-calculating the matrix correction for the change in stoichiometric oxygen. 8)
Many terrestrial minerals contain both Fe2+ and Fe3+. The Smithsonian wet chemical analyses measure Fe gravimetrically to get Fe0, and also a titration is done to determine Fe 2+ vs. 3+
Phases like fayalite may contain small amounts of Fe3+
Andradite garnet by definition is an Fe3+ garnet end member, but almandine is the nominally Fe2+ end member.
Info like this is available on web mineral web pages and in mineralogy texts.
Paul
Quote from: Paul Carpenter on March 10, 2016, 12:28:45 PM
Many terrestrial minerals contain both Fe2+ and Fe3+. The Smithsonian wet chemical analyses measure Fe gravimetrically to get Fe0, and also a titration is done to determine Fe 2+ vs. 3+
Phases like fayalite may contain small amounts of Fe3+
Andradite garnet by definition is an Fe3+ garnet end member, but almandine is the nominally Fe2+ end member.
Info like this is available on web mineral web pages and in mineralogy texts.
I am not explaining myself well enough. I am aware that Fe+2 and Fe+3 coexist in many minerals, but if the ideal chromite formula is FeCr2O4 to MgCr2O4, that is, all Fe is as FeO (Fe+2), then why do the Smithsonian chromite compositions contain some excess oxygen when Fe is calculated as FeO?
I get that Al2O3 can replace Cr2O3 in the chromite formula, alright. But that won't change the cation to oxygen ratio, correct? I guess my question is: can we get a "chromite structure" where all the Cr is replaced by Al such that it is FeAl2O4 and all Fe is still FeO?
More specifically, where is the excess oxygen coming from? Clearly some of the Fe is Fe+3 but which site is it going into? The Cr2O3 site I guess? I assume it's a structural issue.
Jarosewich et al 1980 report this analysis with a comment (#3: Total Fe reported as FeO). However, I agree with others that it is common for spinel-group mineral to have substitution of Al2O3 or Cr2O3 by Fe2O3. You can have a glimpse (estimate) of the FeO vs. Fe2O3 by using an oxygen AND cation normalization (i.e., you fix as a constraint both the total number of anion, 4 for chromite, and the total number of cation, 3). With this, you can perform a charge balance (how many positive charges, assuming only FeO, do you have to compensate for the 8 negative charges? If not enough positive charges, then you "convert" some FeO to Fe2O3 to balance the charge).
I've played with the chromite analysis given in Jarosewich. Here are the results (1st line = cation AND oxygen normalization, 2nd line = oxygen normalization only assuming all Fe as Fe2+):
Al2O3 Cr2O3 Fe2O3 FeO MgO MnO Total
Cation 9.92 60.50 3.45 9.93 15.20 0.11 99.12
Oxygen 9.92 60.50 0.00 13.04 15.20 0.11 98.77
Al Cr Fe3+ Fe2+ Mg Mn Cation O
Cation 0.376 1.540 0.084 0.267 0.730 0.003 3.000 4.000
Oxygen 0.380 1.556 0.000 0.355 0.737 0.003 3.032 4.000
Interestingly enough, with the estimate of Fe2+ / Fe3+, the total number of trivalent cation is now EXACTLY 2.000 (versus 1.937 if you assume all Fe2+ as Fe3+). This is EXACTLY the mineral formula of chromite:
(Fe2+ 0.267, Mg 0.730, Mn 0.003) (Cr 1.540, Al 0.376, Fe3+ 0.084) O4
Sum trivalent = 2.000
Sum divalent = 1.000
Now of course this is the CALCULATED (estimate!!!) of Fe2O3 - not measured. This addition of Fe3+ represent an excess of 0.346 wt-% oxygen, close to the 0.299 you report. Maybe the 0.299 actually reflect a non-published MEASUREMENT of Fe2O3 and FeO by Mössbauer or so...
QED.
Here is a summary spreadsheet of my test... I used my website to run the normalization (http://cub.geoloweb.ch/index.php?page=mineral_formula).
J.
Quote from: Julien on March 10, 2016, 02:13:19 PM
Here is a summary spreadsheet of my test... I used my website to run the normalization (http://cub.geoloweb.ch/index.php?page=mineral_formula).
J.
Thanks Julien,
That makes more sense!
FYI to all: you need to be logged in to see Julien's attachment in the above post.
I understand the purpose of recalculating the oxidation state of multi-valent elements is to satisfy the "assumed chemical formula" of a crystalline phase. In the context of high temperature metallurgy, many phases do not really have fixed "formula", an example is magnetite. At high temperature(~1450oC) in air, magnetite can take significant more Fe3+ so that Fe3+/Fe2+ in it is greater than 2.
I'd appreciate if someone could educate me to what extent mineral phases are close to their the theoretical chemical formulas in geological context.
Quote from: jeffchen on February 07, 2018, 03:37:07 PM
I understand the purpose of recalculating the oxidation state of multi-valent elements is to satisfy the "assumed chemical formula" of a crystalline phase. In the context of high temperature metallurgy, many phases do not really have fixed "formula", an example is magnetite. At high temperature(~1450oC) in air, magnetite can take significant more Fe3+ so that Fe3+/Fe2+ in it is greater than 2.
I'd appreciate if someone could educate me to what extent mineral phases are close to their the theoretical chemical formulas in geological context.
Hi Jeff,
I am not a geologist so I probably can't help you, but I have a question. Are you saying, for example, that in the case of magnetite Fe3O4, which is ideally composed of one FeO molecule and one Fe2O3 molecule, that at high temperatures, the ratio of FeO to Fe2O3 is no longer 1:1? That there are more Fe2O3 molecules than FeO molecules in high temperature magnetite?
john
Bernie Wood, a long time ago, wrote a paper on spinels (I believe) and noted that because the Fe substitution mechanism is known, analysing those elements that Fe 3+ can substitute in for allows you to accurately determine the speciation of iron, with accuracy approaching Mössbauer. Yeah, yeah accurate standards of known Fe2/3+ are required and good probe stats required (Peaksight 6 defeated me last time I attempted this - it had count times the stopped at a fixed precision, now changed I believe), but it works.
The reason I was interested is trying to do Fe speciation by XANES in crystalline materials is a weapons grade pain in the posterior and a drain on life force. Sometimes the old skool probe is just better.
Quote from: Probeman on February 09, 2018, 10:06:37 AM
Quote from: jeffchen on February 07, 2018, 03:37:07 PM
I understand the purpose of recalculating the oxidation state of multi-valent elements is to satisfy the "assumed chemical formula" of a crystalline phase. In the context of high temperature metallurgy, many phases do not really have fixed "formula", an example is magnetite. At high temperature(~1450oC) in air, magnetite can take significant more Fe3+ so that Fe3+/Fe2+ in it is greater than 2.
I'd appreciate if someone could educate me to what extent mineral phases are close to their the theoretical chemical formulas in geological context.
Hi Jeff,
I am not a geologist so I probably can't help you, but I have a question. Are you saying, for example, that in the case of magnetite Fe3O4, which is ideally composed of one FeO molecule and one Fe2O3 molecule, that at high temperatures, the ratio of FeO to Fe2O3 is no longer 1:1? That there are more Fe2O3 molecules than FeO molecules in high temperature magnetite?
john
Hi John,
That's correct, I attached a FeO-Fe2O3 phase diagram at 1atm presssure. As you can see the magnetite can have more Fe2O3 than Fe3O4, it starts at around 1100oC and reaches maximum at around 1800oC.
Hi Jeff,
I learn something new everyday! :)
So in natural magnetites, what is the observed range of compositions? I guess this is what you were asking about in your original post?
Maybe I should ask, if stoichiometric Fe3O4 is 72.36 wt% iron and 27.64 wt.% oxygen, based on that phase diagram you posted, how much would we expect to see the Fe concentration change in natural materials (on Earth!)?
Again apologies for asking basic geology questions.
john
It has taken some time but with the help of a number of friends (Andrew Locock, Anette von der Handt, John Fournelle, and Emma Bullock), we have finally realized Brian Joy's most excellent suggestion to implement a ferrous/ferric calculation into the matrix correction physics.
The implementation of this feature in Probe for EPMA will be described in this post and others soon to follow:
https://smf.probesoftware.com/index.php?topic=92.msg8591#msg8591
But I thought I'd start here with the implementation in CalcZAF. Though both PFE and CalcZAF are using exactly the same code for the ferrous/ferric calculation and subsequent treatment in the ZAFSmp routine as shown here (and hosted on Github):
' Calculate element relative to stoichiometric oxygen based on previous iteration calculation of oxygen
If zaf.il%(zaf.in0%) = 0 Then ' if calculating oxygen by stoichiometry
For i% = 1 To zaf.in1%
If zaf.il%(i%) = 15 Then
r1!(i%) = (r1!(zaf.in0%) / zaf.atwts!(zaf.in0%)) * sample(1).StoichiometryRatio! * zaf.atwts!(i%)
zaf.ksum! = zaf.ksum! + r1!(i%)
zaf.krat!(i%) = r1!(i%)
End If
Next i%
' Calculate amount of stoichiometric oxygen and add to total
r1!(zaf.in0%) = 0#
For i% = 1 To zaf.in1%
r1!(zaf.in0%) = r1!(zaf.in0%) + r1!(i%) * zaf.p1!(i%)
Next i%
' Calculate equivalent oxygen from halogens and subtract from calculated oxygen if flagged
If UseOxygenFromHalogensCorrectionFlag Then
r1!(zaf.in0%) = r1!(zaf.in0%) - ConvertHalogensToOxygen(zaf.in1%, sample(1).Elsyms$(), sample(1).DisableQuantFlag%(), r1!())
If ierror Then Exit Sub
End If
' Calculate excess oxygen from ferric Fe
If sample(1).FerrousFerricCalculationFlag Then
Call ConvertFerrousFerricRatioFromComposition(zaf.in0%, zaf.Z%(), zaf.atwts!(), r1!(), sample(1).numcat%(), sample(1).numoxd%(), zaf.p1!(), sample(1).DisableQuantFlag%(), sample(1).FerrousFerricTotalCations!, sample(1).FerrousFerricTotalOxygens!, tFerricToTotalIronRatio!, tFerricOxygen!, tFe_as_FeO!, tFe_as_Fe2O3!)
If ierror Then Exit Sub
r1(zaf.in0%) = r1(zaf.in0%) + tFerricOxygen! / 100#
End If
' Add to sum
zaf.ksum! = zaf.ksum! + r1!(zaf.in0%)
End If
So here's an example in CalcZAF of a magnetite unknown using magnetite as a standard:
(https://smf.probesoftware.com/gallery/1_19_08_19_3_41_44.png)
And calculating Fe as FeO we see the expected low totals due to some of the iron being Fe2O3:
Magnetite
STANDARD PARAMETERS (TOA= 40):
ELEMENT STDNUM STDCONC STDKFAC Z-BAR ABSCOR FLUCOR ZEDCOR ZAFCOR
Fe Ka 39 72.359 .6810 21.0246 .9969 1.0000 1.0660 1.0626
ELEMENT STP-POW BKS-COR F(x)e F(x)s Eo Ec Eo/Ec
Fe Ka 1.0935 .9748 .9846 .9877 15.00 7.1120 2.1091
SAMPLE: 2, TOA: 40, ITERATIONS: 1, Z-BAR: 21.99148
ELEMENT ABSCOR FLUCOR ZEDCOR ZAFCOR STP-POW BKS-COR F(x)u Ec Eo/Ec MACs uZAF/sZAF
Fe ka .9974 1.0000 1.0530 1.0503 1.0753 .9793 .9871 7.1120 2.1091 53.4587 .9883946
ELEMENT K-RAW K-VALUE ELEMWT% OXIDWT% ATOMIC% FORMULA KILOVOL
Fe ka 1.00023 .68111 71.536 92.031 50.000 1.000 15.00
O .000 .000 .000 .000
O 20.495 ----- 50.000 1.000
TOTAL: 92.031 92.031 100.000 2.000
So by calculating Fe as FeO we get 71.536 wt% Fe, but are missing about 7 wt% excess oxygen from ferric iron. Now clicking on the Calculation Options button in CalcZAF we see a new checkbox and two text fields for the mineral stoichiometry.
(https://smf.probesoftware.com/gallery/1_19_08_19_3_42_15.png)
Since this is a magnetite, that would be 3 total cations and 4 totals oxygens. Other common minerals as follows (I'm no geologist so please let me know if I got these wrong):
Mineral Cations Oxygens
hematite 2 3
olivine 3 4
ilmenite 2 3
garnet 8 12
feldspar 5 8
pyroxene 4 6
With this flag checked and the 3 cations and 4 oxygens specified we now get this result:
Magnetite
STANDARD PARAMETERS (TOA= 40):
ELEMENT STDNUM STDCONC STDKFAC Z-BAR ABSCOR FLUCOR ZEDCOR ZAFCOR
Fe Ka 39 72.359 .6810 21.0246 .9969 1.0000 1.0660 1.0626
ELEMENT STP-POW BKS-COR F(x)e F(x)s Eo Ec Eo/Ec
Fe Ka 1.0935 .9748 .9846 .9877 15.00 7.1120 2.1091
SAMPLE: 2, TOA: 40, ITERATIONS: 2, Z-BAR: 21.02464
ELEMENT ABSCOR FLUCOR ZEDCOR ZAFCOR STP-POW BKS-COR F(x)u Ec Eo/Ec MACs uZAF/sZAF
Fe ka .9969 1.0000 1.0660 1.0626 1.0935 .9748 .9877 7.1120 2.1091 55.6448 1.000000
ELEMENT K-RAW K-VALUE ELEMWT% OXIDWT% ATOMIC% FORMULA KILOVOL
Fe ka 1.00023 .68111 72.376 93.111 42.857 1.000 15.00
O .000 .000 .000 .000
O 27.647 ----- 57.143 1.333
TOTAL: 100.023 100.023 100.000 2.333
Note that not only did our total get much better, but our Fe wt% went from 71.536 to 72.376, just from adding the ferric oxygen into the matrix correction! This is simply because Fe Ka is absorbed more by oxygen, than by iron and therefore needs to be included in the matrix correction phsyics as suggested originally by Brian Joy.
But of course the cool part is the new output as seen here:
FerricIronToTotalIronRatio: .6667
Ferrous Oxide (FeO): 30.6769
Ferric Oxide (Fe2O3): 68.1855
Excess Oxygen from Fe2O3: 6.8316
Right now it outputs this twice (once for each iteration loop), so I'll need to fix that, but the new code calculates the ferric to total iron ratio, the ferrous and ferric iron, and the excess oxygen from the ferric iron.
Please download and try it out and let me know what you think. I have no idea how much of an effect this will have on ones calculated temperatures and pressures, so let us know if you find anything interesting.
Quote from: Julien on March 10, 2016, 02:12:06 PM
Jarosewich et al 1980 report this analysis with a comment (#3: Total Fe reported as FeO). However, I agree with others that it is common for spinel-group mineral to have substitution of Al2O3 or Cr2O3 by Fe2O3. You can have a glimpse (estimate) of the FeO vs. Fe2O3 by using an oxygen AND cation normalization (i.e., you fix as a constraint both the total number of anion, 4 for chromite, and the total number of cation, 3). With this, you can perform a charge balance (how many positive charges, assuming only FeO, do you have to compensate for the 8 negative charges? If not enough positive charges, then you "convert" some FeO to Fe2O3 to balance the charge).
I've played with the chromite analysis given in Jarosewich. Here are the results (1st line = cation AND oxygen normalization, 2nd line = oxygen normalization only assuming all Fe as Fe2+):
Al2O3 Cr2O3 Fe2O3 FeO MgO MnO Total
Cation 9.92 60.50 3.45 9.93 15.20 0.11 99.12
Oxygen 9.92 60.50 0.00 13.04 15.20 0.11 98.77
Al Cr Fe3+ Fe2+ Mg Mn Cation O
Cation 0.376 1.540 0.084 0.267 0.730 0.003 3.000 4.000
Oxygen 0.380 1.556 0.000 0.355 0.737 0.003 3.032 4.000
Interestingly enough, with the estimate of Fe2+ / Fe3+, the total number of trivalent cation is now EXACTLY 2.000 (versus 1.937 if you assume all Fe2+ as Fe3+). This is EXACTLY the mineral formula of chromite:
(Fe2+ 0.267, Mg 0.730, Mn 0.003) (Cr 1.540, Al 0.376, Fe3+ 0.084) O4
Sum trivalent = 2.000
Sum divalent = 1.000
Now of course this is the CALCULATED (estimate!!!) of Fe2O3 - not measured. This addition of Fe3+ represent an excess of 0.346 wt-% oxygen, close to the 0.299 you report. Maybe the 0.299 actually reflect a non-published MEASUREMENT of Fe2O3 and FeO by Mössbauer or so...
QED.
Hi,
Anyone know why the total is so low for this chromite.
The interesting thing about chromites (particularly when considering oxidation state) is that the Mg and Al concentrations are very sensitivity to matrix correction and MACs used, and can change values by 1-2wt.% absolute.
Is there any chromite standard that has been independently measured?
Thanks
Ben
Quote from: Ben Buse on December 03, 2019, 08:45:36 AM
Quote from: Julien on March 10, 2016, 02:12:06 PM
Jarosewich et al 1980 report this analysis with a comment (#3: Total Fe reported as FeO). However, I agree with others that it is common for spinel-group mineral to have substitution of Al2O3 or Cr2O3 by Fe2O3. You can have a glimpse (estimate) of the FeO vs. Fe2O3 by using an oxygen AND cation normalization (i.e., you fix as a constraint both the total number of anion, 4 for chromite, and the total number of cation, 3). With this, you can perform a charge balance (how many positive charges, assuming only FeO, do you have to compensate for the 8 negative charges? If not enough positive charges, then you "convert" some FeO to Fe2O3 to balance the charge).
I've played with the chromite analysis given in Jarosewich. Here are the results (1st line = cation AND oxygen normalization, 2nd line = oxygen normalization only assuming all Fe as Fe2+):
Al2O3 Cr2O3 Fe2O3 FeO MgO MnO Total
Cation 9.92 60.50 3.45 9.93 15.20 0.11 99.12
Oxygen 9.92 60.50 0.00 13.04 15.20 0.11 98.77
Al Cr Fe3+ Fe2+ Mg Mn Cation O
Cation 0.376 1.540 0.084 0.267 0.730 0.003 3.000 4.000
Oxygen 0.380 1.556 0.000 0.355 0.737 0.003 3.032 4.000
Interestingly enough, with the estimate of Fe2+ / Fe3+, the total number of trivalent cation is now EXACTLY 2.000 (versus 1.937 if you assume all Fe2+ as Fe3+). This is EXACTLY the mineral formula of chromite:
(Fe2+ 0.267, Mg 0.730, Mn 0.003) (Cr 1.540, Al 0.376, Fe3+ 0.084) O4
Sum trivalent = 2.000
Sum divalent = 1.000
Now of course this is the CALCULATED (estimate!!!) of Fe2O3 - not measured. This addition of Fe3+ represent an excess of 0.346 wt-% oxygen, close to the 0.299 you report. Maybe the 0.299 actually reflect a non-published MEASUREMENT of Fe2O3 and FeO by Mössbauer or so...
QED.
Hi,
Anyone know why the total is so low for this chromite.
The interesting thing about chromites (particularly when considering oxidation state) is that the Mg and Al concentrations are very sensitivity to matrix correction and MACs used, and can change values by 1-2wt.% absolute.
Is there any chromite standard that has been independently measured?
Thanks
Ben
Hi Ben,
I'm not sure where some of these numbers came from, but my USNM chromite shows this composition along with a 99.64% total, including 0.299 wt% excess oxygen.
St 455 Chromite USNM 117075
TakeOff = 40.0 KiloVolt = 15.0 Density = 4.750 Type = oxide
Analysis (wet chemistry) by Gene Jarosewich
Oxide and Elemental Composition
Average Total Oxygen: 33.208 Average Total Weight%: 99.640
Average Calculated Oxygen: 32.909 Average Atomic Number: 17.187
Average Excess Oxygen: .299 Average Atomic Weight: 27.414
ELEM: Cr2O3 FeO Al2O3 MgO CaO MnO TiO2 NiO SiO2 O
XRAY: ka ka ka ka ka ka ka ka ka ka
OXWT: 60.502 13.040 9.920 15.200 .120 .230 .120 .160 .049 .299
ELWT: 41.395 10.136 5.250 9.166 .086 .178 .072 .126 .023 33.208
KFAC: .3859 .0890 .0317 .0493 .0009 .0016 .0008 .0011 .0002 .2384
ZCOR: 1.0727 1.1388 1.6541 1.8607 .9619 1.1152 .9396 1.1224 1.3950 1.3931
AT% : 21.903 4.993 5.353 10.376 .059 .089 .041 .059 .023 57.103
24 O: 9.206 2.099 2.250 4.361 .025 .037 .017 .025 .009 24.000
Of course everyone knows my feelings about natural standard materials- they just aren't pure enough, nor homogeneous enough, nor inclusion free enough for my "tastes". :P
https://smf.probesoftware.com/index.php?topic=301.msg2405#msg2405
I wonder if there's someone out there that could synthesize a pure end member chromite (FeCr2O4) for us? Wouldn't that be wonderful?
Quote from: Ben Buse on December 03, 2019, 08:45:36 AM
Hi,
Anyone know why the total is so low for this chromite.
The interesting thing about chromites (particularly when considering oxidation state) is that the Mg and Al concentrations are very sensitivity to matrix correction and MACs used, and can change values by 1-2wt.% absolute.
Is there any chromite standard that has been independently measured?
Thanks
Ben
Hello,
Forsythe & Fisk (Proceedings of the Ocean Drilling Program, Scientific Results Vol 135, pp. 585-594, 1994) in their Table 2 reported the following additional oxides (n=62, with standard deviations in parentheses):
SiO2 0.07 (0.01), TiO2 0.12 (0.02), ZnO 0.06 (0.03), NiO 0.17 (0.02), V2O5 0.10 (0.02), Na2O 0.01 (0.01), whose sum is 0.53 wt%.
My own in-house analyses of Tiebaghi chromite USNM 117075 (n=75) give the following additional oxides:
- below the limit of detection are SiO2, CaO, Nb2O5, SO3, CoO, Na2O, and K2O;
- found were TiO2 0.11 (0.01), ZnO 0.03 (0.02), NiO 0.17 (0.01), V2O3 0.08 (0.01), sum 0.38 wt%.
These results are in generally good agreement with Forsythe & Fisk (1994).
[Note that 0.08 V2O3 is essentially equivalent to 0.10 V2O5.]
In particular, I find no CaO above detection, in contrast with the 1980 report of Jarosewich et al.
However, this is in agreement with the most recent on-line posting of the Smithsonian Microprobe Standards datasheets, where CaO is omitted from the chromite entry.
The implication is an additional ~0.4 wt% of elements in addition to those listed on the current SMS datasheet.
And the consequent atomic proportion of ferric iron would be around 24% (around 3.5 wt% Fe2O3).
It would be interesting to know if others have observed these minor elements.
Best regards,
Andrew
Quote from: AndrewLocock on December 17, 2019, 05:09:49 PM
My own in-house analyses of Tiebaghi chromite USNM 117075 (n=75) give the following additional oxides:
- below the limit of detection are SiO2, CaO, Nb2O5, SO3, CoO, Na2O, and K2O;
- found were TiO2 0.11 (0.01), ZnO 0.03 (0.02), NiO 0.17 (0.01), V2O3 0.08 (0.01), sum 0.38 wt%.
These results are in generally good agreement with Forsythe & Fisk (1994).
[Note that 0.08 V2O3 is essentially equivalent to 0.10 V2O5.]
In particular, I find no CaO above detection, in contrast with the 1980 report of Jarosewich et al.
However, this is in agreement with the most recent on-line posting of the Smithsonian Microprobe Standards datasheets, where CaO is omitted from the chromite entry.
The implication is an additional ~0.4 wt% of elements in addition to those listed on the current SMS datasheet.
And the consequent atomic proportion of ferric iron would be around 24% (around 3.5 wt% Fe2O3).
It would be interesting to know if others have observed these minor elements.
Best regards,
Andrew
I often use Tiebaghi chromite as a secondary standard. I find that it is inhomogeneous in Fe and Mg. Using synthetic eskolaite (Cr), synthetic spinel (Mg, Al), synthetic magnetite (Fe), synthetic hausmannite (Mn), natural rutile (Ti), natural gahnite (Zn), synthetic liebenbergite (Ni), and natural wollastonite (Si, Ca) as standards, I get the following averages (and standard deviations) from a subset of 44 analyses collected on nine occasions:
SiO
2: bdl
TiO
2: 0.12 (0.02) wt%
Al
2O
3: 9.75 (0.05)
Cr
2O
3: 61.10 (0.42)
V
2O
3: 0.09 (0.02)
Fe
2O
3: 3.25 (0.44)
FeO: 10.14 (0.31)
MnO: 0.20 (0.01)
MgO: 15.11 (0.21)
ZnO: 0.05 (0.03)
NiO: 0.18 (0.01)
CaO: bdl
Total: 99.98
I've used the PAP atomic number and absorption corrections with Heinrich's (1987) mass absorption coefficients and Reed's (1990) characteristic fluorescence correction. I've recalculated for Fe
2O
3 by charge balance within the iterations used to calculate the matrix correction factors.